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Perl functions A-Z | Perl functions by category | The 'perlfunc' manpage

NAME

perlfunc - Perl builtin functions

DESCRIPTION

The functions in this section can serve as terms in an expression.They fall into two major categories: list operators and named unaryoperators. These differ in their precedence relationship with afollowing comma. (See the precedence table in perlop.) Listoperators take more than one argument, while unary operators can nevertake more than one argument. Thus, a comma terminates the argument ofa unary operator, but merely separates the arguments of a listoperator. A unary operator generally provides scalar context to itsargument, while a list operator may provide either scalar or listcontexts for its arguments. If it does both, scalar arguments come first and list argument follow, and there can only everbe one such list argument. For instance, splice() has three scalararguments followed by a list, whereas gethostbyname() has four scalararguments.

In the syntax descriptions that follow, list operators that expect alist (and provide list context for elements of the list) are shownwith LIST as an argument. Such a list may consist of any combinationof scalar arguments or list values; the list values will be includedin the list as if each individual element were interpolated at thatpoint in the list, forming a longer single-dimensional list value.Commas should separate literal elements of the LIST.

Any function in the list below may be used either with or withoutparentheses around its arguments. (The syntax descriptions omit theparentheses.) If you use parentheses, the simple but occasionally surprising rule is this: It looks like a function, therefore it is afunction, and precedence doesn't matter. Otherwise it's a listoperator or unary operator, and precedence does matter. Whitespacebetween the function and left parenthesis doesn't count, so sometimesyou need to be careful:

  1.  print 1+2+4;  # Prints 7.
    2.  
  2.  print(1+2) + 4;   # Prints 3.
    3.  
  3.  print (1+2)+4; # Also prints 3!
    4.  
  4.  print +(1+2)+4;   # Prints 7.
    5.  
  5.  print ((1+2)+4);  # Prints 7.
    6.

If you run Perl with the -w switch it can warn you about this. Forexample, the third line above produces:

  1.  print (...) interpreted as function at - line 1.
    2.  
  2.  Useless use of integer addition in void context at - line 1.
    3.

A few functions take no arguments at all, and therefore work as neitherunary nor list operators. These include such functions as timeand endpwent. For example, time+86_400 always meanstime() + 86_400.

For functions that can be used in either a scalar or list context,nonabortive failure is generally indicated in scalar context byreturning the undefined value, and in list context by returning theempty list.

Remember the following important rule: There is no rule that relatesthe behavior of an expression in list context to its behavior in scalarcontext, or vice versa. It might do two totally different things.Each operator and function decides which sort of value would be mostappropriate to return in scalar context. Some operators return thelength of the list that would have been returned in list context. Someoperators return the first value in the list. Some operators return thelast value in the list. Some operators return a count of successfuloperations. In general, they do what you want, unless you wantconsistency.

A named array in scalar context is quite different from what would atfirst glance appear to be a list in scalar context. You can't get a listlike (1,2,3) into being in scalar context, because the compiler knowsthe context at compile time. It would generate the scalar comma operatorthere, not the list construction version of the comma. That means itwas never a list to start with.

In general, functions in Perl that serve as wrappers for system calls ("syscalls")of the same name (like chown(2), fork(2), closedir(2), etc.) returntrue when they succeed and undef otherwise, as is usually mentionedin the descriptions below. This is different from the C interfaces,which return -1 on failure. Exceptions to this rule include wait,waitpid, and syscall. System calls also set the special $!variable on failure. Other functions do not, except accidentally.

Extension modules can also hook into the Perl parser to define newkinds of keyword-headed expression. These may look like functions, butmay also look completely different. The syntax following the keywordis defined entirely by the extension. If you are an implementor, seePL_keyword_plugin in perlapi for the mechanism. If you are using sucha module, see the module's documentation for details of the syntax thatit defines.

Perl Functions by Category

Here are Perl's functions (including things that look likefunctions, like some keywords and named operators)arranged by category. Some functions appear in morethan one place.

Portability

Perl was born in Unix and can therefore access all common Unixsystem calls. In non-Unix environments, the functionality of someUnix system calls may not be available or details of the availablefunctionality may differ slightly. The Perl functions affectedby this are:

-X, binmode, chmod, chown, chroot, crypt,dbmclose, dbmopen, dump, endgrent, endhostent,endnetent, endprotoent, endpwent, endservent, exec,fcntl, flock, fork, getgrent, getgrgid, gethostbyname,gethostent, getlogin, getnetbyaddr, getnetbyname, getnetent,getppid, getpgrp, getpriority, getprotobynumber,getprotoent, getpwent, getpwnam, getpwuid,getservbyport, getservent, getsockopt, glob, ioctl,kill, link, lstat, msgctl, msgget, msgrcv,msgsnd, open, pipe, readlink, rename, select, semctl,semget, semop, setgrent, sethostent, setnetent,setpgrp, setpriority, setprotoent, setpwent,setservent, setsockopt, shmctl, shmget, shmread,shmwrite, socket, socketpair,stat, symlink, syscall, sysopen, system,times, truncate, umask, unlink,utime, wait, waitpid

For more information about the portability of these functions, seeperlport and other available platform-specific documentation.

Alphabetical Listing of Perl Functions

  • -X FILEHANDLE
  • -X EXPR
  • -X DIRHANDLE
  • -X

    A file test, where X is one of the letters listed below. This unaryoperator takes one argument, either a filename, a filehandle, or a dirhandle, and tests the associated file to see if something is true about it. If theargument is omitted, tests $_, except for -t, which tests STDIN.Unless otherwise documented, it returns 1 for true and '' for false, orthe undefined value if the file doesn't exist. Despite the funnynames, precedence is the same as any other named unary operator. Theoperator may be any of:

    1.  -r  File is readable by effective uid/gid.
      2.  
    2.  -w  File is writable by effective uid/gid.
      3.  
    3.  -x  File is executable by effective uid/gid.
      4.  
    4.  -o  File is owned by effective uid.
      5.  
    5.  
    6.  -R  File is readable by real uid/gid.
      7.  
    7.  -W  File is writable by real uid/gid.
      8.  
    8.  -X  File is executable by real uid/gid.
      9.  
    9.  -O  File is owned by real uid.
      10.  
    10.  
    11.  -e  File exists.
      12.  
    12.  -z  File has zero size (is empty).
      13.  
    13.  -s  File has nonzero size (returns size in bytes).
      14.  
    14.  
    15.  -f  File is a plain file.
      16.  
    16.  -d  File is a directory.
      17.  
    17.  -l  File is a symbolic link.
      18.  
    18.  -p  File is a named pipe (FIFO), or Filehandle is a pipe.
      19.  
    19.  -S  File is a socket.
      20.  
    20.  -b  File is a block special file.
      21.  
    21.  -c  File is a character special file.
      22.  
    22.  -t  Filehandle is opened to a tty.
      23.  
    23.  
    24.  -u  File has setuid bit set.
      25.  
    25.  -g  File has setgid bit set.
      26.  
    26.  -k  File has sticky bit set.
      27.  
    27.  
    28.  -T  File is an ASCII text file (heuristic guess).
      29.  
    29.  -B  File is a "binary" file (opposite of -T).
      30.  
    30.  
    31.  -M  Script start time minus file modification time, in days.
      32.  
    32.  -A  Same for access time.
      33.  
    33.  -C  Same for inode change time (Unix, may differ for other platforms)
      34.

    Example:

    1.  while (<>) {
      2.  
    2.  chomp;
      3.  
    3.  next unless -f $_;  # ignore specials
      4.  
    4.  #...
      5.  
    5.  }
      6.

    Note that -s/a/b/ does not do a negated substitution. Saying-exp($foo) still works as expected, however: only single lettersfollowing a minus are interpreted as file tests.

    These operators are exempt from the "looks like a function rule" describedabove. That is, an opening parenthesis after the operator does not affecthow much of the following code constitutes the argument. Put the openingparentheses before the operator to separate it from code that follows (thisapplies only to operators with higher precedence than unary operators, ofcourse):

    1.  -s($file) + 1024   # probably wrong; same as -s($file + 1024)
      2.  
    2.  (-s $file) + 1024  # correct
      3.

    The interpretation of the file permission operators -r, -R,-w, -W, -x, and -X is by default based solely on the modeof the file and the uids and gids of the user. There may be otherreasons you can't actually read, write, or execute the file: forexample network filesystem access controls, ACLs (access control lists),read-only filesystems, and unrecognized executable formats. Notethat the use of these six specific operators to verify if some operationis possible is usually a mistake, because it may be open to raceconditions.

    Also note that, for the superuser on the local filesystems, the -r,-R, -w, and -W tests always return 1, and -x and -X return 1if any execute bit is set in the mode. Scripts run by the superusermay thus need to do a stat() to determine the actual mode of the file,or temporarily set their effective uid to something else.

    If you are using ACLs, there is a pragma called filetest that mayproduce more accurate results than the bare stat() mode bits.When under use filetest 'access' the above-mentioned fileteststest whether the permission can(not) be granted using theaccess(2) family of system calls. Also note that the -x and -X mayunder this pragma return true even if there are no execute permissionbits set (nor any extra execute permission ACLs). This strangeness isdue to the underlying system calls' definitions. Note also that, due tothe implementation of use filetest 'access', the _ specialfilehandle won't cache the results of the file tests when this pragma isin effect. Read the documentation for the filetest pragma for moreinformation.

    The -T and -B switches work as follows. The first block or so of thefile is examined for odd characters such as strange control codes orcharacters with the high bit set. If too many strange characters (>30%)are found, it's a -B file; otherwise it's a -T file. Also, any filecontaining a zero byte in the first block is considered a binary file. If -Tor -B is used on a filehandle, the current IO buffer is examinedrather than the first block. Both -T and -B return true on an emptyfile, or a file at EOF when testing a filehandle. Because you have toread a file to do the -T test, on most occasions you want to use a -fagainst the file first, as in next unless -f $file && -T $file.

    If any of the file tests (or either the stat or lstat operator) is giventhe special filehandle consisting of a solitary underline, then the statstructure of the previous file test (or stat operator) is used, savinga system call. (This doesn't work with -t, and you need to rememberthat lstat() and -l leave values in the stat structure for thesymbolic link, not the real file.) (Also, if the stat buffer was filled byan lstat call, -T and -B will reset it with the results of stat _).Example:

    1.  print "Can do.\n" if -r $a || -w _ || -x _;
      2.  
    2.  
    3.  stat($filename);
      4.  
    4.  print "Readable\n" if -r _;
      5.  
    5.  print "Writable\n" if -w _;
      6.  
    6.  print "Executable\n" if -x _;
      7.  
    7.  print "Setuid\n" if -u _;
      8.  
    8.  print "Setgid\n" if -g _;
      9.  
    9.  print "Sticky\n" if -k _;
      10.  
    10.  print "Text\n" if -T _;
      11.  
    11.  print "Binary\n" if -B _;
      12.

    As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack filetest operators, in a way that -f -w -x $file is equivalent to-x $file && -w _ && -f _. (This is only fancy fancy: if you usethe return value of -f $file as an argument to another filetestoperator, no special magic will happen.)

    Portability issues: -X in perlport.

    To avoid confusing would-be users of your code with mysterioussyntax errors, put something like this at the top of your script:

    1.  use 5.010;  # so filetest ops can stack
      2.
  • abs VALUE
  • abs

    Returns the absolute value of its argument.If VALUE is omitted, uses $_.

  • accept NEWSOCKET,GENERICSOCKET

    Accepts an incoming socket connect, just as accept(2) does. Returns the packed address if it succeeded, false otherwise.See the example in Sockets: Client/Server Communication in perlipc.

    On systems that support a close-on-exec flag on files, the flag willbe set for the newly opened file descriptor, as determined by thevalue of $^F. See $^F in perlvar.

  • alarm SECONDS
  • alarm

    Arranges to have a SIGALRM delivered to this process after thespecified number of wallclock seconds has elapsed. If SECONDS is notspecified, the value stored in $_ is used. (On some machines,unfortunately, the elapsed time may be up to one second less or morethan you specified because of how seconds are counted, and processscheduling may delay the delivery of the signal even further.)

    Only one timer may be counting at once. Each call disables theprevious timer, and an argument of 0 may be supplied to cancel theprevious timer without starting a new one. The returned value is theamount of time remaining on the previous timer.

    For delays of finer granularity than one second, the Time::HiRes module(from CPAN, and starting from Perl 5.8 part of the standarddistribution) provides ualarm(). You may also use Perl's four-argumentversion of select() leaving the first three arguments undefined, or youmight be able to use the syscall interface to access setitimer(2) ifyour system supports it. See perlfaq8 for details.

    It is usually a mistake to intermix alarm and sleep calls, becausesleep may be internally implemented on your system with alarm.

    If you want to use alarm to time out a system call you need to use aneval/die pair. You can't rely on the alarm causing the system call tofail with $! set to EINTR because Perl sets up signal handlers torestart system calls on some systems. Using eval/die always works,modulo the caveats given in Signals in perlipc.

    1.  eval {
      2.  
    2.  local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
      3.  
    3.  alarm $timeout;
      4.  
    4.  $nread = sysread SOCKET, $buffer, $size;
      5.  
    5.  alarm 0;
      6.  
    6.  };
      7.  
    7.  if ($@) {
      8.  
    8.  die unless $@ eq "alarm\n";   # propagate unexpected errors
      9.  
    9.  # timed out
      10.  
    10.  }
      11.  
    11.  else {
      12.  
    12.  # didn't
      13.  
    13.  }
      14.

    For more information see perlipc.

    Portability issues: alarm in perlport.

  • atan2 Y,X

    Returns the arctangent of Y/X in the range -PI to PI.

    For the tangent operation, you may use the Math::Trig::tanfunction, or use the familiar relation:

    1.  sub tan { sin($_[0]) / cos($_[0])  }
      2.

    The return value for atan2(0,0) is implementation-defined; consultyour atan2(3) manpage for more information.

    Portability issues: atan2 in perlport.

  • bind SOCKET,NAME

    Binds a network address to a socket, just as bind(2)does. Returns true if it succeeded, false otherwise. NAME should be apacked address of the appropriate type for the socket. See the examples inSockets: Client/Server Communication in perlipc.

  • binmode FILEHANDLE, LAYER
  • binmode FILEHANDLE

    Arranges for FILEHANDLE to be read or written in "binary" or "text"mode on systems where the run-time libraries distinguish betweenbinary and text files. If FILEHANDLE is an expression, the value istaken as the name of the filehandle. Returns true on success,otherwise it returns undef and sets $! (errno).

    On some systems (in general, DOS- and Windows-based systems) binmode()is necessary when you're not working with a text file. For the sakeof portability it is a good idea always to use it when appropriate,and never to use it when it isn't appropriate. Also, people canset their I/O to be by default UTF8-encoded Unicode, not bytes.

    In other words: regardless of platform, use binmode() on binary data,like images, for example.

    If LAYER is present it is a single string, but may contain multipledirectives. The directives alter the behaviour of the filehandle.When LAYER is present, using binmode on a text file makes sense.

    If LAYER is omitted or specified as :raw the filehandle is madesuitable for passing binary data. This includes turning off possible CRLFtranslation and marking it as bytes (as opposed to Unicode characters).Note that, despite what may be implied in "Programming Perl" (theCamel, 3rd edition) or elsewhere, :raw is not simply the inverse of :crlf.Other layers that would affect the binary nature of the stream arealso disabled. See PerlIO, perlrun, and the discussion about thePERLIO environment variable.

    The :bytes, :crlf, :utf8, and any other directives of theform :..., are called I/O layers. The open pragma can be used toestablish default I/O layers. See open.

    The LAYER parameter of the binmode() function is described as "DISCIPLINE"in "Programming Perl, 3rd Edition". However, since the publishing of thisbook, by many known as "Camel III", the consensus of the naming of thisfunctionality has moved from "discipline" to "layer". All documentationof this version of Perl therefore refers to "layers" rather than to"disciplines". Now back to the regularly scheduled documentation...

    To mark FILEHANDLE as UTF-8, use :utf8 or :encoding(UTF-8).:utf8 just marks the data as UTF-8 without further checking,while :encoding(UTF-8) checks the data for actually being validUTF-8. More details can be found in PerlIO::encoding.

    In general, binmode() should be called after open() but before any I/Ois done on the filehandle. Calling binmode() normally flushes anypending buffered output data (and perhaps pending input data) on thehandle. An exception to this is the :encoding layer thatchanges the default character encoding of the handle; see open.The :encoding layer sometimes needs to be called inmid-stream, and it doesn't flush the stream. The :encodingalso implicitly pushes on top of itself the :utf8 layer becauseinternally Perl operates on UTF8-encoded Unicode characters.

    The operating system, device drivers, C libraries, and Perl run-timesystem all conspire to let the programmer treat a singlecharacter (\n) as the line terminator, irrespective of externalrepresentation. On many operating systems, the native text filerepresentation matches the internal representation, but on someplatforms the external representation of \n is made up of more thanone character.

    All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS usea single character to end each line in the external representation of text(even though that single character is CARRIAGE RETURN on old, pre-Darwinflavors of Mac OS, and is LINE FEED on Unix and most VMS files). In othersystems like OS/2, DOS, and the various flavors of MS-Windows, your programsees a \n as a simple \cJ, but what's stored in text files are thetwo characters \cM\cJ. That means that if you don't use binmode() onthese systems, \cM\cJ sequences on disk will be converted to \n oninput, and any \n in your program will be converted back to \cM\cJ onoutput. This is what you want for text files, but it can be disastrous forbinary files.

    Another consequence of using binmode() (on some systems) is thatspecial end-of-file markers will be seen as part of the data stream.For systems from the Microsoft family this means that, if your binarydata contain \cZ, the I/O subsystem will regard it as the end ofthe file, unless you use binmode().

    binmode() is important not only for readline() and print() operations,but also when using read(), seek(), sysread(), syswrite() and tell()(see perlport for more details). See the $/ and $\ variablesin perlvar for how to manually set your input and outputline-termination sequences.

    Portability issues: binmode in perlport.

  • bless REF,CLASSNAME
  • bless REF

    This function tells the thingy referenced by REF that it is now an objectin the CLASSNAME package. If CLASSNAME is omitted, the current packageis used. Because a bless is often the last thing in a constructor,it returns the reference for convenience. Always use the two-argumentversion if a derived class might inherit the function doing the blessing.Seeperlobj for more about the blessing (and blessings) of objects.

    Consider always blessing objects in CLASSNAMEs that are mixed case.Namespaces with all lowercase names are considered reserved forPerl pragmata. Builtin types have all uppercase names. To preventconfusion, you may wish to avoid such package names as well. Make surethat CLASSNAME is a true value.

    See Perl Modules in perlmod.

  • break

    Break out of a given() block.

    This keyword is enabled by the "switch" feature: seefeature for more information. You can also access it byprefixing it with CORE::. Alternately, include a usev5.10 or later to the current scope.

  • caller EXPR
  • caller

    Returns the context of the current subroutine call. In scalar context,returns the caller's package name if there is a caller (that is, ifwe're in a subroutine or eval or require) and the undefined valueotherwise. In list context, returns

    1.  # 0 1  2
      2.  
    2.  ($package, $filename, $line) = caller;
      3.

    With EXPR, it returns some extra information that the debugger uses toprint a stack trace. The value of EXPR indicates how many call framesto go back before the current one.

    1.  #  0 1  2  3 4
      2.  
    2.  ($package, $filename, $line, $subroutine, $hasargs,
      3.  
    3.  
    4.  #  5  6  7 8   9 10
      5.  
    5.  $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
      6.  
    6.  = caller($i);
      7.

    Here $subroutine may be (eval) if the frame is not a subroutinecall, but an eval. In such a case additional elements $evaltext and$is_require are set: $is_require is true if the frame is created by arequire or use statement, $evaltext contains the text of theeval EXPR statement. In particular, for an eval BLOCK statement,$subroutine is (eval), but $evaltext is undefined. (Note also thateach use statement creates a require frame inside an eval EXPRframe.) $subroutine may also be (unknown) if this particularsubroutine happens to have been deleted from the symbol table.$hasargs is true if a new instance of @_ was set up for the frame.$hints and $bitmask contain pragmatic hints that the caller wascompiled with. The $hints and $bitmask values are subject to changebetween versions of Perl, and are not meant for external use.

    $hinthash is a reference to a hash containing the value of %^H when thecaller was compiled, or undef if %^H was empty. Do not modify the valuesof this hash, as they are the actual values stored in the optree.

    Furthermore, when called from within the DB package inlist context, and with an argument, caller returns moredetailed information: it sets the list variable @DB::args to be thearguments with which the subroutine was invoked.

    Be aware that the optimizer might have optimized call frames away beforecaller had a chance to get the information. That means that caller(N)might not return information about the call frame you expect it to, forN > 1. In particular, @DB::args might have information from theprevious time caller was called.

    Be aware that setting @DB::args is best effort, intended fordebugging or generating backtraces, and should not be relied upon. Inparticular, as @_ contains aliases to the caller's arguments, Perl doesnot take a copy of @_, so @DB::args will contain modifications thesubroutine makes to @_ or its contents, not the original values at calltime. @DB::args, like @_, does not hold explicit references to itselements, so under certain cases its elements may have become freed andreallocated for other variables or temporary values. Finally, a side effectof the current implementation is that the effects of shift @_ cannormally be undone (but not pop @_ or other splicing, and not if areference to @_ has been taken, and subject to the caveat about reallocatedelements), so @DB::args is actually a hybrid of the current state andinitial state of @_. Buyer beware.

  • chdir EXPR
  • chdir FILEHANDLE
  • chdir DIRHANDLE
  • chdir

    Changes the working directory to EXPR, if possible. If EXPR is omitted,changes to the directory specified by $ENV{HOME}, if set; if not,changes to the directory specified by $ENV{LOGDIR}. (Under VMS, thevariable $ENV{SYS$LOGIN} is also checked, and used if it is set.) Ifneither is set, chdir does nothing. It returns true on success,false otherwise. See the example under die.

    On systems that support fchdir(2), you may pass a filehandle ordirectory handle as the argument. On systems that don't support fchdir(2),passing handles raises an exception.

  • chmod LIST

    Changes the permissions of a list of files. The first element of thelist must be the numeric mode, which should probably be an octalnumber, and which definitely should not be a string of octal digits:0644 is okay, but "0644" is not. Returns the number of filessuccessfully changed. See also oct if all you have is a string.

    1.  $cnt = chmod 0755, "foo", "bar";
      2.  
    2.  chmod 0755, @executables;
      3.  
    3.  $mode = "0644"; chmod $mode, "foo";  # !!! sets mode to
      4.  
    4.  # --w----r-T
      5.  
    5.  $mode = "0644"; chmod oct($mode), "foo"; # this is better
      6.  
    6.  $mode = 0644;   chmod $mode, "foo";  # this is best
      7.

    On systems that support fchmod(2), you may pass filehandles among thefiles. On systems that don't support fchmod(2), passing filehandles raisesan exception. Filehandles must be passed as globs or glob references to berecognized; barewords are considered filenames.

    1.  open(my $fh, "<", "foo");
      2.  
    2.  my $perm = (stat $fh)[2] & 07777;
      3.  
    3.  chmod($perm | 0600, $fh);
      4.

    You can also import the symbolic S_I* constants from the Fcntlmodule:

    1.  use Fcntl qw( :mode );
      2.  
    2.  chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
      3.  
    3.  # Identical to the chmod 0755 of the example above.
      4.

    Portability issues: chmod in perlport.

  • chomp VARIABLE
  • chomp( LIST )
  • chomp

    This safer version of chop removes any trailing stringthat corresponds to the current value of $/ (also known as$INPUT_RECORD_SEPARATOR in the English module). It returns the totalnumber of characters removed from all its arguments. It's often used toremove the newline from the end of an input record when you're worriedthat the final record may be missing its newline. When in paragraphmode ($/ = ""), it removes all trailing newlines from the string.When in slurp mode ($/ = undef) or fixed-length record mode ($/ isa reference to an integer or the like; see perlvar) chomp() won'tremove anything.If VARIABLE is omitted, it chomps $_. Example:

    1.  while (<>) {
      2.  
    2.  chomp;  # avoid \n on last field
      3.  
    3.  @array = split(/:/);
      4.  
    4.  # ...
      5.  
    5.  }
      6.

    If VARIABLE is a hash, it chomps the hash's values, but not its keys.

    You can actually chomp anything that's an lvalue, including an assignment:

    1.  chomp($cwd = `pwd`);
      2.  
    2.  chomp($answer = <STDIN>);
      3.

    If you chomp a list, each element is chomped, and the total number ofcharacters removed is returned.

    Note that parentheses are necessary when you're chomping anythingthat is not a simple variable. This is because chomp $cwd = `pwd`;is interpreted as (chomp $cwd) = `pwd`;, rather than aschomp( $cwd = `pwd` ) which you might expect. Similarly,chomp $a, $b is interpreted as chomp($a), $b rather thanas chomp($a, $b).

  • chop VARIABLE
  • chop( LIST )
  • chop

    Chops off the last character of a string and returns the characterchopped. It is much more efficient than s/.$//s because it neitherscans nor copies the string. If VARIABLE is omitted, chops $_.If VARIABLE is a hash, it chops the hash's values, but not its keys.

    You can actually chop anything that's an lvalue, including an assignment.

    If you chop a list, each element is chopped. Only the value of thelast chop is returned.

    Note that chop returns the last character. To return all but the lastcharacter, use substr($string, 0, -1).

    See also chomp.

  • chown LIST

    Changes the owner (and group) of a list of files. The first twoelements of the list must be the numeric uid and gid, in thatorder. A value of -1 in either position is interpreted by mostsystems to leave that value unchanged. Returns the number of filessuccessfully changed.

    1.  $cnt = chown $uid, $gid, 'foo', 'bar';
      2.  
    2.  chown $uid, $gid, @filenames;
      3.

    On systems that support fchown(2), you may pass filehandles among thefiles. On systems that don't support fchown(2), passing filehandles raisesan exception. Filehandles must be passed as globs or glob references to berecognized; barewords are considered filenames.

    Here's an example that looks up nonnumeric uids in the passwd file:

    1.  print "User: ";
      2.  
    2.  chomp($user = <STDIN>);
      3.  
    3.  print "Files: ";
      4.  
    4.  chomp($pattern = <STDIN>);
      5.  
    5.  
    6.  ($login,$pass,$uid,$gid) = getpwnam($user)
      7.  
    7.  or die "$user not in passwd file";
      8.  
    8.  
    9.  @ary = glob($pattern);  # expand filenames
      10.  
    10.  chown $uid, $gid, @ary;
      11.

    On most systems, you are not allowed to change the ownership of thefile unless you're the superuser, although you should be able to changethe group to any of your secondary groups. On insecure systems, theserestrictions may be relaxed, but this is not a portable assumption.On POSIX systems, you can detect this condition this way:

    1.  use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
      2.  
    2.  $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
      3.

    Portability issues: chmod in perlport.

  • chr NUMBER
  • chr

    Returns the character represented by that NUMBER in the character set.For example, chr(65) is "A" in either ASCII or Unicode, andchr(0x263a) is a Unicode smiley face.

    Negative values give the Unicode replacement character (chr(0xfffd)),except under the bytes pragma, where the low eight bits of the value(truncated to an integer) are used.

    If NUMBER is omitted, uses $_.

    For the reverse, use ord.

    Note that characters from 128 to 255 (inclusive) are by defaultinternally not encoded as UTF-8 for backward compatibility reasons.

    See perlunicode for more about Unicode.

  • chroot FILENAME
  • chroot

    This function works like the system call by the same name: it makes thenamed directory the new root directory for all further pathnames thatbegin with a / by your process and all its children. (It doesn'tchange your current working directory, which is unaffected.) For securityreasons, this call is restricted to the superuser. If FILENAME isomitted, does a chroot to $_.

    Portability issues: chroot in perlport.

  • close FILEHANDLE
  • close

    Closes the file or pipe associated with the filehandle, flushes the IObuffers, and closes the system file descriptor. Returns true if thoseoperations succeed and if no error was reported by any PerlIOlayer. Closes the currently selected filehandle if the argument isomitted.

    You don't have to close FILEHANDLE if you are immediately going to doanother open on it, because open closes it for you. (Seeopen.) However, an explicit close on an input file resets the linecounter ($.), while the implicit close done by open does not.

    If the filehandle came from a piped open, close returns false if one ofthe other syscalls involved fails or if its program exits with non-zerostatus. If the only problem was that the program exited non-zero, $!will be set to 0. Closing a pipe also waits for the process executingon the pipe to exit--in case you wish to look at the output of the pipeafterwards--and implicitly puts the exit status value of that command into$? and ${^CHILD_ERROR_NATIVE}.

    If there are multiple threads running, close on a filehandle from apiped open returns true without waiting for the child process to terminate,if the filehandle is still open in another thread.

    Closing the read end of a pipe before the process writing to it at theother end is done writing results in the writer receiving a SIGPIPE. Ifthe other end can't handle that, be sure to read all the data beforeclosing the pipe.

    Example:

    1.  open(OUTPUT, '|sort >foo')  # pipe to sort
      2.  
    2.  or die "Can't start sort: $!";
      3.  
    3.  #... # print stuff to output
      4.  
    4.  close OUTPUT # wait for sort to finish
      5.  
    5.  or warn $! ? "Error closing sort pipe: $!"
      6.  
    6.    : "Exit status $? from sort";
      7.  
    7.  open(INPUT, 'foo')  # get sort's results
      8.  
    8.  or die "Can't open 'foo' for input: $!";
      9.

    FILEHANDLE may be an expression whose value can be used as an indirectfilehandle, usually the real filehandle name or an autovivified handle.

  • closedir DIRHANDLE

    Closes a directory opened by opendir and returns the success of thatsystem call.

  • connect SOCKET,NAME

    Attempts to connect to a remote socket, just like connect(2).Returns true if it succeeded, false otherwise. NAME should be apacked address of the appropriate type for the socket. See the examples inSockets: Client/Server Communication in perlipc.

  • continue BLOCK
  • continue

    When followed by a BLOCK, continue is actually aflow control statement rather than a function. Ifthere is a continue BLOCK attached to a BLOCK (typically in a while orforeach), it is always executed just before the conditional is about tobe evaluated again, just like the third part of a for loop in C. Thusit can be used to increment a loop variable, even when the loop has beencontinued via the next statement (which is similar to the C continuestatement).

    last, next, or redo may appear within a continueblock; last and redo behave as if they had been executed withinthe main block. So will next, but since it will execute a continueblock, it may be more entertaining.

    1.  while (EXPR) {
      2.  
    2.  ### redo always comes here
      3.  
    3.  do_something;
      4.  
    4.  } continue {
      5.  
    5.  ### next always comes here
      6.  
    6.  do_something_else;
      7.  
    7.  # then back the top to re-check EXPR
      8.  
    8.  }
      9.  
    9.  ### last always comes here
      10.

    Omitting the continue section is equivalent to using anempty one, logically enough, so next goes directly backto check the condition at the top of the loop.

    When there is no BLOCK, continue is a function thatfalls through the current when or default block instead of iteratinga dynamically enclosing foreach or exiting a lexically enclosing given.In Perl 5.14 and earlier, this form of continue wasonly available when the "switch" feature was enabled.See feature and Switch Statements in perlsyn for moreinformation.

  • cos EXPR
  • cos

    Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,takes the cosine of $_.

    For the inverse cosine operation, you may use the Math::Trig::acos()function, or use this relation:

    1.  sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
      2.
  • crypt PLAINTEXT,SALT

    Creates a digest string exactly like the crypt(3) function in the Clibrary (assuming that you actually have a version there that has notbeen extirpated as a potential munition).

    crypt() is a one-way hash function. The PLAINTEXT and SALT are turnedinto a short string, called a digest, which is returned. The samePLAINTEXT and SALT will always return the same string, but there is no(known) way to get the original PLAINTEXT from the hash. Smallchanges in the PLAINTEXT or SALT will result in large changes in thedigest.

    There is no decrypt function. This function isn't all that useful forcryptography (for that, look for Crypt modules on your nearby CPANmirror) and the name "crypt" is a bit of a misnomer. Instead it isprimarily used to check if two pieces of text are the same withouthaving to transmit or store the text itself. An example is checkingif a correct password is given. The digest of the password is stored,not the password itself. The user types in a password that iscrypt()'d with the same salt as the stored digest. If the two digestsmatch, the password is correct.

    When verifying an existing digest string you should use the digest asthe salt (like crypt($plain, $digest) eq $digest). The SALT usedto create the digest is visible as part of the digest. This ensurescrypt() will hash the new string with the same salt as the digest.This allows your code to work with the standard crypt andwith more exotic implementations. In other words, assumenothing about the returned string itself nor about how many bytes of SALT may matter.

    Traditionally the result is a string of 13 bytes: two first bytes ofthe salt, followed by 11 bytes from the set [./0-9A-Za-z], and onlythe first eight bytes of PLAINTEXT mattered. But alternativehashing schemes (like MD5), higher level security schemes (like C2),and implementations on non-Unix platforms may produce differentstrings.

    When choosing a new salt create a random two character string whosecharacters come from the set [./0-9A-Za-z] (like join '', ('.','/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]). This set ofcharacters is just a recommendation; the characters allowed inthe salt depend solely on your system's crypt library, and Perl can'trestrict what salts crypt() accepts.

    Here's an example that makes sure that whoever runs this program knowstheir password:

    1.  $pwd = (getpwuid($<))[1];
      2.  
    2.  
    3.  system "stty -echo";
      4.  
    4.  print "Password: ";
      5.  
    5.  chomp($word = <STDIN>);
      6.  
    6.  print "\n";
      7.  
    7.  system "stty echo";
      8.  
    8.  
    9.  if (crypt($word, $pwd) ne $pwd) {
      10.  
    10.  die "Sorry...\n";
      11.  
    11.  } else {
      12.  
    12.  print "ok\n";
      13.  
    13.  }
      14.

    Of course, typing in your own password to whoever asks youfor it is unwise.

    The crypt function is unsuitable for hashing large quantitiesof data, not least of all because you can't get the informationback. Look at the Digest module for more robust algorithms.

    If using crypt() on a Unicode string (which potentially hascharacters with codepoints above 255), Perl tries to make senseof the situation by trying to downgrade (a copy of)the string back to an eight-bit byte string before calling crypt()(on that copy). If that works, good. If not, crypt() dies withWide character in crypt.

    Portability issues: crypt in perlport.

  • dbmclose HASH

    [This function has been largely superseded by the untie function.]

    Breaks the binding between a DBM file and a hash.

    Portability issues: dbmclose in perlport.

  • dbmopen HASH,DBNAME,MASK

    [This function has been largely superseded by thetie function.]

    This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to ahash. HASH is the name of the hash. (Unlike normal open, the firstargument is not a filehandle, even though it looks like one). DBNAMEis the name of the database (without the .dir or .pag extension ifany). If the database does not exist, it is created with protectionspecified by MASK (as modified by the umask). To prevent creation ofthe database if it doesn't exist, you may specify a MODEof 0, and the function will return a false value if itcan't find an existing database. If your system supportsonly the older DBM functions, you may make only one dbmopen call in yourprogram. In older versions of Perl, if your system had neither DBM norndbm, calling dbmopen produced a fatal error; it now falls back tosdbm(3).

    If you don't have write access to the DBM file, you can only read hashvariables, not set them. If you want to test whether you can write,either use file tests or try setting a dummy hash entry inside an eval to trap the error.

    Note that functions such as keys and values may return huge listswhen used on large DBM files. You may prefer to use the eachfunction to iterate over large DBM files. Example:

    1.  # print out history file offsets
      2.  
    2.  dbmopen(%HIST,'/usr/lib/news/history',0666);
      3.  
    3.  while (($key,$val) = each %HIST) {
      4.  
    4.  print $key, ' = ', unpack('L',$val), "\n";
      5.  
    5.  }
      6.  
    6.  dbmclose(%HIST);
      7.

    See also AnyDBM_File for a more general description of the pros andcons of the various dbm approaches, as well as DB_File for a particularlyrich implementation.

    You can control which DBM library you use by loading that librarybefore you call dbmopen():

    1.  use DB_File;
      2.  
    2.  dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
      3.  
    3.  or die "Can't open netscape history file: $!";
      4.

    Portability issues: dbmopen in perlport.

  • defined EXPR
  • defined

    Returns a Boolean value telling whether EXPR has a value other thanthe undefined value undef. If EXPR is not present, $_ ischecked.

    Many operations return undef to indicate failure, end of file,system error, uninitialized variable, and other exceptionalconditions. This function allows you to distinguish undef fromother values. (A simple Boolean test will not distinguish amongundef, zero, the empty string, and "0", which are all equallyfalse.) Note that since undef is a valid scalar, its presencedoesn't necessarily indicate an exceptional condition: popreturns undef when its argument is an empty array, or when theelement to return happens to be undef.

    You may also use defined(&func) to check whether subroutine &funchas ever been defined. The return value is unaffected by any forwarddeclarations of &func. A subroutine that is not definedmay still be callable: its package may have an AUTOLOAD method thatmakes it spring into existence the first time that it is called; seeperlsub.

    Use of defined on aggregates (hashes and arrays) is deprecated. Itused to report whether memory for that aggregate had ever beenallocated. This behavior may disappear in future versions of Perl.You should instead use a simple test for size:

    1.  if (@an_array) { print "has array elements\n" }
      2.  
    2.  if (%a_hash)   { print "has hash members\n"   }
      3.

    When used on a hash element, it tells you whether the value is defined,not whether the key exists in the hash. Use exists for the latterpurpose.

    Examples:

    1.  print if defined $switch{D};
      2.  
    2.  print "$val\n" while defined($val = pop(@ary));
      3.  
    3.  die "Can't readlink $sym: $!"
      4.  
    4.  unless defined($value = readlink $sym);
      5.  
    5.  sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
      6.  
    6.  $debugging = 0 unless defined $debugging;
      7.

    Note: Many folks tend to overuse defined and are then surprised todiscover that the number 0 and "" (the zero-length string) are, in fact,defined values. For example, if you say

    1.  "ab" =~ /a(.*)b/;
      2.

    The pattern match succeeds and $1 is defined, although itmatched "nothing". It didn't really fail to match anything. Rather, itmatched something that happened to be zero characters long. This is allvery above-board and honest. When a function returns an undefined value,it's an admission that it couldn't give you an honest answer. So youshould use defined only when questioning the integrity of whatyou're trying to do. At other times, a simple comparison to 0 or "" iswhat you want.

    See also undef, exists, ref.

  • delete EXPR

    Given an expression that specifies an element or slice of a hash, deletedeletes the specified elements from that hash so that exists() on that elementno longer returns true. Setting a hash element to the undefined value doesnot remove its key, but deleting it does; see exists.

    In list context, returns the value or values deleted, or the last suchelement in scalar context. The return list's length always matches that ofthe argument list: deleting non-existent elements returns the undefined valuein their corresponding positions.

    delete() may also be used on arrays and array slices, but its behavior is lessstraightforward. Although exists() will return false for deleted entries,deleting array elements never changes indices of existing values; use shift()or splice() for that. However, if all deleted elements fall at the end of anarray, the array's size shrinks to the position of the highest element thatstill tests true for exists(), or to 0 if none do.

    WARNING: Calling delete on array values is deprecated and likely tobe removed in a future version of Perl.

    Deleting from %ENV modifies the environment. Deleting from a hash tied toa DBM file deletes the entry from the DBM file. Deleting from a tied hashor array may not necessarily return anything; it depends on the implementationof the tied package's DELETE method, which may do whatever it pleases.

    The delete local EXPR construct localizes the deletion to the currentblock at run time. Until the block exits, elements locally deletedtemporarily no longer exist. See Localized deletion of elements of composite types in perlsub.

    1.  %hash = (foo => 11, bar => 22, baz => 33);
      2.  
    2.  $scalar = delete $hash{foo}; # $scalar is 11
      3.  
    3.  $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
      4.  
    4.  @array  = delete @hash{qw(foo bar baz)}; # @array  is (undef,undef,33)
      5.

    The following (inefficiently) deletes all the values of %HASH and @ARRAY:

    1.  foreach $key (keys %HASH) {
      2.  
    2.  delete $HASH{$key};
      3.  
    3.  }
      4.  
    4.  
    5.  foreach $index (0 .. $#ARRAY) {
      6.  
    6.  delete $ARRAY[$index];
      7.  
    7.  }
      8.

    And so do these:

    1.  delete @HASH{keys %HASH};
      2.  
    2.  
    3.  delete @ARRAY[0 .. $#ARRAY];
      4.

    But both are slower than assigning the empty listor undefining %HASH or @ARRAY, which is the customary way to empty out an aggregate:

    1.  %HASH = (); # completely empty %HASH
      2.  
    2.  undef %HASH; # forget %HASH ever existed
      3.  
    3.  
    4.  @ARRAY = (); # completely empty @ARRAY
      5.  
    5.  undef @ARRAY;   # forget @ARRAY ever existed
      6.

    The EXPR can be arbitrarily complicated provided itsfinal operation is an element or slice of an aggregate:

    1.  delete $ref->[$x][$y]{$key};
      2.  
    2.  delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
      3.  
    3.  
    4.  delete $ref->[$x][$y][$index];
      5.  
    5.  delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
      6.
  • die LIST

    die raises an exception. Inside an eval the error message is stuffedinto $@ and the eval is terminated with the undefined value.If the exception is outside of all enclosing evals, then the uncaughtexception prints LIST to STDERR and exits with a non-zero value. If youneed to exit the process with a specific exit code, see exit.

    Equivalent examples:

    1.  die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
      2.  
    2.  chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
      3.

    If the last element of LIST does not end in a newline, the currentscript line number and input line number (if any) are also printed,and a newline is supplied. Note that the "input line number" (alsoknown as "chunk") is subject to whatever notion of "line" happens tobe currently in effect, and is also available as the special variable$.. See $/ in perlvar and $. in perlvar.

    Hint: sometimes appending ", stopped" to your message will cause itto make better sense when the string "at foo line 123" is appended.Suppose you are running script "canasta".

    1.  die "/etc/games is no good";
      2.  
    2.  die "/etc/games is no good, stopped";
      3.

    produce, respectively

    1.  /etc/games is no good at canasta line 123.
      2.  
    2.  /etc/games is no good, stopped at canasta line 123.
      3.

    If the output is empty and $@ already contains a value (typically from aprevious eval) that value is reused after appending "\t...propagated".This is useful for propagating exceptions:

    1.  eval { ... };
      2.  
    2.  die unless $@ =~ /Expected exception/;
      3.

    If the output is empty and $@ contains an object reference that has aPROPAGATE method, that method will be called with additional fileand line number parameters. The return value replaces the value in$@; i.e., as if $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };were called.

    If $@ is empty then the string "Died" is used.

    If an uncaught exception results in interpreter exit, the exit code isdetermined from the values of $! and $? with this pseudocode:

    1.  exit $! if $!;  # errno
      2.  
    2.  exit $? >> 8 if $? >> 8; # child exit status
      3.  
    3.  exit 255;   # last resort
      4.

    The intent is to squeeze as much possible information about the likely causeinto the limited space of the system exitcode. However, as $! is the valueof C's errno, which can be set by any system call, this means that the valueof the exit code used by die can be non-predictable, so should not be reliedupon, other than to be non-zero.

    You can also call die with a reference argument, and if this is trappedwithin an eval, $@ contains that reference. This permits moreelaborate exception handling using objects that maintain arbitrary stateabout the exception. Such a scheme is sometimes preferable to matchingparticular string values of $@ with regular expressions. Because $@ is a global variable and eval may be used within object implementations,be careful that analyzing the error object doesn't replace the reference inthe global variable. It's easiest to make a local copy of the referencebefore any manipulations. Here's an example:

    1.  use Scalar::Util "blessed";
      2.  
    2.  
    3.  eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
      4.  
    4.  if (my $ev_err = $@) {
      5.  
    5.  if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
      6.  
    6.  # handle Some::Module::Exception
      7.  
    7.  }
      8.  
    8.  else {
      9.  
    9.  # handle all other possible exceptions
      10.  
    10.  }
      11.  
    11.  }
      12.

    Because Perl stringifies uncaught exception messages before display,you'll probably want to overload stringification operations onexception objects. See overload for details about that.

    You can arrange for a callback to be run just before the diedoes its deed, by setting the $SIG{__DIE__} hook. The associatedhandler is called with the error text and can change the errormessage, if it sees fit, by calling die again. See%SIG in perlvar for details on setting %SIG entries, andeval BLOCK for some examples. Although this feature was to be run only right before your program was to exit, this is notcurrently so: the $SIG{__DIE__} hook is currently calledeven inside eval()ed blocks/strings! If one wants the hook to donothing in such situations, put

    1.  die @_ if $^S;
      2.

    as the first line of the handler (see $^S in perlvar). Becausethis promotes strange action at a distance, this counterintuitivebehavior may be fixed in a future release.

    See also exit(), warn(), and the Carp module.

  • do BLOCK

    Not really a function. Returns the value of the last command in thesequence of commands indicated by BLOCK. When modified by the while oruntil loop modifier, executes the BLOCK once before testing the loopcondition. (On other statements the loop modifiers test the conditionalfirst.)

    do BLOCK does not count as a loop, so the loop control statementsnext, last, or redo cannot be used to leave or restart the block.See perlsyn for alternative strategies.

  • do SUBROUTINE(LIST)

    This form of subroutine call is deprecated. SUBROUTINE can be a bareword,a scalar variable or a subroutine beginning with &.

  • do EXPR

    Uses the value of EXPR as a filename and executes the contents of thefile as a Perl script.

    1.  do 'stat.pl';
      2.

    is just like

    1.  eval `cat stat.pl`;
      2.

    except that it's more efficient and concise, keeps track of the currentfilename for error messages, searches the @INC directories, and updates%INC if the file is found. See @INC in perlvar and %INC in perlvar forthese variables. It also differs in that code evaluated with do FILENAMEcannot see lexicals in the enclosing scope; eval STRING does. It's thesame, however, in that it does reparse the file every time you call it,so you probably don't want to do this inside a loop.

    If do can read the file but cannot compile it, it returns undef and setsan error message in $@. If do cannot read the file, it returns undefand sets $! to the error. Always check $@ first, as compilationcould fail in a way that also sets $!. If the file is successfullycompiled, do returns the value of the last expression evaluated.

    Inclusion of library modules is better done with theuse and require operators, which also do automatic error checkingand raise an exception if there's a problem.

    You might like to use do to read in a program configurationfile. Manual error checking can be done this way:

    1.  # read in config files: system first, then user
      2.  
    2.  for $file ("/share/prog/defaults.rc",
      3.  
    3.    "$ENV{HOME}/.someprogrc")
      4.  
    4.  {
      5.  
    5.  unless ($return = do $file) {
      6.  
    6.  warn "couldn't parse $file: $@" if $@;
      7.  
    7.  warn "couldn't do $file: $!" unless defined $return;
      8.  
    8.  warn "couldn't run $file"   unless $return;
      9.  
    9.  }
      10.  
    10.  }
      11.
  • dump LABEL
  • dump

    This function causes an immediate core dump. See also the -ucommand-line switch in perlrun, which does the same thing.Primarily this is so that you can use the undump program (notsupplied) to turn your core dump into an executable binary afterhaving initialized all your variables at the beginning of theprogram. When the new binary is executed it will begin by executinga goto LABEL (with all the restrictions that goto suffers).Think of it as a goto with an intervening core dump and reincarnation.If LABEL is omitted, restarts the program from the top.

    WARNING: Any files opened at the time of the dump will notbe open any more when the program is reincarnated, with possibleresulting confusion by Perl.

    This function is now largely obsolete, mostly because it's very hard toconvert a core file into an executable. That's why you should now invokeit as CORE::dump(), if you don't want to be warned against a possibletypo.

    Portability issues: dump in perlport.

  • each HASH
  • each ARRAY
  • each EXPR

    When called on a hash in list context, returns a 2-element listconsisting of the key and value for the next element of a hash. In Perl5.12 and later only, it will also return the index and value for the nextelement of an array so that you can iterate over it; older Perls considerthis a syntax error. When called in scalar context, returns only the key(not the value) in a hash, or the index in an array.

    Hash entries are returned in an apparently random order. The actual randomorder is subject to change in future versions of Perl, but it isguaranteed to be in the same order as either the keys or valuesfunction would produce on the same (unmodified) hash. Since Perl5.8.2 the ordering can be different even between different runs of Perlfor security reasons (see Algorithmic Complexity Attacks in perlsec).

    After each has returned all entries from the hash or array, the nextcall to each returns the empty list in list context and undef inscalar context; the next call following that one restarts iteration.Each hash or array has its own internal iterator, accessed by each,keys, and values. The iterator is implicitly reset when each hasreached the end as just described; it can be explicitly reset by callingkeys or values on the hash or array. If you add or delete a hash'selements while iterating over it, entries may be skipped or duplicated--sodon't do that. Exception: In the current implementation, it is always safeto delete the item most recently returned by each(), so the followingcode works properly:

    1.  while (($key, $value) = each %hash) {
      2.  
    2.   print $key, "\n";
      3.  
    3.   delete $hash{$key};   # This is safe
      4.  
    4.  }
      5.

    This prints out your environment like the printenv(1) program,but in a different order:

    1.  while (($key,$value) = each %ENV) {
      2.  
    2.  print "$key=$value\n";
      3.  
    3.  }
      4.

    Starting with Perl 5.14, each can take a scalar EXPR, which must holdreference to an unblessed hash or array. The argument will be dereferencedautomatically. This aspect of each is considered highly experimental.The exact behaviour may change in a future version of Perl.

    1.  while (($key,$value) = each $hashref) { ... }
      2.

    To avoid confusing would-be users of your code who are running earlierversions of Perl with mysterious syntax errors, put this sort of thing atthe top of your file to signal that your code will work only on Perls ofa recent vintage:

    1.  use 5.012;# so keys/values/each work on arrays
      2.  
    2.  use 5.014;# so keys/values/each work on scalars (experimental)
      3.

    See also keys, values, and sort.

  • eof FILEHANDLE
  • eof ()
  • eof

    Returns 1 if the next read on FILEHANDLE will return end of file or ifFILEHANDLE is not open. FILEHANDLE may be an expression whose valuegives the real filehandle. (Note that this function actuallyreads a character and then ungetcs it, so isn't useful in aninteractive context.) Do not read from a terminal file (or calleof(FILEHANDLE) on it) after end-of-file is reached. File types suchas terminals may lose the end-of-file condition if you do.

    An eof without an argument uses the last file read. Using eof()with empty parentheses is different. It refers to the pseudo fileformed from the files listed on the command line and accessed via the<> operator. Since <> isn't explicitly opened,as a normal filehandle is, an eof() before <> has beenused will cause @ARGV to be examined to determine if input isavailable. Similarly, an eof() after <> has returnedend-of-file will assume you are processing another @ARGV list,and if you haven't set @ARGV, will read input from STDIN;see I/O Operators in perlop.

    In a while (<>) loop, eof or eof(ARGV) can be used todetect the end of each file, whereas eof() will detect the end of the very last file only. Examples:

    1.  # reset line numbering on each input file
      2.  
    2.  while (<>) {
      3.  
    3.  next if /^\s*#/;  # skip comments
      4.  
    4.  print "$.\t$_";
      5.  
    5.  } continue {
      6.  
    6.  close ARGV if eof;  # Not eof()!
      7.  
    7.  }
      8.  
    8.  
    9.  # insert dashes just before last line of last file
      10.  
    10.  while (<>) {
      11.  
    11.  if (eof()) {  # check for end of last file
      12.  
    12.  print "--------------\n";
      13.  
    13.  }
      14.  
    14.  print;
      15.  
    15.  last if eof();  # needed if we're reading from a terminal
      16.  
    16.  }
      17.

    Practical hint: you almost never need to use eof in Perl, because theinput operators typically return undef when they run out of data or encounter an error.

  • eval EXPR
  • eval BLOCK
  • eval

    In the first form, the return value of EXPR is parsed and executed as if itwere a little Perl program. The value of the expression (which is itselfdetermined within scalar context) is first parsed, and if there were noerrors, executed as a block within the lexical context of the current Perlprogram. This means, that in particular, any outer lexical variables arevisible to it, and any package variable settings or subroutine and formatdefinitions remain afterwards.

    Note that the value is parsed every time the eval executes.If EXPR is omitted, evaluates $_. This form is typically used todelay parsing and subsequent execution of the text of EXPR until run time.

    If the unicode_eval feature is enabled (which is the default under ause 5.16 or higher declaration), EXPR or $_ is treated as a string ofcharacters, so use utf8 declarations have no effect, and source filtersare forbidden. In the absence of the unicode_eval feature, the stringwill sometimes be treated as characters and sometimes as bytes, dependingon the internal encoding, and source filters activated within the evalexhibit the erratic, but historical, behaviour of affecting some outer filescope that is still compiling. See also the evalbytes keyword, whichalways treats its input as a byte stream and works properly with sourcefilters, and the feature pragma.

    In the second form, the code within the BLOCK is parsed only once--at thesame time the code surrounding the eval itself was parsed--and executedwithin the context of the current Perl program. This form is typicallyused to trap exceptions more efficiently than the first (see below), whilealso providing the benefit of checking the code within BLOCK at compiletime.

    The final semicolon, if any, may be omitted from the value of EXPR or withinthe BLOCK.

    In both forms, the value returned is the value of the last expressionevaluated inside the mini-program; a return statement may be also used, justas with subroutines. The expression providing the return value is evaluatedin void, scalar, or list context, depending on the context of the eval itself. See wantarray for more on how the evaluation context can be determined.

    If there is a syntax error or runtime error, or a die statement isexecuted, eval returns undef in scalar contextor an empty list in list context, and $@ is set to the errormessage. (Prior to 5.16, a bug caused undef to be returnedin list context for syntax errors, but not for runtime errors.)If there was no error, $@ is set to the empty string. Acontrol flow operator like last or goto can bypass the setting of$@. Beware that using eval neither silences Perl from printingwarnings to STDERR, nor does it stuff the text of warning messages into $@.To do either of those, you have to use the $SIG{__WARN__} facility, orturn off warnings inside the BLOCK or EXPR using no warnings 'all'.See warn, perlvar, warnings and perllexwarn.

    Note that, because eval traps otherwise-fatal errors, it is useful fordetermining whether a particular feature (such as socket or symlink)is implemented. It is also Perl's exception-trapping mechanism, wherethe die operator is used to raise exceptions.

    If you want to trap errors when loading an XS module, some problems withthe binary interface (such as Perl version skew) may be fatal even witheval unless $ENV{PERL_DL_NONLAZY} is set. See perlrun.

    If the code to be executed doesn't vary, you may use the eval-BLOCKform to trap run-time errors without incurring the penalty ofrecompiling each time. The error, if any, is still returned in $@.Examples:

    1.  # make divide-by-zero nonfatal
      2.  
    2.  eval { $answer = $a / $b; }; warn $@ if $@;
      3.  
    3.  
    4.  # same thing, but less efficient
      5.  
    5.  eval '$answer = $a / $b'; warn $@ if $@;
      6.  
    6.  
    7.  # a compile-time error
      8.  
    8.  eval { $answer = }; # WRONG
      9.  
    9.  
    10.  # a run-time error
      11.  
    11.  eval '$answer =';   # sets $@
      12.

    Using the eval{} form as an exception trap in libraries does have someissues. Due to the current arguably broken state of __DIE__ hooks, youmay wish not to trigger any __DIE__ hooks that user code may have installed.You can use the local $SIG{__DIE__} construct for this purpose,as this example shows:

    1.  # a private exception trap for divide-by-zero
      2.  
    2.  eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
      3.  
    3.  warn $@ if $@;
      4.

    This is especially significant, given that __DIE__ hooks can calldie again, which has the effect of changing their error messages:

    1.  # __DIE__ hooks may modify error messages
      2.  
    2.  {
      3.  
    3.    local $SIG{'__DIE__'} =
      4.  
    4.   sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
      5.  
    5.    eval { die "foo lives here" };
      6.  
    6.    print $@ if $@; # prints "bar lives here"
      7.  
    7.  }
      8.

    Because this promotes action at a distance, this counterintuitive behaviormay be fixed in a future release.

    With an eval, you should be especially careful to remember what'sbeing looked at when:

    1.  eval $x; # CASE 1
      2.  
    2.  eval "$x";  # CASE 2
      3.  
    3.  
    4.  eval '$x';  # CASE 3
      5.  
    5.  eval { $x }; # CASE 4
      6.  
    6.  
    7.  eval "\$$x++";  # CASE 5
      8.  
    8.  $$x++;  # CASE 6
      9.

    Cases 1 and 2 above behave identically: they run the code contained inthe variable $x. (Although case 2 has misleading double quotes makingthe reader wonder what else might be happening (nothing is).) Cases 3and 4 likewise behave in the same way: they run the code '$x', whichdoes nothing but return the value of $x. (Case 4 is preferred forpurely visual reasons, but it also has the advantage of compiling atcompile-time instead of at run-time.) Case 5 is a place wherenormally you would like to use double quotes, except that in thisparticular situation, you can just use symbolic references instead, asin case 6.

    Before Perl 5.14, the assignment to $@ occurred before restoration of localized variables, which means that for your code to run on olderversions, a temporary is required if you want to mask some but not allerrors:

    1.  # alter $@ on nefarious repugnancy only
      2.  
    2.  {
      3.  
    3.    my $e;
      4.  
    4.    {
      5.  
    5.   local $@; # protect existing $@
      6.  
    6.   eval { test_repugnancy() };
      7.  
    7.   # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
      8.  
    8.   $@ =~ /nefarious/ and $e = $@;
      9.  
    9.    }
      10.  
    10.    die $e if defined $e
      11.  
    11.  }
      12.

    eval BLOCK does not count as a loop, so the loop control statementsnext, last, or redo cannot be used to leave or restart the block.

    An eval '' executed within the DB package doesn't see the usualsurrounding lexical scope, but rather the scope of the first non-DB pieceof code that called it. You don't normally need to worry about this unlessyou are writing a Perl debugger.

  • evalbytes EXPR
  • evalbytes

    This function is like eval with a string argument, except it alwaysparses its argument, or $_ if EXPR is omitted, as a string of bytes. Astring containing characters whose ordinal value exceeds 255 results in anerror. Source filters activated within the evaluated code apply to thecode itself.

    This function is only available under the evalbytes feature, ause v5.16 (or higher) declaration, or with a CORE:: prefix. Seefeature for more information.

  • exec LIST
  • exec PROGRAM LIST

    The exec function executes a system command and never returns;use system instead of exec if you want it to return. It fails andreturns false only if the command does not exist and it is executeddirectly instead of via your system's command shell (see below).

    Since it's a common mistake to use exec instead of system, Perlwarns you if exec is called in void context and if there is a followingstatement that isn't die, warn, or exit (if -w is set--butyou always do that, right?). If you really want to follow an execwith some other statement, you can use one of these styles to avoid the warning:

    1.  exec ('foo')   or print STDERR "couldn't exec foo: $!";
      2.  
    2.  { exec ('foo') }; print STDERR "couldn't exec foo: $!";
      3.

    If there is more than one argument in LIST, or if LIST is an arraywith more than one value, calls execvp(3) with the arguments in LIST.If there is only one scalar argument or an array with one element in it,the argument is checked for shell metacharacters, and if there are any,the entire argument is passed to the system's command shell for parsing(this is /bin/sh -c on Unix platforms, but varies on other platforms).If there are no shell metacharacters in the argument, it is split intowords and passed directly to execvp, which is more efficient.Examples:

    1.  exec '/bin/echo', 'Your arguments are: ', @ARGV;
      2.  
    2.  exec "sort $outfile | uniq";
      3.

    If you don't really want to execute the first argument, but want to lieto the program you are executing about its own name, you can specifythe program you actually want to run as an "indirect object" (without acomma) in front of the LIST. (This always forces interpretation of theLIST as a multivalued list, even if there is only a single scalar inthe list.) Example:

    1.  $shell = '/bin/csh';
      2.  
    2.  exec $shell '-sh'; # pretend it's a login shell
      3.

    or, more directly,

    1.  exec {'/bin/csh'} '-sh';  # pretend it's a login shell
      2.

    When the arguments get executed via the system shell, results aresubject to its quirks and capabilities. See `STRING` in perlopfor details.

    Using an indirect object with exec or system is also moresecure. This usage (which also works fine with system()) forcesinterpretation of the arguments as a multivalued list, even if thelist had just one argument. That way you're safe from the shellexpanding wildcards or splitting up words with whitespace in them.

    1.  @args = ( "echo surprise" );
      2.  
    2.  
    3.  exec @args;   # subject to shell escapes
      4.  
    4.  # if @args == 1
      5.  
    5.  exec { $args[0] } @args;  # safe even with one-arg list
      6.

    The first version, the one without the indirect object, ran the echoprogram, passing it "surprise" an argument. The second version didn't;it tried to run a program named "echo surprise", didn't find it, and set$? to a non-zero value indicating failure.

    Beginning with v5.6.0, Perl attempts to flush all files opened foroutput before the exec, but this may not be supported on some platforms(see perlport). To be safe, you may need to set $| ($AUTOFLUSHin English) or call the autoflush() method of IO::Handle on anyopen handles to avoid lost output.

    Note that exec will not call your END blocks, nor will it invokeDESTROY methods on your objects.

    Portability issues: exec in perlport.

  • exists EXPR

    Given an expression that specifies an element of a hash, returns true if thespecified element in the hash has ever been initialized, even if thecorresponding value is undefined.

    1.  print "Exists\n" if exists $hash{$key};
      2.  
    2.  print "Defined\n"   if defined $hash{$key};
      3.  
    3.  print "True\n"  if $hash{$key};
      4.

    exists may also be called on array elements, but its behavior is much lessobvious and is strongly tied to the use of delete on arrays. Be awarethat calling exists on array values is deprecated and likely to be removed ina future version of Perl.

    1.  print "Exists\n" if exists $array[$index];
      2.  
    2.  print "Defined\n"   if defined $array[$index];
      3.  
    3.  print "True\n"  if $array[$index];
      4.

    A hash or array element can be true only if it's defined and defined only ifit exists, but the reverse doesn't necessarily hold true.

    Given an expression that specifies the name of a subroutine,returns true if the specified subroutine has ever been declared, evenif it is undefined. Mentioning a subroutine name for exists or defineddoes not count as declaring it. Note that a subroutine that does notexist may still be callable: its package may have an AUTOLOADmethod that makes it spring into existence the first time that it iscalled; see perlsub.

    1.  print "Exists\n"  if exists &subroutine;
      2.  
    2.  print "Defined\n" if defined &subroutine;
      3.

    Note that the EXPR can be arbitrarily complicated as long as the finaloperation is a hash or array key lookup or subroutine name:

    1.  if (exists $ref->{A}->{B}->{$key})  { }
      2.  
    2.  if (exists $hash{A}{B}{$key})   { }
      3.  
    3.  
    4.  if (exists $ref->{A}->{B}->[$ix])   { }
      5.  
    5.  if (exists $hash{A}{B}[$ix]) { }
      6.  
    6.  
    7.  if (exists &{$ref->{A}{B}{$key}})   { }
      8.

    Although the most deeply nested array or hash element will not spring intoexistence just because its existence was tested, any intervening ones will.Thus $ref->{"A"} and $ref->{"A"}->{"B"} will springinto existence due to the existence test for the $key element above.This happens anywhere the arrow operator is used, including even here:

    1.  undef $ref;
      2.  
    2.  if (exists $ref->{"Some key"}) { }
      3.  
    3.  print $ref;  # prints HASH(0x80d3d5c)
      4.

    This surprising autovivification in what does not at first--or evensecond--glance appear to be an lvalue context may be fixed in a futurerelease.

    Use of a subroutine call, rather than a subroutine name, as an argumentto exists() is an error.

    1.  exists &sub; # OK
      2.  
    2.  exists &sub();  # Error
      3.
  • exit EXPR
  • exit

    Evaluates EXPR and exits immediately with that value. Example:

    1.  $ans = <STDIN>;
      2.  
    2.  exit 0 if $ans =~ /^[Xx]/;
      3.

    See also die. If EXPR is omitted, exits with 0 status. The onlyuniversally recognized values for EXPR are 0 for success and 1for error; other values are subject to interpretation depending on theenvironment in which the Perl program is running. For example, exiting69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter will causethe mailer to return the item undelivered, but that's not true everywhere.

    Don't use exit to abort a subroutine if there's any chance thatsomeone might want to trap whatever error happened. Use die instead,which can be trapped by an eval.

    The exit() function does not always exit immediately. It calls anydefined END routines first, but these END routines may notthemselves abort the exit. Likewise any object destructors that need tobe called are called before the real exit. END routines and destructorscan change the exit status by modifying $?. If this is a problem, youcan call POSIX::_exit($status) to avoid END and destructor processing.See perlmod for details.

    Portability issues: exit in perlport.

  • exp EXPR
  • exp

    Returns e (the natural logarithm base) to the power of EXPR.If EXPR is omitted, gives exp($_).

  • fc EXPR
  • fc

    Returns the casefolded version of EXPR. This is the internal functionimplementing the \F escape in double-quoted strings.

    Casefolding is the process of mapping strings to a form where casedifferences are erased; comparing two strings in their casefoldedform is effectively a way of asking if two strings are equal,regardless of case.

    Roughly, if you ever found yourself writing this

    1.  lc($this) eq lc($that)  # Wrong!
      2.  
    2.  # or
      3.  
    3.  uc($this) eq uc($that)  # Also wrong!
      4.  
    4.  # or
      5.  
    5.  $this =~ /\Q$that/i # Right!
      6.

    Now you can write

    1.  fc($this) eq fc($that)
      2.

    And get the correct results.

    Perl only implements the full form of casefolding.For further information on casefolding, refer tothe Unicode Standard, specifically sections 3.13 Default Case Operations,4.2 Case-Normative, and 5.18 Case Mappings,available at http://www.unicode.org/versions/latest/, as well as theCase Charts available at http://www.unicode.org/charts/case/.

    If EXPR is omitted, uses $_.

    This function behaves the same way under various pragma, such as in a locale,as lc does.

    While the Unicode Standard defines two additional forms of casefolding,one for Turkic languages and one that never maps one character into multiplecharacters, these are not provided by the Perl core; However, the CPAN moduleUnicode::Casing may be used to provide an implementation.

    This keyword is available only when the "fc" feature is enabled,or when prefixed with CORE::; See feature. Alternately,include a use v5.16 or later to the current scope.

  • fcntl FILEHANDLE,FUNCTION,SCALAR

    Implements the fcntl(2) function. You'll probably have to say

    1.  use Fcntl;
      2.

    first to get the correct constant definitions. Argument processing andvalue returned work just like ioctl below.For example:

    1.  use Fcntl;
      2.  
    2.  fcntl($filehandle, F_GETFL, $packed_return_buffer)
      3.  
    3.  or die "can't fcntl F_GETFL: $!";
      4.

    You don't have to check for defined on the return from fcntl.Like ioctl, it maps a 0 return from the system call into"0 but true" in Perl. This string is true in boolean context and 0in numeric context. It is also exempt from the normal -w warningson improper numeric conversions.

    Note that fcntl raises an exception if used on a machine thatdoesn't implement fcntl(2). See the Fcntl module or your fcntl(2)manpage to learn what functions are available on your system.

    Here's an example of setting a filehandle named REMOTE to benon-blocking at the system level. You'll have to negotiate $|on your own, though.

    1.  use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
      2.  
    2.  
    3.  $flags = fcntl(REMOTE, F_GETFL, 0)
      4.  
    4.  or die "Can't get flags for the socket: $!\n";
      5.  
    5.  
    6.  $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
      7.  
    7.  or die "Can't set flags for the socket: $!\n";
      8.

    Portability issues: fcntl in perlport.

  • __FILE__

    A special token that returns the name of the file in which it occurs.

  • fileno FILEHANDLE

    Returns the file descriptor for a filehandle, or undefined if thefilehandle is not open. If there is no real file descriptor at the OSlevel, as can happen with filehandles connected to memory objects viaopen with a reference for the third argument, -1 is returned.

    This is mainly useful for constructingbitmaps for select and low-level POSIX tty-handling operations.If FILEHANDLE is an expression, the value is taken as an indirectfilehandle, generally its name.

    You can use this to find out whether two handles refer to thesame underlying descriptor:

    1.  if (fileno(THIS) == fileno(THAT)) {
      2.  
    2.  print "THIS and THAT are dups\n";
      3.  
    3.  }
      4.
  • flock FILEHANDLE,OPERATION

    Calls flock(2), or an emulation of it, on FILEHANDLE. Returns truefor success, false on failure. Produces a fatal error if used on amachine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).flock is Perl's portable file-locking interface, although it locksentire files only, not records.

    Two potentially non-obvious but traditional flock semantics arethat it waits indefinitely until the lock is granted, and that its locksare merely advisory. Such discretionary locks are more flexible, butoffer fewer guarantees. This means that programs that do not also useflock may modify files locked with flock. See perlport, your port's specific documentation, and your system-specific local manpagesfor details. It's best to assume traditional behavior if you're writingportable programs. (But if you're not, you should as always feel perfectlyfree to write for your own system's idiosyncrasies (sometimes called"features"). Slavish adherence to portability concerns shouldn't getin the way of your getting your job done.)

    OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined withLOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, butyou can use the symbolic names if you import them from the Fcntl module,either individually, or as a group using the :flock tag. LOCK_SHrequests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UNreleases a previously requested lock. If LOCK_NB is bitwise-or'ed withLOCK_SH or LOCK_EX, then flock returns immediately rather than blockingwaiting for the lock; check the return status to see if you got it.

    To avoid the possibility of miscoordination, Perl now flushes FILEHANDLEbefore locking or unlocking it.

    Note that the emulation built with lockf(3) doesn't provide sharedlocks, and it requires that FILEHANDLE be open with write intent. Theseare the semantics that lockf(3) implements. Most if not all systemsimplement lockf(3) in terms of fcntl(2) locking, though, so thediffering semantics shouldn't bite too many people.

    Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLEbe open with read intent to use LOCK_SH and requires that it be openwith write intent to use LOCK_EX.

    Note also that some versions of flock cannot lock things over thenetwork; you would need to use the more system-specific fcntl forthat. If you like you can force Perl to ignore your system's flock(2)function, and so provide its own fcntl(2)-based emulation, by passingthe switch -Ud_flock to the Configure program when you configureand build a new Perl.

    Here's a mailbox appender for BSD systems.

    1.  use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
      2.  
    2.  
    3.  sub lock {
      4.  
    4.  my ($fh) = @_;
      5.  
    5.  flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
      6.  
    6.  
    7.  # and, in case someone appended while we were waiting...
      8.  
    8.  seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
      9.  
    9.  }
      10.  
    10.  
    11.  sub unlock {
      12.  
    12.  my ($fh) = @_;
      13.  
    13.  flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
      14.  
    14.  }
      15.  
    15.  
    16.  open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
      17.  
    17.  or die "Can't open mailbox: $!";
      18.  
    18.  
    19.  lock($mbox);
      20.  
    20.  print $mbox $msg,"\n\n";
      21.  
    21.  unlock($mbox);
      22.

    On systems that support a real flock(2), locks are inherited across fork()calls, whereas those that must resort to the more capricious fcntl(2)function lose their locks, making it seriously harder to write servers.

    See also DB_File for other flock() examples.

    Portability issues: flock in perlport.

  • fork

    Does a fork(2) system call to create a new process running thesame program at the same point. It returns the child pid to theparent process, 0 to the child process, or undef if the fork isunsuccessful. File descriptors (and sometimes locks on those descriptors)are shared, while everything else is copied. On most systems supportingfork(), great care has gone into making it extremely efficient (forexample, using copy-on-write technology on data pages), making it thedominant paradigm for multitasking over the last few decades.

    Beginning with v5.6.0, Perl attempts to flush all files opened foroutput before forking the child process, but this may not be supportedon some platforms (see perlport). To be safe, you may need to set$| ($AUTOFLUSH in English) or call the autoflush() method ofIO::Handle on any open handles to avoid duplicate output.

    If you fork without ever waiting on your children, you willaccumulate zombies. On some systems, you can avoid this by setting$SIG{CHLD} to "IGNORE". See also perlipc for more examples offorking and reaping moribund children.

    Note that if your forked child inherits system file descriptors likeSTDIN and STDOUT that are actually connected by a pipe or socket, evenif you exit, then the remote server (such as, say, a CGI script or abackgrounded job launched from a remote shell) won't think you're done.You should reopen those to /dev/null if it's any issue.

    On some platforms such as Windows, where the fork() system call is not available,Perl can be built to emulate fork() in the Perl interpreter.The emulation is designed, at the level of the Perl program,to be as compatible as possible with the "Unix" fork().However it has limitations that have to be considered in code intended to be portable.See perlfork for more details.

    Portability issues: fork in perlport.

  • format

    Declare a picture format for use by the write function. Forexample:

    1.  format Something =
      2.  
    2.  Test: @<<<<<<<< @||||| @>>>>>
      3.  
    3.   $str, $%, '$' . int($num)
      4.  
    4.  .
      5.  
    5.  
    6.  $str = "widget";
      7.  
    7.  $num = $cost/$quantity;
      8.  
    8.  $~ = 'Something';
      9.  
    9.  write;
      10.

    See perlform for many details and examples.

  • formline PICTURE,LIST

    This is an internal function used by formats, though you may call it,too. It formats (see perlform) a list of values according to thecontents of PICTURE, placing the output into the format outputaccumulator, $^A (or $ACCUMULATOR in English).Eventually, when a write is done, the contents of$^A are written to some filehandle. You could also read $^Aand then set $^A back to "". Note that a format typicallydoes one formline per line of form, but the formline function itselfdoesn't care how many newlines are embedded in the PICTURE. This meansthat the ~ and ~~ tokens treat the entire PICTURE as a single line.You may therefore need to use multiple formlines to implement a singlerecord format, just like the format compiler.

    Be careful if you put double quotes around the picture, because an @character may be taken to mean the beginning of an array name.formline always returns true. See perlform for other examples.

    If you are trying to use this instead of write to capture the output,you may find it easier to open a filehandle to a scalar(open $fh, ">", \$output) and write to that instead.

  • getc FILEHANDLE
  • getc

    Returns the next character from the input file attached to FILEHANDLE,or the undefined value at end of file or if there was an error (inthe latter case $! is set). If FILEHANDLE is omitted, reads fromSTDIN. This is not particularly efficient. However, it cannot beused by itself to fetch single characters without waiting for the userto hit enter. For that, try something more like:

    1.  if ($BSD_STYLE) {
      2.  
    2.  system "stty cbreak </dev/tty >/dev/tty 2>&1";
      3.  
    3.  }
      4.  
    4.  else {
      5.  
    5.  system "stty", '-icanon', 'eol', "\001";
      6.  
    6.  }
      7.  
    7.  
    8.  $key = getc(STDIN);
      9.  
    9.  
    10.  if ($BSD_STYLE) {
      11.  
    11.  system "stty -cbreak </dev/tty >/dev/tty 2>&1";
      12.  
    12.  }
      13.  
    13.  else {
      14.  
    14.  system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
      15.  
    15.  }
      16.  
    16.  print "\n";
      17.

    Determination of whether $BSD_STYLE should be setis left as an exercise to the reader.

    The POSIX::getattr function can do this more portably onsystems purporting POSIX compliance. See also the Term::ReadKeymodule from your nearest CPAN site; details on CPAN can be found underCPAN in perlmodlib.

  • getlogin

    This implements the C library function of the same name, which on mostsystems returns the current login from /etc/utmp, if any. If itreturns the empty string, use getpwuid.

    1.  $login = getlogin || getpwuid($<) || "Kilroy";
      2.

    Do not consider getlogin for authentication: it is not assecure as getpwuid.

    Portability issues: getlogin in perlport.

  • getpeername SOCKET

    Returns the packed sockaddr address of the other end of the SOCKETconnection.

    1.  use Socket;
      2.  
    2.  $hersockaddr = getpeername(SOCK);
      3.  
    3.  ($port, $iaddr) = sockaddr_in($hersockaddr);
      4.  
    4.  $herhostname = gethostbyaddr($iaddr, AF_INET);
      5.  
    5.  $herstraddr = inet_ntoa($iaddr);
      6.
  • getpgrp PID

    Returns the current process group for the specified PID. Usea PID of 0 to get the current process group for thecurrent process. Will raise an exception if used on a machine thatdoesn't implement getpgrp(2). If PID is omitted, returns the processgroup of the current process. Note that the POSIX version of getpgrpdoes not accept a PID argument, so only PID==0 is truly portable.

    Portability issues: getpgrp in perlport.

  • getppid

    Returns the process id of the parent process.

    Note for Linux users: Between v5.8.1 and v5.16.0 Perl would workaround non-POSIX thread semantics the minority of Linux systems (andDebian GNU/kFreeBSD systems) that used LinuxThreads, this emulationhas since been removed. See the documentation for $$ fordetails.

    Portability issues: getppid in perlport.

  • getpriority WHICH,WHO

    Returns the current priority for a process, a process group, or a user.(See getpriority(2).) Will raise a fatal exception if used on amachine that doesn't implement getpriority(2).

    Portability issues: getpriority in perlport.

  • getpwnam NAME
  • getgrnam NAME
  • gethostbyname NAME
  • getnetbyname NAME
  • getprotobyname NAME
  • getpwuid UID
  • getgrgid GID
  • getservbyname NAME,PROTO
  • gethostbyaddr ADDR,ADDRTYPE
  • getnetbyaddr ADDR,ADDRTYPE
  • getprotobynumber NUMBER
  • getservbyport PORT,PROTO
  • getpwent
  • getgrent
  • gethostent
  • getnetent
  • getprotoent
  • getservent
  • setpwent
  • setgrent
  • sethostent STAYOPEN
  • setnetent STAYOPEN
  • setprotoent STAYOPEN
  • setservent STAYOPEN
  • endpwent
  • endgrent
  • endhostent
  • endnetent
  • endprotoent
  • endservent

    These routines are the same as their counterparts in thesystem C library. In list context, the return values from thevarious get routines are as follows:

    1.  ($name,$passwd,$uid,$gid,
      2.  
    2.    $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
      3.  
    3.  ($name,$passwd,$gid,$members) = getgr*
      4.  
    4.  ($name,$aliases,$addrtype,$length,@addrs) = gethost*
      5.  
    5.  ($name,$aliases,$addrtype,$net) = getnet*
      6.  
    6.  ($name,$aliases,$proto) = getproto*
      7.  
    7.  ($name,$aliases,$port,$proto) = getserv*
      8.

    (If the entry doesn't exist you get an empty list.)

    The exact meaning of the $gcos field varies but it usually containsthe real name of the user (as opposed to the login name) and otherinformation pertaining to the user. Beware, however, that in manysystem users are able to change this information and therefore itcannot be trusted and therefore the $gcos is tainted (seeperlsec). The $passwd and $shell, user's encrypted password andlogin shell, are also tainted, for the same reason.

    In scalar context, you get the name, unless the function was alookup by name, in which case you get the other thing, whatever it is.(If the entry doesn't exist you get the undefined value.) For example:

    1.  $uid   = getpwnam($name);
      2.  
    2.  $name  = getpwuid($num);
      3.  
    3.  $name  = getpwent();
      4.  
    4.  $gid   = getgrnam($name);
      5.  
    5.  $name  = getgrgid($num);
      6.  
    6.  $name  = getgrent();
      7.  
    7.  #etc.
      8.

    In getpw*() the fields $quota, $comment, and $expire are specialin that they are unsupported on many systems. If the$quota is unsupported, it is an empty scalar. If it is supported, itusually encodes the disk quota. If the $comment field is unsupported,it is an empty scalar. If it is supported it usually encodes someadministrative comment about the user. In some systems the $quotafield may be $change or $age, fields that have to do with passwordaging. In some systems the $comment field may be $class. The $expirefield, if present, encodes the expiration period of the account or thepassword. For the availability and the exact meaning of these fieldsin your system, please consult getpwnam(3) and your system's pwd.h file. You can also find out from within Perl what your$quota and $comment fields mean and whether you have the $expire fieldby using the Config module and the values d_pwquota, d_pwage,d_pwchange, d_pwcomment, and d_pwexpire. Shadow passwordfiles are supported only if your vendor has implemented them in theintuitive fashion that calling the regular C library routines gets theshadow versions if you're running under privilege or if there existsthe shadow(3) functions as found in System V (this includes Solarisand Linux). Those systems that implement a proprietary shadow passwordfacility are unlikely to be supported.

    The $members value returned by getgr*() is a space-separated list ofthe login names of the members of the group.

    For the gethost*() functions, if the h_errno variable is supported inC, it will be returned to you via $? if the function call fails. The@addrs value returned by a successful call is a list of rawaddresses returned by the corresponding library call. In theInternet domain, each address is four bytes long; you can unpack itby saying something like:

    1.  ($a,$b,$c,$d) = unpack('W4',$addr[0]);
      2.

    The Socket library makes this slightly easier:

    1.  use Socket;
      2.  
    2.  $iaddr = inet_aton("127.1"); # or whatever address
      3.  
    3.  $name  = gethostbyaddr($iaddr, AF_INET);
      4.  
    4.  
    5.  # or going the other way
      6.  
    6.  $straddr = inet_ntoa($iaddr);
      7.

    In the opposite way, to resolve a hostname to the IP addressyou can write this:

    1.  use Socket;
      2.  
    2.  $packed_ip = gethostbyname("www.perl.org");
      3.  
    3.  if (defined $packed_ip) {
      4.  
    4.  $ip_address = inet_ntoa($packed_ip);
      5.  
    5.  }
      6.

    Make sure gethostbyname() is called in SCALAR context and thatits return value is checked for definedness.

    The getprotobynumber function, even though it only takes one argument,has the precedence of a list operator, so beware:

    1.  getprotobynumber $number eq 'icmp'   # WRONG
      2.  
    2.  getprotobynumber($number eq 'icmp')  # actually means this
      3.  
    3.  getprotobynumber($number) eq 'icmp'  # better this way
      4.

    If you get tired of remembering which element of the return listcontains which return value, by-name interfaces are providedin standard modules: File::stat, Net::hostent, Net::netent,Net::protoent, Net::servent, Time::gmtime, Time::localtime,and User::grent. These override the normal built-ins, supplyingversions that return objects with the appropriate namesfor each field. For example:

    1.    use File::stat;
      2.  
    2.    use User::pwent;
      3.  
    3.    $is_his = (stat($filename)->uid == pwent($whoever)->uid);
      4.

    Even though it looks as though they're the same method calls (uid),they aren't, because a File::stat object is different froma User::pwent object.

    Portability issues: getpwnam in perlport to endservent in perlport.

  • getsockname SOCKET

    Returns the packed sockaddr address of this end of the SOCKET connection,in case you don't know the address because you have several differentIPs that the connection might have come in on.

    1.  use Socket;
      2.  
    2.  $mysockaddr = getsockname(SOCK);
      3.  
    3.  ($port, $myaddr) = sockaddr_in($mysockaddr);
      4.  
    4.  printf "Connect to %s [%s]\n",
      5.  
    5.    scalar gethostbyaddr($myaddr, AF_INET),
      6.  
    6.    inet_ntoa($myaddr);
      7.
  • getsockopt SOCKET,LEVEL,OPTNAME

    Queries the option named OPTNAME associated with SOCKET at a given LEVEL.Options may exist at multiple protocol levels depending on the sockettype, but at least the uppermost socket level SOL_SOCKET (defined in theSocket module) will exist. To query options at another level theprotocol number of the appropriate protocol controlling the optionshould be supplied. For example, to indicate that an option is to beinterpreted by the TCP protocol, LEVEL should be set to the protocolnumber of TCP, which you can get using getprotobyname.

    The function returns a packed string representing the requested socketoption, or undef on error, with the reason for the error placed in$!. Just what is in the packed string depends on LEVEL and OPTNAME;consult getsockopt(2) for details. A common case is that the option is aninteger, in which case the result is a packed integer, which you can decodeusing unpack with the i (or I) format.

    Here's an example to test whether Nagle's algorithm is enabled on a socket:

    1.  use Socket qw(:all);
      2.  
    2.  
    3.  defined(my $tcp = getprotobyname("tcp"))
      4.  
    4.  or die "Could not determine the protocol number for tcp";
      5.  
    5.  # my $tcp = IPPROTO_TCP; # Alternative
      6.  
    6.  my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
      7.  
    7.  or die "getsockopt TCP_NODELAY: $!";
      8.  
    8.  my $nodelay = unpack("I", $packed);
      9.  
    9.  print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
      10.

    Portability issues: getsockopt in perlport.

  • glob EXPR
  • glob

    In list context, returns a (possibly empty) list of filename expansions onthe value of EXPR such as the standard Unix shell /bin/csh would do. Inscalar context, glob iterates through such filename expansions, returningundef when the list is exhausted. This is the internal functionimplementing the <*.c> operator, but you can use it directly. IfEXPR is omitted, $_ is used. The <*.c> operator is discussed inmore detail in I/O Operators in perlop.

    Note that glob splits its arguments on whitespace and treatseach segment as separate pattern. As such, glob("*.c *.h") matches all files with a .c or .h extension. The expressionglob(".* *") matches all files in the current working directory.If you want to glob filenames that might contain whitespace, you'llhave to use extra quotes around the spacey filename to protect it.For example, to glob filenames that have an e followed by a spacefollowed by an f, use either of:

    1.  @spacies = <"*e f*">;
      2.  
    2.  @spacies = glob '"*e f*"';
      3.  
    3.  @spacies = glob q("*e f*");
      4.

    If you had to get a variable through, you could do this:

    1.  @spacies = glob "'*${var}e f*'";
      2.  
    2.  @spacies = glob qq("*${var}e f*");
      3.

    If non-empty braces are the only wildcard characters used in theglob, no filenames are matched, but potentially many stringsare returned. For example, this produces nine strings, one foreach pairing of fruits and colors:

    1.  @many =  glob "{apple,tomato,cherry}={green,yellow,red}";
      2.

    Beginning with v5.6.0, this operator is implemented using the standardFile::Glob extension. See File::Glob for details, includingbsd_glob which does not treat whitespace as a pattern separator.

    Portability issues: glob in perlport.

  • gmtime EXPR
  • gmtime

    Works just like localtime but the returned values arelocalized for the standard Greenwich time zone.

    Note: When called in list context, $isdst, the last valuereturned by gmtime, is always 0. There is noDaylight Saving Time in GMT.

    Portability issues: gmtime in perlport.

  • goto LABEL
  • goto EXPR
  • goto &NAME

    The goto-LABEL form finds the statement labeled with LABEL andresumes execution there. It can't be used to get out of a block orsubroutine given to sort. It can be used to go almost anywhereelse within the dynamic scope, including out of subroutines, but it'susually better to use some other construct such as last or die.The author of Perl has never felt the need to use this form of goto(in Perl, that is; C is another matter). (The difference is that Cdoes not offer named loops combined with loop control. Perl does, andthis replaces most structured uses of goto in other languages.)

    The goto-EXPR form expects a label name, whose scope will be resolveddynamically. This allows for computed gotos per FORTRAN, but isn'tnecessarily recommended if you're optimizing for maintainability:

    1.  goto ("FOO", "BAR", "GLARCH")[$i];
      2.

    As shown in this example, goto-EXPR is exempt from the "looks like afunction" rule. A pair of parentheses following it does not (necessarily)delimit its argument. goto("NE")."XT" is equivalent to goto NEXT.

    Use of goto-LABEL or goto-EXPR to jump into a construct isdeprecated and will issue a warning. Even then, it may not be used togo into any construct that requires initialization, such as asubroutine or a foreach loop. It also can't be used to go into aconstruct that is optimized away.

    The goto-&NAME form is quite different from the other forms ofgoto. In fact, it isn't a goto in the normal sense at all, anddoesn't have the stigma associated with other gotos. Instead, itexits the current subroutine (losing any changes set by local()) andimmediately calls in its place the named subroutine using the currentvalue of @_. This is used by AUTOLOAD subroutines that wish toload another subroutine and then pretend that the other subroutine hadbeen called in the first place (except that any modifications to @_in the current subroutine are propagated to the other subroutine.)After the goto, not even caller will be able to tell that thisroutine was called first.

    NAME needn't be the name of a subroutine; it can be a scalar variablecontaining a code reference or a block that evaluates to a codereference.

  • grep BLOCK LIST
  • grep EXPR,LIST

    This is similar in spirit to, but not the same as, grep(1) and itsrelatives. In particular, it is not limited to using regular expressions.

    Evaluates the BLOCK or EXPR for each element of LIST (locally setting$_ to each element) and returns the list value consisting of thoseelements for which the expression evaluated to true. In scalarcontext, returns the number of times the expression was true.

    1.  @foo = grep(!/^#/, @bar); # weed out comments
      2.

    or equivalently,

    1.  @foo = grep {!/^#/} @bar; # weed out comments
      2.

    Note that $_ is an alias to the list value, so it can be used tomodify the elements of the LIST. While this is useful and supported,it can cause bizarre results if the elements of LIST are not variables.Similarly, grep returns aliases into the original list, much as a forloop's index variable aliases the list elements. That is, modifying anelement of a list returned by grep (for example, in a foreach, mapor another grep) actually modifies the element in the original list.This is usually something to be avoided when writing clear code.

    If $_ is lexical in the scope where the grep appears (because it hasbeen declared with my $_) then, in addition to being locally aliased tothe list elements, $_ keeps being lexical inside the block; i.e., itcan't be seen from the outside, avoiding any potential side-effects.

    See also map for a list composed of the results of the BLOCK or EXPR.

  • hex EXPR
  • hex

    Interprets EXPR as a hex string and returns the corresponding value.(To convert strings that might start with either 0, 0x, or 0b, seeoct.) If EXPR is omitted, uses $_.

    1.  print hex '0xAf'; # prints '175'
      2.  
    2.  print hex 'aF';   # same
      3.

    Hex strings may only represent integers. Strings that would causeinteger overflow trigger a warning. Leading whitespace is not stripped,unlike oct(). To present something as hex, look into printf,sprintf, and unpack.

  • import LIST

    There is no builtin import function. It is just an ordinarymethod (subroutine) defined (or inherited) by modules that wish to exportnames to another module. The use function calls the import methodfor the package used. See also use, perlmod, and Exporter.

  • index STR,SUBSTR,POSITION
  • index STR,SUBSTR

    The index function searches for one string within another, but withoutthe wildcard-like behavior of a full regular-expression pattern match.It returns the position of the first occurrence of SUBSTR in STR ator after POSITION. If POSITION is omitted, starts searching from thebeginning of the string. POSITION before the beginning of the stringor after its end is treated as if it were the beginning or the end,respectively. POSITION and the return value are based at zero.If the substring is not found, index returns -1.

  • int EXPR
  • int

    Returns the integer portion of EXPR. If EXPR is omitted, uses $_.You should not use this function for rounding: one because it truncatestowards 0, and two because machine representations of floating-pointnumbers can sometimes produce counterintuitive results. For example,int(-6.725/0.025) produces -268 rather than the correct -269; that'sbecause it's really more like -268.99999999999994315658 instead. Usually,the sprintf, printf, or the POSIX::floor and POSIX::ceilfunctions will serve you better than will int().

  • ioctl FILEHANDLE,FUNCTION,SCALAR

    Implements the ioctl(2) function. You'll probably first have to say

    1.  require "sys/ioctl.ph";  # probably in $Config{archlib}/sys/ioctl.ph
      2.

    to get the correct function definitions. If sys/ioctl.ph doesn'texist or doesn't have the correct definitions you'll have to roll yourown, based on your C header files such as <sys/ioctl.h>.(There is a Perl script called h2ph that comes with the Perl kit thatmay help you in this, but it's nontrivial.) SCALAR will be read and/orwritten depending on the FUNCTION; a C pointer to the string value of SCALARwill be passed as the third argument of the actual ioctl call. (If SCALARhas no string value but does have a numeric value, that value will bepassed rather than a pointer to the string value. To guarantee this to betrue, add a 0 to the scalar before using it.) The pack and unpackfunctions may be needed to manipulate the values of structures used byioctl.

    The return value of ioctl (and fcntl) is as follows:

    1.  if OS returns:  then Perl returns:
      2.  
    2.  -1   undefined value
      3.  
    3.  0  string "0 but true"
      4.  
    4.  anything else   that number
      5.

    Thus Perl returns true on success and false on failure, yet you canstill easily determine the actual value returned by the operatingsystem:

    1.  $retval = ioctl(...) || -1;
      2.  
    2.  printf "System returned %d\n", $retval;
      3.

    The special string "0 but true" is exempt from -w complaintsabout improper numeric conversions.

    Portability issues: ioctl in perlport.

  • join EXPR,LIST

    Joins the separate strings of LIST into a single string with fieldsseparated by the value of EXPR, and returns that new string. Example:

    1.  $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
      2.

    Beware that unlike split, join doesn't take a pattern as itsfirst argument. Compare split.

  • keys HASH
  • keys ARRAY
  • keys EXPR

    Called in list context, returns a list consisting of all the keys of thenamed hash, or in Perl 5.12 or later only, the indices of an array. Perlreleases prior to 5.12 will produce a syntax error if you try to use anarray argument. In scalar context, returns the number of keys or indices.

    The keys of a hash are returned in an apparently random order. The actualrandom order is subject to change in future versions of Perl, but itis guaranteed to be the same order as either the values or eachfunction produces (given that the hash has not been modified). SincePerl 5.8.1 the ordering can be different even between different runs ofPerl for security reasons (see Algorithmic Complexity Attacks in perlsec).

    As a side effect, calling keys() resets the internal interator of the HASH or ARRAY(see each). In particular, calling keys() in void context resetsthe iterator with no other overhead.

    Here is yet another way to print your environment:

    1.  @keys = keys %ENV;
      2.  
    2.  @values = values %ENV;
      3.  
    3.  while (@keys) {
      4.  
    4.  print pop(@keys), '=', pop(@values), "\n";
      5.  
    5.  }
      6.

    or how about sorted by key:

    1.  foreach $key (sort(keys %ENV)) {
      2.  
    2.  print $key, '=', $ENV{$key}, "\n";
      3.  
    3.  }
      4.

    The returned values are copies of the original keys in the hash, somodifying them will not affect the original hash. Compare values.

    To sort a hash by value, you'll need to use a sort function.Here's a descending numeric sort of a hash by its values:

    1.  foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
      2.  
    2.  printf "%4d %s\n", $hash{$key}, $key;
      3.  
    3.  }
      4.

    Used as an lvalue, keys allows you to increase the number of hash bucketsallocated for the given hash. This can gain you a measure of efficiency ifyou know the hash is going to get big. (This is similar to pre-extendingan array by assigning a larger number to $#array.) If you say

    1.  keys %hash = 200;
      2.

    then %hash will have at least 200 buckets allocated for it--256 of them,in fact, since it rounds up to the next power of two. Thesebuckets will be retained even if you do %hash = (), use undef%hash if you want to free the storage while %hash is still in scope.You can't shrink the number of buckets allocated for the hash usingkeys in this way (but you needn't worry about doing this by accident,as trying has no effect). keys @array in an lvalue context is a syntaxerror.

    Starting with Perl 5.14, keys can take a scalar EXPR, which must containa reference to an unblessed hash or array. The argument will bedereferenced automatically. This aspect of keys is considered highlyexperimental. The exact behaviour may change in a future version of Perl.

    1.  for (keys $hashref) { ... }
      2.  
    2.  for (keys $obj->get_arrayref) { ... }
      3.

    To avoid confusing would-be users of your code who are running earlierversions of Perl with mysterious syntax errors, put this sort of thing atthe top of your file to signal that your code will work only on Perls ofa recent vintage:

    1.  use 5.012;# so keys/values/each work on arrays
      2.  
    2.  use 5.014;# so keys/values/each work on scalars (experimental)
      3.

    See also each, values, and sort.

  • kill SIGNAL, LIST
  • kill SIGNAL

    Sends a signal to a list of processes. Returns the number ofprocesses successfully signaled (which is not necessarily thesame as the number actually killed).

    1.  $cnt = kill 1, $child1, $child2;
      2.  
    2.  kill 9, @goners;
      3.

    If SIGNAL is zero, no signal is sent to the process, but killchecks whether it's possible to send a signal to it (thatmeans, to be brief, that the process is owned by the same user, or we arethe super-user). This is useful to check that a child process is stillalive (even if only as a zombie) and hasn't changed its UID. Seeperlport for notes on the portability of this construct.

    Unlike in the shell, if SIGNAL is negative, it kills process groups insteadof processes. That means you usuallywant to use positive not negative signals.You may also use a signal name in quotes.

    The behavior of kill when a PROCESS number is zero or negative depends onthe operating system. For example, on POSIX-conforming systems, zero willsignal the current process group and -1 will signal all processes.

    See Signals in perlipc for more details.

    On some platforms such as Windows where the fork() system call is not available.Perl can be built to emulate fork() at the interpreter level.This emulation has limitations related to kill that have to be considered,for code running on Windows and in code intended to be portable.

    See perlfork for more details.

    If there is no LIST of processes, no signal is sent, and the returnvalue is 0. This form is sometimes used, however, because it causestainting checks to be run. But seeLaundering and Detecting Tainted Data in perlsec.

    Portability issues: kill in perlport.

  • last LABEL
  • last

    The last command is like the break statement in C (as used inloops); it immediately exits the loop in question. If the LABEL isomitted, the command refers to the innermost enclosing loop. Thecontinue block, if any, is not executed:

    1.  LINE: while (<STDIN>) {
      2.  
    2.  last LINE if /^$/;  # exit when done with header
      3.  
    3.  #...
      4.  
    4.  }
      5.

    last cannot be used to exit a block that returns a value such aseval {}, sub {}, or do {}, and should not be used to exita grep() or map() operation.

    Note that a block by itself is semantically identical to a loopthat executes once. Thus last can be used to effect an earlyexit out of such a block.

    See also continue for an illustration of how last, next, andredo work.

  • lc EXPR
  • lc

    Returns a lowercased version of EXPR. This is the internal functionimplementing the \L escape in double-quoted strings.

    If EXPR is omitted, uses $_.

    What gets returned depends on several factors:

    • If use bytes is in effect:
      • On EBCDIC platforms

        The results are what the C language system call tolower() returns.

      • On ASCII platforms

        The results follow ASCII semantics. Only characters A-Z change, to a-zrespectively.

    • Otherwise, if use locale (but not use locale ':not_characters') is in effect:

      Respects current LC_CTYPE locale for code points < 256; and uses Unicodesemantics for the remaining code points (this last can only happen ifthe UTF8 flag is also set). See perllocale.

      A deficiency in this is that case changes that cross the 255/256boundary are not well-defined. For example, the lower case of LATIN CAPITALLETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCIIplatforms). But under use locale, the lower case of U+1E9E isitself, because 0xDF may not be LATIN SMALL LETTER SHARP S in thecurrent locale, and Perl has no way of knowing if that character evenexists in the locale, much less what code point it is. Perl returnsthe input character unchanged, for all instances (and there aren'tmany) where the 255/256 boundary would otherwise be crossed.

    • Otherwise, If EXPR has the UTF8 flag set:

      Unicode semantics are used for the case change.

    • Otherwise, if use feature 'unicode_strings' or use locale ':not_characters') is in effect:

      Unicode semantics are used for the case change.

    • Otherwise:
      • On EBCDIC platforms

        The results are what the C language system call tolower() returns.

      • On ASCII platforms

        ASCII semantics are used for the case change. The lowercase of any characteroutside the ASCII range is the character itself.

  • lcfirst EXPR
  • lcfirst

    Returns the value of EXPR with the first character lowercased. Thisis the internal function implementing the \l escape indouble-quoted strings.

    If EXPR is omitted, uses $_.

    This function behaves the same way under various pragmata, such as in a locale,as lc does.

  • length EXPR
  • length

    Returns the length in characters of the value of EXPR. If EXPR isomitted, returns the length of $_. If EXPR is undefined, returnsundef.

    This function cannot be used on an entire array or hash to find out howmany elements these have. For that, use scalar @array and scalar keys%hash, respectively.

    Like all Perl character operations, length() normally deals in logicalcharacters, not physical bytes. For how many bytes a string encoded asUTF-8 would take up, use length(Encode::encode_utf8(EXPR)) (you'll haveto use Encode first). See Encode and perlunicode.

  • __LINE__

    A special token that compiles to the current line number.

  • link OLDFILE,NEWFILE

    Creates a new filename linked to the old filename. Returns true forsuccess, false otherwise.

    Portability issues: link in perlport.

  • listen SOCKET,QUEUESIZE

    Does the same thing that the listen(2) system call does. Returns true ifit succeeded, false otherwise. See the example inSockets: Client/Server Communication in perlipc.

  • local EXPR

    You really probably want to be using my instead, because local isn'twhat most people think of as "local". SeePrivate Variables via my() in perlsub for details.

    A local modifies the listed variables to be local to the enclosingblock, file, or eval. If more than one value is listed, the list mustbe placed in parentheses. See Temporary Values via local() in perlsubfor details, including issues with tied arrays and hashes.

    The delete local EXPR construct can also be used to localize the deletionof array/hash elements to the current block.See Localized deletion of elements of composite types in perlsub.

  • localtime EXPR
  • localtime

    Converts a time as returned by the time function to a 9-element listwith the time analyzed for the local time zone. Typically used asfollows:

    1.  #  0 1 2 3 4 5 6 7 8
      2.  
    2.  ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
      3.  
    3.  localtime(time);
      4.

    All list elements are numeric and come straight out of the C `structtm'. $sec, $min, and $hour are the seconds, minutes, and hoursof the specified time.

    $mday is the day of the month and $mon the month inthe range 0..11, with 0 indicating January and 11 indicating December.This makes it easy to get a month name from a list:

    1.  my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
      2.  
    2.  print "$abbr[$mon] $mday";
      3.  
    3.  # $mon=9, $mday=18 gives "Oct 18"
      4.

    $year contains the number of years since 1900. To get a 4-digityear write:

    1.  $year += 1900;
      2.

    To get the last two digits of the year (e.g., "01" in 2001) do:

    1.  $year = sprintf("%02d", $year % 100);
      2.

    $wday is the day of the week, with 0 indicating Sunday and 3 indicatingWednesday. $yday is the day of the year, in the range 0..364(or 0..365 in leap years.)

    $isdst is true if the specified time occurs during Daylight SavingTime, false otherwise.

    If EXPR is omitted, localtime() uses the current time (as returnedby time(3)).

    In scalar context, localtime() returns the ctime(3) value:

    1.  $now_string = localtime;  # e.g., "Thu Oct 13 04:54:34 1994"
      2.

    The format of this scalar value is not locale-dependentbut built into Perl. For GMT instead of localtime use the gmtime builtin. See also theTime::Local module (for converting seconds, minutes, hours, and such back tothe integer value returned by time()), and the POSIX module's strftime(3)and mktime(3) functions.

    To get somewhat similar but locale-dependent date strings, set up yourlocale environment variables appropriately (please see perllocale) andtry for example:

    1.  use POSIX qw(strftime);
      2.  
    2.  $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
      3.  
    3.  # or for GMT formatted appropriately for your locale:
      4.  
    4.  $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
      5.

    Note that the %a and %b, the short forms of the day of the weekand the month of the year, may not necessarily be three characters wide.

    The Time::gmtime and Time::localtime modules provide a convenient,by-name access mechanism to the gmtime() and localtime() functions,respectively.

    For a comprehensive date and time representation look at theDateTime module on CPAN.

    Portability issues: localtime in perlport.

  • lock THING

    This function places an advisory lock on a shared variable or referencedobject contained in THING until the lock goes out of scope.

    The value returned is the scalar itself, if the argument is a scalar, or areference, if the argument is a hash, array or subroutine.

    lock() is a "weak keyword" : this means that if you've defined a functionby this name (before any calls to it), that function will be calledinstead. If you are not under use threads::shared this does nothing.See threads::shared.

  • log EXPR
  • log

    Returns the natural logarithm (base e) of EXPR. If EXPR is omitted,returns the log of $_. To get thelog of another base, use basic algebra:The base-N log of a number is equal to the natural log of that numberdivided by the natural log of N. For example:

    1.  sub log10 {
      2.  
    2.  my $n = shift;
      3.  
    3.  return log($n)/log(10);
      4.  
    4.  }
      5.

    See also exp for the inverse operation.

  • lstat FILEHANDLE
  • lstat EXPR
  • lstat DIRHANDLE
  • lstat

    Does the same thing as the stat function (including setting thespecial _ filehandle) but stats a symbolic link instead of the filethe symbolic link points to. If symbolic links are unimplemented onyour system, a normal stat is done. For much more detailedinformation, please see the documentation for stat.

    If EXPR is omitted, stats $_.

    Portability issues: lstat in perlport.

  • m//

    The match operator. See Regexp Quote-Like Operators in perlop.

  • map BLOCK LIST
  • map EXPR,LIST

    Evaluates the BLOCK or EXPR for each element of LIST (locally setting$_ to each element) and returns the list value composed of theresults of each such evaluation. In scalar context, returns thetotal number of elements so generated. Evaluates BLOCK or EXPR inlist context, so each element of LIST may produce zero, one, ormore elements in the returned value.

    1.  @chars = map(chr, @numbers);
      2.

    translates a list of numbers to the corresponding characters.

    1.  my @squares = map { $_ * $_ } @numbers;
      2.

    translates a list of numbers to their squared values.

    1.  my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
      2.

    shows that number of returned elements can differ from the number ofinput elements. To omit an element, return an empty list ().This could also be achieved by writing

    1.  my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
      2.

    which makes the intention more clear.

    Map always returns a list, which can beassigned to a hash such that the elementsbecome key/value pairs. See perldata for more details.

    1.  %hash = map { get_a_key_for($_) => $_ } @array;
      2.

    is just a funny way to write

    1.  %hash = ();
      2.  
    2.  foreach (@array) {
      3.  
    3.  $hash{get_a_key_for($_)} = $_;
      4.  
    4.  }
      5.

    Note that $_ is an alias to the list value, so it can be used tomodify the elements of the LIST. While this is useful and supported,it can cause bizarre results if the elements of LIST are not variables.Using a regular foreach loop for this purpose would be clearer inmost cases. See also grep for an array composed of those items ofthe original list for which the BLOCK or EXPR evaluates to true.

    If $_ is lexical in the scope where the map appears (because it hasbeen declared with my $_), then, in addition to being locally aliased tothe list elements, $_ keeps being lexical inside the block; that is, itcan't be seen from the outside, avoiding any potential side-effects.

    { starts both hash references and blocks, so map { ... could be eitherthe start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't lookahead for the closing } it has to take a guess at which it's dealing withbased on what it finds just after the{. Usually it gets it right, but if itdoesn't it won't realize something is wrong until it gets to the } andencounters the missing (or unexpected) comma. The syntax error will bereported close to the }, but you'll need to change something near the {such as using a unary + to give Perl some help:

    1.  %hash = map {  "\L$_" => 1  } @array  # perl guesses EXPR.  wrong
      2.  
    2.  %hash = map { +"\L$_" => 1  } @array  # perl guesses BLOCK. right
      3.  
    3.  %hash = map { ("\L$_" => 1) } @array  # this also works
      4.  
    4.  %hash = map {  lc($_) => 1  } @array  # as does this.
      5.  
    5.  %hash = map +( lc($_) => 1 ), @array  # this is EXPR and works!
      6.  
    6.  
    7.  %hash = map  ( lc($_), 1 ),   @array  # evaluates to (1, @array)
      8.

    or to force an anon hash constructor use +{:

    1.    @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
      2.

    to get a list of anonymous hashes each with only one entry apiece.

  • mkdir FILENAME,MASK
  • mkdir FILENAME
  • mkdir

    Creates the directory specified by FILENAME, with permissionsspecified by MASK (as modified by umask). If it succeeds itreturns true; otherwise it returns false and sets $! (errno).MASK defaults to 0777 if omitted, and FILENAME defaultsto $_ if omitted.

    In general, it is better to create directories with a permissive MASKand let the user modify that with their umask than it is to supplya restrictive MASK and give the user no way to be more permissive.The exceptions to this rule are when the file or directory should bekept private (mail files, for instance). The perlfunc(1) entry onumask discusses the choice of MASK in more detail.

    Note that according to the POSIX 1003.1-1996 the FILENAME may have anynumber of trailing slashes. Some operating and filesystems do not getthis right, so Perl automatically removes all trailing slashes to keepeveryone happy.

    To recursively create a directory structure, look atthe mkpath function of the File::Path module.

  • msgctl ID,CMD,ARG

    Calls the System V IPC function msgctl(2). You'll probably have to say

    1.  use IPC::SysV;
      2.

    first to get the correct constant definitions. If CMD is IPC_STAT,then ARG must be a variable that will hold the returned msqid_dsstructure. Returns like ioctl: the undefined value for error,"0 but true" for zero, or the actual return value otherwise. See alsoSysV IPC in perlipc and the documentation for IPC::SysV andIPC::Semaphore.

    Portability issues: msgctl in perlport.

  • msgget KEY,FLAGS

    Calls the System V IPC function msgget(2). Returns the message queueid, or undef on error. See alsoSysV IPC in perlipc and the documentation for IPC::SysV andIPC::Msg.

    Portability issues: msgget in perlport.

  • msgrcv ID,VAR,SIZE,TYPE,FLAGS

    Calls the System V IPC function msgrcv to receive a message frommessage queue ID into variable VAR with a maximum message size ofSIZE. Note that when a message is received, the message type as anative long integer will be the first thing in VAR, followed by theactual message. This packing may be opened with unpack("l! a*").Taints the variable. Returns true if successful, false on error. See also SysV IPC in perlipc and the documentation forIPC::SysV and IPC::SysV::Msg.

    Portability issues: msgrcv in perlport.

  • msgsnd ID,MSG,FLAGS

    Calls the System V IPC function msgsnd to send the message MSG to themessage queue ID. MSG must begin with the native long integer messagetype, be followed by the length of the actual message, and then finallythe message itself. This kind of packing can be achieved withpack("l! a*", $type, $message). Returns true if successful,false on error. See also the IPC::SysVand IPC::SysV::Msg documentation.

    Portability issues: msgsnd in perlport.

  • my EXPR
  • my TYPE EXPR
  • my EXPR : ATTRS
  • my TYPE EXPR : ATTRS

    A my declares the listed variables to be local (lexically) to theenclosing block, file, or eval. If more than one value is listed,the list must be placed in parentheses.

    The exact semantics and interface of TYPE and ATTRS are stillevolving. TYPE is currently bound to the use of the fields pragma,and attributes are handled using the attributes pragma, or startingfrom Perl 5.8.0 also via the Attribute::Handlers module. SeePrivate Variables via my() in perlsub for details, and fields,attributes, and Attribute::Handlers.

  • next LABEL
  • next

    The next command is like the continue statement in C; it startsthe next iteration of the loop:

    1.  LINE: while (<STDIN>) {
      2.  
    2.  next LINE if /^#/;  # discard comments
      3.  
    3.  #...
      4.  
    4.  }
      5.

    Note that if there were a continue block on the above, it would getexecuted even on discarded lines. If LABEL is omitted, the commandrefers to the innermost enclosing loop.

    next cannot be used to exit a block which returns a value such aseval {}, sub {}, or do {}, and should not be used to exita grep() or map() operation.

    Note that a block by itself is semantically identical to a loopthat executes once. Thus next will exit such a block early.

    See also continue for an illustration of how last, next, andredo work.

  • no MODULE VERSION LIST
  • no MODULE VERSION
  • no MODULE LIST
  • no MODULE
  • no VERSION

    See the use function, of which no is the opposite.

  • oct EXPR
  • oct

    Interprets EXPR as an octal string and returns the correspondingvalue. (If EXPR happens to start off with 0x, interprets it as ahex string. If EXPR starts off with 0b, it is interpreted as abinary string. Leading whitespace is ignored in all three cases.)The following will handle decimal, binary, octal, and hex in standardPerl notation:

    1.  $val = oct($val) if $val =~ /^0/;
      2.

    If EXPR is omitted, uses $_. To go the other way (produce a numberin octal), use sprintf() or printf():

    1.  $dec_perms = (stat("filename"))[2] & 07777;
      2.  
    2.  $oct_perm_str = sprintf "%o", $perms;
      3.

    The oct() function is commonly used when a string such as 644 needsto be converted into a file mode, for example. Although Perl automatically converts strings into numbers as needed, this automaticconversion assumes base 10.

    Leading white space is ignored without warning, as too are any trailing non-digits, such as a decimal point (oct only handles non-negativeintegers, not negative integers or floating point).

  • open FILEHANDLE,EXPR
  • open FILEHANDLE,MODE,EXPR
  • open FILEHANDLE,MODE,EXPR,LIST
  • open FILEHANDLE,MODE,REFERENCE
  • open FILEHANDLE

    Opens the file whose filename is given by EXPR, and associates it withFILEHANDLE.

    Simple examples to open a file for reading:

    1.  open(my $fh, "<", "input.txt") 
      2.  
    2. or die "cannot open < input.txt: $!";
      3.

    and for writing:

    1.  open(my $fh, ">", "output.txt") 
      2.  
    2. or die "cannot open > output.txt: $!";
      3.

    (The following is a comprehensive reference to open(): for a gentlerintroduction you may consider perlopentut.)

    If FILEHANDLE is an undefined scalar variable (or array or hash element), anew filehandle is autovivified, meaning that the variable is assigned areference to a newly allocated anonymous filehandle. Otherwise ifFILEHANDLE is an expression, its value is the real filehandle. (This isconsidered a symbolic reference, so use strict "refs" should not bein effect.)

    If EXPR is omitted, the global (package) scalar variable of the samename as the FILEHANDLE contains the filename. (Note that lexical variables--those declared with my or state--will not work for thispurpose; so if you're using my or state, specify EXPR in yourcall to open.)

    If three (or more) arguments are specified, the open mode (includingoptional encoding) in the second argument are distinct from the filename inthe third. If MODE is < or nothing, the file is opened for input.If MODE is >, the file is opened for output, with existing filesfirst being truncated ("clobbered") and nonexisting files newly created.If MODE is >>, the file is opened for appending, again beingcreated if necessary.

    You can put a + in front of the > or < toindicate that you want both read and write access to the file; thus+< is almost always preferred for read/write updates--the +> mode would clobber the file first. You can't usually useeither read-write mode for updating textfiles, since they havevariable-length records. See the -i switch in perlrun for abetter approach. The file is created with permissions of 0666modified by the process's umask value.

    These various prefixes correspond to the fopen(3) modes of r,r+, w, w+, a, and a+.

    In the one- and two-argument forms of the call, the mode and filenameshould be concatenated (in that order), preferably separated by whitespace. You can--but shouldn't--omit the mode in these forms when that modeis <. It is always safe to use the two-argument form of open ifthe filename argument is a known literal.

    For three or more arguments if MODE is |-, the filename isinterpreted as a command to which output is to be piped, and if MODEis -|, the filename is interpreted as a command that pipesoutput to us. In the two-argument (and one-argument) form, one shouldreplace dash (-) with the command.See Using open() for IPC in perlipc for more examples of this.(You are not allowed to open to a command that pipes both in andout, but see IPC::Open2, IPC::Open3, andBidirectional Communication with Another Process in perlipc foralternatives.)

    In the form of pipe opens taking three or more arguments, if LIST is specified(extra arguments after the command name) then LIST becomes argumentsto the command invoked if the platform supports it. The meaning ofopen with more than three arguments for non-pipe modes is not yetdefined, but experimental "layers" may give extra LIST argumentsmeaning.

    In the two-argument (and one-argument) form, opening <- or - opens STDIN and opening >- opens STDOUT.

    You may (and usually should) use the three-argument form of open to specifyI/O layers (sometimes referred to as "disciplines") to apply to the handlethat affect how the input and output are processed (see open andPerlIO for more details). For example:

    1.   open(my $fh, "<:encoding(UTF-8)", "filename")
      2.  
    2.  || die "can't open UTF-8 encoded filename: $!";
      3.

    opens the UTF8-encoded file containing Unicode characters;see perluniintro. Note that if layers are specified in thethree-argument form, then default layers stored in ${^OPEN} (see perlvar;usually set by the open pragma or the switch -CioD) are ignored.Those layers will also be ignored if you specifying a colon with no namefollowing it. In that case the default layer for the operating system(:raw on Unix, :crlf on Windows) is used.

    Open returns nonzero on success, the undefined value otherwise. Ifthe open involved a pipe, the return value happens to be the pid ofthe subprocess.

    If you're running Perl on a system that distinguishes between textfiles and binary files, then you should check out binmode for tipsfor dealing with this. The key distinction between systems that needbinmode and those that don't is their text file formats. Systemslike Unix, Mac OS, and Plan 9, that end lines with a singlecharacter and encode that character in C as "\n" do notneed binmode. The rest need it.

    When opening a file, it's seldom a good idea to continue if the request failed, so open is frequently used withdie. Even if die won't do what you want (say, in a CGI script,where you want to format a suitable error message (but there aremodules that can help with that problem)) always checkthe return value from opening a file.

    As a special case the three-argument form with a read/write mode and the thirdargument being undef:

    1.  open(my $tmp, "+>", undef) or die ...
      2.

    opens a filehandle to an anonymous temporary file. Also using +<works for symmetry, but you really should consider writing somethingto the temporary file first. You will need to seek() to do thereading.

    Since v5.8.0, Perl has built using PerlIO by default. Unless you'vechanged this (such as building Perl with Configure -Uuseperlio), you canopen filehandles directly to Perl scalars via:

    1.  open($fh, ">", \$variable) || ..
      2.

    To (re)open STDOUT or STDERR as an in-memory file, close it first:

    1.  close STDOUT;
      2.  
    2.  open(STDOUT, ">", \$variable)
      3.  
    3. or die "Can't open STDOUT: $!";
      4.

    General examples:

    1.  $ARTICLE = 100;
      2.  
    2.  open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
      3.  
    3.  while (<ARTICLE>) {...
      4.  
    4.  
    5.  open(LOG, ">>/usr/spool/news/twitlog");  # (log is reserved)
      6.  
    6.  # if the open fails, output is discarded
      7.  
    7.  
    8.  open(my $dbase, "+<", "dbase.mine")  # open for update
      9.  
    9.  or die "Can't open 'dbase.mine' for update: $!";
      10.  
    10.  
    11.  open(my $dbase, "+<dbase.mine")  # ditto
      12.  
    12.  or die "Can't open 'dbase.mine' for update: $!";
      13.  
    13.  
    14.  open(ARTICLE, "-|", "caesar <$article")  # decrypt article
      15.  
    15.  or die "Can't start caesar: $!";
      16.  
    16.  
    17.  open(ARTICLE, "caesar <$article |")  # ditto
      18.  
    18.  or die "Can't start caesar: $!";
      19.  
    19.  
    20.  open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
      21.  
    21.  or die "Can't start sort: $!";
      22.  
    22.  
    23.  # in-memory files
      24.  
    24.  open(MEMORY, ">", \$var)
      25.  
    25.  or die "Can't open memory file: $!";
      26.  
    26.  print MEMORY "foo!\n";   # output will appear in $var
      27.  
    27.  
    28.  # process argument list of files along with any includes
      29.  
    29.  
    30.  foreach $file (@ARGV) {
      31.  
    31.  process($file, "fh00");
      32.  
    32.  }
      33.  
    33.  
    34.  sub process {
      35.  
    35.  my($filename, $input) = @_;
      36.  
    36.  $input++; # this is a string increment
      37.  
    37.  unless (open($input, "<", $filename)) {
      38.  
    38.  print STDERR "Can't open $filename: $!\n";
      39.  
    39.  return;
      40.  
    40.  }
      41.  
    41.  
    42.  local $_;
      43.  
    43.  while (<$input>) { # note use of indirection
      44.  
    44.  if (/^#include "(.*)"/) {
      45.  
    45.  process($1, $input);
      46.  
    46.  next;
      47.  
    47.  }
      48.  
    48.  #...  # whatever
      49.  
    49.  }
      50.  
    50.  }
      51.

    See perliol for detailed info on PerlIO.

    You may also, in the Bourne shell tradition, specify an EXPR beginningwith >&, in which case the rest of the string is interpretedas the name of a filehandle (or file descriptor, if numeric) to beduped (as dup(2)) and opened. You may use & after >,>>, <, +>, +>>, and +<.The mode you specify should match the mode of the original filehandle.(Duping a filehandle does not take into account any existing contentsof IO buffers.) If you use the three-argumentform, then you can pass either anumber, the name of a filehandle, or the normal "reference to a glob".

    Here is a script that saves, redirects, and restores STDOUT andSTDERR using various methods:

    1.  #!/usr/bin/perl
      2.  
    2.  open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
      3.  
    3.  open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
      4.  
    4.  
    5.  open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
      6.  
    6.  open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
      7.  
    7.  
    8.  select STDERR; $| = 1;  # make unbuffered
      9.  
    9.  select STDOUT; $| = 1;  # make unbuffered
      10.  
    10.  
    11.  print STDOUT "stdout 1\n";  # this works for
      12.  
    12.  print STDERR "stderr 1\n";  # subprocesses too
      13.  
    13.  
    14.  open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
      15.  
    15.  open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
      16.  
    16.  
    17.  print STDOUT "stdout 2\n";
      18.  
    18.  print STDERR "stderr 2\n";
      19.

    If you specify '<&=X', where X is a file descriptor numberor a filehandle, then Perl will do an equivalent of C's fdopen ofthat file descriptor (and not call dup(2)); this is moreparsimonious of file descriptors. For example:

    1.  # open for input, reusing the fileno of $fd
      2.  
    2.  open(FILEHANDLE, "<&=$fd")
      3.

    or

    1.  open(FILEHANDLE, "<&=", $fd)
      2.

    or

    1.  # open for append, using the fileno of OLDFH
      2.  
    2.  open(FH, ">>&=", OLDFH)
      3.

    or

    1.  open(FH, ">>&=OLDFH")
      2.

    Being parsimonious on filehandles is also useful (besides beingparsimonious) for example when something is dependent on filedescriptors, like for example locking using flock(). If you do justopen(A, ">>&B"), the filehandle A will not have the same filedescriptor as B, and therefore flock(A) will not flock(B) nor viceversa. But with open(A, ">>&=B"), the filehandles will sharethe same underlying system file descriptor.

    Note that under Perls older than 5.8.0, Perl uses the standard C library's'fdopen() to implement the = functionality. On many Unix systems,fdopen() fails when file descriptors exceed a certain value, typically 255.For Perls 5.8.0 and later, PerlIO is (most often) the default.

    You can see whether your Perl was built with PerlIO by running perl -Vand looking for the useperlio= line. If useperlio is define, youhave PerlIO; otherwise you don't.

    If you open a pipe on the command - (that is, specify either |- or -|with the one- or two-argument forms of open), an implicit fork is done, so open returns twice: in the parentprocess it returns the pidof the child process, and in the child process it returns (a defined) 0.Use defined($pid) or // to determine whether the open was successful.

    For example, use either

    1.  $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
      2.

    or $child_pid = open(TO_KID, "|-") // die "can't fork: $!";

    followed by

    1.  if ($child_pid) {
      2.  
    2. # am the parent:
      3.  
    3. # either write TO_KID or else read FROM_KID
      4.  
    4. ...
      5.  
    5. wait $child_pid;
      6.  
    6.  } else {
      7.  
    7. # am the child; use STDIN/STDOUT normally
      8.  
    8. ...
      9.  
    9. exit;
      10.  
    10.  }
      11.

    The filehandle behaves normally for the parent, but I/O to thatfilehandle is piped from/to the STDOUT/STDIN of the child process.In the child process, the filehandle isn't opened--I/O happens from/tothe new STDOUT/STDIN. Typically this is used like the normalpiped open when you want to exercise more control over just how thepipe command gets executed, such as when running setuid andyou don't want to have to scan shell commands for metacharacters.

    The following blocks are more or less equivalent:

    1.  open(FOO, "|tr '[a-z]' '[A-Z]'");
      2.  
    2.  open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
      3.  
    3.  open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
      4.  
    4.  open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
      5.  
    5.  
    6.  open(FOO, "cat -n '$file'|");
      7.  
    7.  open(FOO, "-|", "cat -n '$file'");
      8.  
    8.  open(FOO, "-|") || exec "cat", "-n", $file;
      9.  
    9.  open(FOO, "-|", "cat", "-n", $file);
      10.

    The last two examples in each block show the pipe as "list form", which isnot yet supported on all platforms. A good rule of thumb is that ifyour platform has a real fork() (in other words, if your platform isUnix, including Linux and MacOS X), you can use the list form. You would want to use the list form of the pipe so you can pass literal argumentsto the command without risk of the shell interpreting any shell metacharactersin them. However, this also bars you from opening pipes to commandsthat intentionally contain shell metacharacters, such as:

    1.  open(FOO, "|cat -n | expand -4 | lpr")
      2.  
    2. // die "Can't open pipeline to lpr: $!";
      3.

    See Safe Pipe Opens in perlipc for more examples of this.

    Beginning with v5.6.0, Perl will attempt to flush all files opened foroutput before any operation that may do a fork, but this may not besupported on some platforms (see perlport). To be safe, you may needto set $| ($AUTOFLUSH in English) or call the autoflush() methodof IO::Handle on any open handles.

    On systems that support a close-on-exec flag on files, the flag willbe set for the newly opened file descriptor as determined by the valueof $^F. See $^F in perlvar.

    Closing any piped filehandle causes the parent process to wait for thechild to finish, then returns the status value in $? and${^CHILD_ERROR_NATIVE}.

    The filename passed to the one- and two-argument forms of open() willhave leading and trailing whitespace deleted and normalredirection characters honored. This property, known as "magic open",can often be used to good effect. A user could specify a filename of"rsh cat file |", or you could change certain filenames as needed:

    1.  $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
      2.  
    2.  open(FH, $filename) or die "Can't open $filename: $!";
      3.

    Use the three-argument form to open a file with arbitrary weird characters in it,

    1.  open(FOO, "<", $file)
      2.  
    2. || die "can't open < $file: $!";
      3.

    otherwise it's necessary to protect any leading and trailing whitespace:

    1.  $file =~ s#^(\s)#./$1#;
      2.  
    2.  open(FOO, "< $file\0")
      3.  
    3. || die "open failed: $!";
      4.

    (this may not work on some bizarre filesystems). One shouldconscientiously choose between the magic and three-argument formof open():

    1.  open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
      2.

    will allow the user to specify an argument of the form "rsh cat file |",but will not work on a filename that happens to have a trailing space, while

    1.  open(IN, "<", $ARGV[0])
      2.  
    2. || die "can't open < $ARGV[0]: $!";
      3.

    will have exactly the opposite restrictions.

    If you want a "real" C open (see open(2) on your system), then youshould use the sysopen function, which involves no such magic (but mayuse subtly different filemodes than Perl open(), which is mapped to Cfopen()). This is another way to protect your filenames frominterpretation. For example:

    1.  use IO::Handle;
      2.  
    2.  sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
      3.  
    3.  or die "sysopen $path: $!";
      4.  
    4.  $oldfh = select(HANDLE); $| = 1; select($oldfh);
      5.  
    5.  print HANDLE "stuff $$\n";
      6.  
    6.  seek(HANDLE, 0, 0);
      7.  
    7.  print "File contains: ", <HANDLE>;
      8.

    Using the constructor from the IO::Handle package (or one of itssubclasses, such as IO::File or IO::Socket), you can generate anonymousfilehandles that have the scope of the variables used to hold them, thenautomatically (but silently) close once their reference counts becomezero, typically at scope exit:

    1.  use IO::File;
      2.  
    2.  #...
      3.  
    3.  sub read_myfile_munged {
      4.  
    4.  my $ALL = shift;
      5.  
    5. # or just leave it undef to autoviv
      6.  
    6.  my $handle = IO::File->new;
      7.  
    7.  open($handle, "<", "myfile") or die "myfile: $!";
      8.  
    8.  $first = <$handle>
      9.  
    9.  or return (); # Automatically closed here.
      10.  
    10.  mung($first) or die "mung failed";  # Or here.
      11.  
    11.  return (first, <$handle>) if $ALL;  # Or here.
      12.  
    12.  return $first;  # Or here.
      13.  
    13.  }
      14.

    WARNING: The previous example has a bug because the automaticclose that happens when the refcount on handle does notproperly detect and report failures. Always close the handleyourself and inspect the return value.

    1.  close($handle) 
      2.  
    2. || warn "close failed: $!";
      3.

    See seek for some details about mixing reading and writing.

    Portability issues: open in perlport.

  • opendir DIRHANDLE,EXPR

    Opens a directory named EXPR for processing by readdir, telldir,seekdir, rewinddir, and closedir. Returns true if successful.DIRHANDLE may be an expression whose value can be used as an indirectdirhandle, usually the real dirhandle name. If DIRHANDLE is an undefinedscalar variable (or array or hash element), the variable is assigned areference to a new anonymous dirhandle; that is, it's autovivified.DIRHANDLEs have their own namespace separate from FILEHANDLEs.

    See the example at readdir.

  • ord EXPR
  • ord

    Returns the numeric value of the first character of EXPR.If EXPR is an empty string, returns 0. If EXPR is omitted, uses $_.(Note character, not byte.)

    For the reverse, see chr.See perlunicode for more about Unicode.

  • our EXPR
  • our TYPE EXPR
  • our EXPR : ATTRS
  • our TYPE EXPR : ATTRS

    our associates a simple name with a package variable in the currentpackage for use within the current scope. When use strict 'vars' is ineffect, our lets you use declared global variables without qualifyingthem with package names, within the lexical scope of the our declaration.In this way our differs from use vars, which is package-scoped.

    Unlike my or state, which allocates storage for a variable andassociates a simple name with that storage for use within the currentscope, our associates a simple name with a package (read: global)variable in the current package, for use within the current lexical scope.In other words, our has the same scoping rules as my or state, butdoes not necessarily create a variable.

    If more than one value is listed, the list must be placedin parentheses.

    1.  our $foo;
      2.  
    2.  our($bar, $baz);
      3.

    An our declaration declares a global variable that will be visibleacross its entire lexical scope, even across package boundaries. Thepackage in which the variable is entered is determined at the pointof the declaration, not at the point of use. This means the followingbehavior holds:

    1.  package Foo;
      2.  
    2.  our $bar;  # declares $Foo::bar for rest of lexical scope
      3.  
    3.  $bar = 20;
      4.  
    4.  
    5.  package Bar;
      6.  
    6.  print $bar; # prints 20, as it refers to $Foo::bar
      7.

    Multiple our declarations with the same name in the same lexicalscope are allowed if they are in different packages. If they happento be in the same package, Perl will emit warnings if you have askedfor them, just like multiple my declarations. Unlike a secondmy declaration, which will bind the name to a fresh variable, asecond our declaration in the same package, in the same scope, ismerely redundant.

    1.  use warnings;
      2.  
    2.  package Foo;
      3.  
    3.  our $bar;  # declares $Foo::bar for rest of lexical scope
      4.  
    4.  $bar = 20;
      5.  
    5.  
    6.  package Bar;
      7.  
    7.  our $bar = 30; # declares $Bar::bar for rest of lexical scope
      8.  
    8.  print $bar; # prints 30
      9.  
    9.  
    10.  our $bar;  # emits warning but has no other effect
      11.  
    11.  print $bar; # still prints 30
      12.

    An our declaration may also have a list of attributes associatedwith it.

    The exact semantics and interface of TYPE and ATTRS are stillevolving. TYPE is currently bound to the use of the fields pragma,and attributes are handled using the attributes pragma, or, startingfrom Perl 5.8.0, also via the Attribute::Handlers module. SeePrivate Variables via my() in perlsub for details, and fields,attributes, and Attribute::Handlers.

  • pack TEMPLATE,LIST

    Takes a LIST of values and converts it into a string using the rulesgiven by the TEMPLATE. The resulting string is the concatenation ofthe converted values. Typically, each converted value lookslike its machine-level representation. For example, on 32-bit machinesan integer may be represented by a sequence of 4 bytes, which will inPerl be presented as a string that's 4 characters long.

    See perlpacktut for an introduction to this function.

    The TEMPLATE is a sequence of characters that give the order and typeof values, as follows:

    1.  a  A string with arbitrary binary data, will be null padded.
      2.  
    2.  A  A text (ASCII) string, will be space padded.
      3.  
    3.  Z  A null-terminated (ASCIZ) string, will be null padded.
      4.  
    4.  
    5.  b  A bit string (ascending bit order inside each byte,
      6.  
    6.    like vec()).
      7.  
    7.  B  A bit string (descending bit order inside each byte).
      8.  
    8.  h  A hex string (low nybble first).
      9.  
    9.  H  A hex string (high nybble first).
      10.  
    10.  
    11.  c  A signed char (8-bit) value.
      12.  
    12.  C  An unsigned char (octet) value.
      13.  
    13.  W  An unsigned char value (can be greater than 255).
      14.  
    14.  
    15.  s  A signed short (16-bit) value.
      16.  
    16.   S  An unsigned short value.
      17.  
    17.  
    18.   l  A signed long (32-bit) value.
      19.  
    19.  L  An unsigned long value.
      20.  
    20.  
    21.  q  A signed quad (64-bit) value.
      22.  
    22.   Q  An unsigned quad value.
      23.  
    23.  (Quads are available only if your system supports 64-bit
      24.  
    24.   integer values _and_ if Perl has been compiled to support
      25.  
    25.   those.  Raises an exception otherwise.)
      26.  
    26.  
    27.  i  A signed integer value.
      28.  
    28.  I  A unsigned integer value.
      29.  
    29.  (This 'integer' is _at_least_ 32 bits wide.  Its exact
      30.  
    30.   size depends on what a local C compiler calls 'int'.)
      31.  
    31.  
    32.  n  An unsigned short (16-bit) in "network" (big-endian) order.
      33.  
    33.  N  An unsigned long (32-bit) in "network" (big-endian) order.
      34.  
    34.  v  An unsigned short (16-bit) in "VAX" (little-endian) order.
      35.  
    35.  V  An unsigned long (32-bit) in "VAX" (little-endian) order.
      36.  
    36.  
    37.  j  A Perl internal signed integer value (IV).
      38.  
    38.  J  A Perl internal unsigned integer value (UV).
      39.  
    39.  
    40.  f  A single-precision float in native format.
      41.  
    41.  d  A double-precision float in native format.
      42.  
    42.  
    43.  F  A Perl internal floating-point value (NV) in native format
      44.  
    44.  D  A float of long-double precision in native format.
      45.  
    45.  (Long doubles are available only if your system supports
      46.  
    46.   long double values _and_ if Perl has been compiled to
      47.  
    47.   support those.  Raises an exception otherwise.)
      48.  
    48.  
    49.  p  A pointer to a null-terminated string.
      50.  
    50.  P  A pointer to a structure (fixed-length string).
      51.  
    51.  
    52.  u  A uuencoded string.
      53.  
    53.  U  A Unicode character number.  Encodes to a character in char-
      54.  
    54.    acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
      55.  
    55.    byte mode.
      56.  
    56.  
    57.  w  A BER compressed integer (not an ASN.1 BER, see perlpacktut
      58.  
    58.    for details).  Its bytes represent an unsigned integer in
      59.  
    59.    base 128, most significant digit first, with as few digits
      60.  
    60.    as possible.  Bit eight (the high bit) is set on each byte
      61.  
    61.    except the last.
      62.  
    62.  
    63.  x  A null byte (a.k.a ASCII NUL, "\000", chr(0))
      64.  
    64.  X  Back up a byte.
      65.  
    65.  @  Null-fill or truncate to absolute position, counted from the
      66.  
    66.    start of the innermost ()-group.
      67.  
    67.  .  Null-fill or truncate to absolute position specified by
      68.  
    68.    the value.
      69.  
    69.  (  Start of a ()-group.
      70.

    One or more modifiers below may optionally follow certain letters in theTEMPLATE (the second column lists letters for which the modifier is valid):

    1.  !   sSlLiI Forces native (short, long, int) sizes instead
      2.  
    2.    of fixed (16-/32-bit) sizes.
      3.  
    3.  
    4.   xX Make x and X act as alignment commands.
      5.  
    5.  
    6.   nNvV   Treat integers as signed instead of unsigned.
      7.  
    7.  
    8.   @. Specify position as byte offset in the internal
      9.  
    9.       representation of the packed string.  Efficient
      10.  
    10.       but dangerous.
      11.  
    11.  
    12.   >   sSiIlLqQ   Force big-endian byte-order on the type.
      13.  
    13.   jJfFdDpP   (The "big end" touches the construct.)
      14.  
    14.  
    15.   <   sSiIlLqQ   Force little-endian byte-order on the type.
      16.  
    16.   jJfFdDpP   (The "little end" touches the construct.)
      17.

    The > and < modifiers can also be used on () groups to force a particular byte-order on all components in that group, including all its subgroups.

    The following rules apply:

    • Each letter may optionally be followed by a number indicating the repeatcount. A numeric repeat count may optionally be enclosed in brackets, asin pack("C[80]", @arr). The repeat count gobbles that many values fromthe LIST when used with all format types other than a, A, Z, b,B, h, H, @, ., x, X, and P, where it meanssomething else, described below. Supplying a * for the repeat countinstead of a number means to use however many items are left, except for:

      • @, x, and X, where it is equivalent to 0.

      • <.>, where it means relative to the start of the string.

      • u, where it is equivalent to 1 (or 45, which here is equivalent).

      One can replace a numeric repeat count with a template letter enclosed inbrackets to use the packed byte length of the bracketed template for therepeat count.

      For example, the template x[L] skips as many bytes as in a packed long,and the template "$t X[$t] $t" unpacks twice whatever $t (whenvariable-expanded) unpacks. If the template in brackets contains alignmentcommands (such as x![d]), its packed length is calculated as if thestart of the template had the maximal possible alignment.

      When used with Z, a * as the repeat count is guaranteed to add atrailing null byte, so the resulting string is always one byte longer thanthe byte length of the item itself.

      When used with @, the repeat count represents an offset from the startof the innermost () group.

      When used with ., the repeat count determines the starting position tocalculate the value offset as follows:

      • If the repeat count is 0, it's relative to the current position.

      • If the repeat count is *, the offset is relative to the start of thepacked string.

      • And if it's an integer n, the offset is relative to the start of thenth innermost ( ) group, or to the start of the string if n isbigger then the group level.

      The repeat count for u is interpreted as the maximal number of bytesto encode per line of output, with 0, 1 and 2 replaced by 45. The repeat count should not be more than 65.

    • The a, A, and Z types gobble just one value, but pack it as astring of length count, padding with nulls or spaces as needed. Whenunpacking, A strips trailing whitespace and nulls, Z strips everythingafter the first null, and a returns data with no stripping at all.

      If the value to pack is too long, the result is truncated. If it's toolong and an explicit count is provided, Z packs only $count-1 bytes,followed by a null byte. Thus Z always packs a trailing null, exceptwhen the count is 0.

    • Likewise, the b and B formats pack a string that's that many bits long.Each such format generates 1 bit of the result. These are typically followedby a repeat count like B8 or B64.

      Each result bit is based on the least-significant bit of the correspondinginput character, i.e., on ord($char)%2. In particular, characters "0"and "1" generate bits 0 and 1, as do characters "\000" and "\001".

      Starting from the beginning of the input string, each 8-tupleof characters is converted to 1 character of output. With format b,the first character of the 8-tuple determines the least-significant bit of acharacter; with format B, it determines the most-significant bit ofa character.

      If the length of the input string is not evenly divisible by 8, theremainder is packed as if the input string were padded by null charactersat the end. Similarly during unpacking, "extra" bits are ignored.

      If the input string is longer than needed, remaining characters are ignored.

      A * for the repeat count uses all characters of the input field. On unpacking, bits are converted to a string of 0s and 1s.

    • The h and H formats pack a string that many nybbles (4-bit groups,representable as hexadecimal digits, "0".."9" "a".."f") long.

      For each such format, pack() generates 4 bits of result.With non-alphabetical characters, the result is based on the 4 least-significantbits of the input character, i.e., on ord($char)%16. In particular,characters "0" and "1" generate nybbles 0 and 1, as do bytes"\000" and "\001". For characters "a".."f" and "A".."F", the resultis compatible with the usual hexadecimal digits, so that "a" and"A" both generate the nybble 0xA==10. Use only these specific hex characters with this format.

      Starting from the beginning of the template to pack(), each pairof characters is converted to 1 character of output. With format h, thefirst character of the pair determines the least-significant nybble of theoutput character; with format H, it determines the most-significantnybble.

      If the length of the input string is not even, it behaves as if padded bya null character at the end. Similarly, "extra" nybbles are ignored duringunpacking.

      If the input string is longer than needed, extra characters are ignored.

      A * for the repeat count uses all characters of the input field. Forunpack(), nybbles are converted to a string of hexadecimal digits.

    • The p format packs a pointer to a null-terminated string. You areresponsible for ensuring that the string is not a temporary value, as thatcould potentially get deallocated before you got around to using the packedresult. The P format packs a pointer to a structure of the size indicatedby the length. A null pointer is created if the corresponding value forp or P is undef; similarly with unpack(), where a null pointerunpacks into undef.

      If your system has a strange pointer size--meaning a pointer is neither asbig as an int nor as big as a long--it may not be possible to pack orunpack pointers in big- or little-endian byte order. Attempting to doso raises an exception.

    • The / template character allows packing and unpacking of a sequence ofitems where the packed structure contains a packed item count followed bythe packed items themselves. This is useful when the structure you'reunpacking has encoded the sizes or repeat counts for some of its fieldswithin the structure itself as separate fields.

      For pack, you write length-item/sequence-item, and thelength-item describes how the length value is packed. Formats likelyto be of most use are integer-packing ones like n for Java strings,w for ASN.1 or SNMP, and N for Sun XDR.

      For pack, sequence-item may have a repeat count, in which casethe minimum of that and the number of available items is used as the argumentfor length-item. If it has no repeat count or uses a '*', the numberof available items is used.

      For unpack, an internal stack of integer arguments unpacked so far isused. You write /sequence-item and the repeat count is obtained bypopping off the last element from the stack. The sequence-item must nothave a repeat count.

      If sequence-item refers to a string type ("A", "a", or "Z"),the length-item is the string length, not the number of strings. Withan explicit repeat count for pack, the packed string is adjusted to thatlength. For example:

      1.  This code:  gives this result:
        2.  
      2.  
        3.  
      3.   unpack("W/a", "\004Gurusamy")  ("Guru")
        4.  
      4.   unpack("a3/A A*", "007 Bond  J ")  (" Bond", "J")
        5.  
      5.   unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
        6.  
      6.  
      7.   pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
        8.  
      8.   pack("a/W2", ord("a") .. ord("z")) "2ab"
        9.

      The length-item is not returned explicitly from unpack.

      Supplying a count to the length-item format letter is only useful withA, a, or Z. Packing with a length-item of a or Z mayintroduce "\000" characters, which Perl does not regard as legal innumeric strings.

    • The integer types s, S, l, and L may befollowed by a ! modifier to specify native shorts orlongs. As shown in the example above, a bare l meansexactly 32 bits, although the native long as seen by the local C compilermay be larger. This is mainly an issue on 64-bit platforms. You cansee whether using ! makes any difference this way:

      1.  printf "format s is %d, s! is %d\n", 
        2.  
      2. length pack("s"), length pack("s!");
        3.  
      3.  
      4.  printf "format l is %d, l! is %d\n", 
        5.  
      5. length pack("l"), length pack("l!");
        6.

      i! and I! are also allowed, but only for completeness' sake:they are identical to i and I.

      The actual sizes (in bytes) of native shorts, ints, longs, and longlongs on the platform where Perl was built are also available fromthe command line:

      1.  $ perl -V:{short,int,long{,long}}size
        2.  
      2.  shortsize='2';
        3.  
      3.  intsize='4';
        4.  
      4.  longsize='4';
        5.  
      5.  longlongsize='8';
        6.

      or programmatically via the Config module:

      1.    use Config;
        2.  
      2.    print $Config{shortsize}, "\n";
        3.  
      3.    print $Config{intsize},  "\n";
        4.  
      4.    print $Config{longsize}, "\n";
        5.  
      5.    print $Config{longlongsize}, "\n";
        6.

      $Config{longlongsize} is undefined on systems without long long support.

    • The integer formats s, S, i, I, l, L, j, and J areinherently non-portable between processors and operating systems becausethey obey native byteorder and endianness. For example, a 4-byte integer0x12345678 (305419896 decimal) would be ordered natively (arranged in andhandled by the CPU registers) into bytes as

      1.  0x12 0x34 0x56 0x78  # big-endian
        2.  
      2.  0x78 0x56 0x34 0x12  # little-endian
        3.

      Basically, Intel and VAX CPUs are little-endian, while everybody else,including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, arebig-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used) them in little-endian mode, but SGI/Cray uses them in big-endian mode.

      The names big-endian and little-endian are comic references to theegg-eating habits of the little-endian Lilliputians and the big-endianBlefuscudians from the classic Jonathan Swift satire, Gulliver's Travels.This entered computer lingo via the paper "On Holy Wars and a Plea forPeace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.

      Some systems may have even weirder byte orders such as

      1.    0x56 0x78 0x12 0x34
        2.  
      2.    0x34 0x12 0x78 0x56
        3.

      You can determine your system endianness with this incantation:

      1.    printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
        2.

      The byteorder on the platform where Perl was built is also availablevia Config:

      1.  use Config;
        2.  
      2.  print "$Config{byteorder}\n";
        3.

      or from the command line:

      1.  $ perl -V:byteorder
        2.

      Byteorders "1234" and "12345678" are little-endian; "4321"and "87654321" are big-endian.

      For portably packed integers, either use the formats n, N, v, and V or else use the > and < modifiers describedimmediately below. See also perlport.

    • Starting with Perl 5.9.2, integer and floating-point formats, along withthe p and P formats and () groups, may all be followed by the > or < endianness modifiers to respectively enforce big-or little-endian byte-order. These modifiers are especially useful given how n, N, v, and V don't cover signed integers, 64-bit integers, or floating-point values.

      Here are some concerns to keep in mind when using an endianness modifier:

      • Exchanging signed integers between different platforms works only when all platforms store them in the same format. Most platforms storesigned integers in two's-complement notation, so usually this is not an issue.

      • The > or < modifiers can only be used on floating-pointformats on big- or little-endian machines. Otherwise, attempting touse them raises an exception.

      • Forcing big- or little-endian byte-order on floating-point values fordata exchange can work only if all platforms use the samebinary representation such as IEEE floating-point. Even if allplatforms are using IEEE, there may still be subtle differences. Being ableto use > or < on floating-point values can be useful,but also dangerous if you don't know exactly what you're doing.It is not a general way to portably store floating-point values.

      • When using > or < on a () group, this affectsall types inside the group that accept byte-order modifiers,including all subgroups. It is silently ignored for all othertypes. You are not allowed to override the byte-order within a groupthat already has a byte-order modifier suffix.

    • Real numbers (floats and doubles) are in native machine format only.Due to the multiplicity of floating-point formats and the lack of astandard "network" representation for them, no facility for interchange has beenmade. This means that packed floating-point data written on one machinemay not be readable on another, even if both use IEEE floating-pointarithmetic (because the endianness of the memory representation is not partof the IEEE spec). See also perlport.

      If you know exactly what you're doing, you can use the > or <modifiers to force big- or little-endian byte-order on floating-point values.

      Because Perl uses doubles (or long doubles, if configured) internally forall numeric calculation, converting from double into float and thence to double again loses precision, so unpack("f", pack("f", $foo))will not in general equal $foo.

    • Pack and unpack can operate in two modes: character mode (C0 mode) wherethe packed string is processed per character, and UTF-8 mode (U0 mode)where the packed string is processed in its UTF-8-encoded Unicode form ona byte-by-byte basis. Character mode is the defaultunless the format string starts with U. Youcan always switch mode mid-format with an explicit C0 or U0 in the format. This mode remains in effect until the next mode change, or until the end of the () group it (directly) applies to.

      Using C0 to get Unicode characters while using U0 to get non-Unicode bytes is not necessarily obvious. Probably only the first of theseis what you want:

      1.  $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | 
        2.  
      2.   perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
        3.  
      3.  03B1.03C9
        4.  
      4.  $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | 
        5.  
      5.   perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
        6.  
      6.  CE.B1.CF.89
        7.  
      7.  $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | 
        8.  
      8.   perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
        9.  
      9.  CE.B1.CF.89
        10.  
      10.  $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | 
        11.  
      11.   perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
        12.  
      12.  C3.8E.C2.B1.C3.8F.C2.89
        13.

      Those examples also illustrate that you should not try to usepack/unpack as a substitute for the Encode module.

    • You must yourself do any alignment or padding by inserting, for example,enough "x"es while packing. There is no way for pack() and unpack()to know where characters are going to or coming from, so they handle their output and input as flat sequences of characters.

    • A () group is a sub-TEMPLATE enclosed in parentheses. A group maytake a repeat count either as postfix, or for unpack(), also via the /template character. Within each repetition of a group, positioning with@ starts over at 0. Therefore, the result of

      1.  pack("@1A((@2A)@3A)", qw[X Y Z])
        2.

      is the string "\0X\0\0YZ".

    • x and X accept the ! modifier to act as alignment commands: theyjump forward or back to the closest position aligned at a multiple of countcharacters. For example, to pack() or unpack() a C structure like

      1.  struct {
        2.  
      2. char   c; /* one signed, 8-bit character */
        3.  
      3. double d; 
        4.  
      4. char   cc[2];
        5.  
      5.  }
        6.

      one may need to use the template c x![d] d c[2]. This assumes thatdoubles must be aligned to the size of double.

      For alignment commands, a count of 0 is equivalent to a count of 1;both are no-ops.

    • n, N, v and V accept the ! modifier torepresent signed 16-/32-bit integers in big-/little-endian order.This is portable only when all platforms sharing packed data use thesame binary representation for signed integers; for example, when allplatforms use two's-complement representation.

    • Comments can be embedded in a TEMPLATE using # through the end of line.White space can separate pack codes from each other, but modifiers andrepeat counts must follow immediately. Breaking complex templates intoindividual line-by-line components, suitably annotated, can do as much toimprove legibility and maintainability of pack/unpack formats as /x canfor complicated pattern matches.

    • If TEMPLATE requires more arguments than pack() is given, pack()assumes additional "" arguments. If TEMPLATE requires fewer argumentsthan given, extra arguments are ignored.

    Examples:

    1.  $foo = pack("WWWW",65,66,67,68);
      2.  
    2.  # foo eq "ABCD"
      3.  
    3.  $foo = pack("W4",65,66,67,68);
      4.  
    4.  # same thing
      5.  
    5.  $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
      6.  
    6.  # same thing with Unicode circled letters.
      7.  
    7.  $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
      8.  
    8.  # same thing with Unicode circled letters.  You don't get the
      9.  
    9.  # UTF-8 bytes because the U at the start of the format caused
      10.  
    10.  # a switch to U0-mode, so the UTF-8 bytes get joined into
      11.  
    11.  # characters
      12.  
    12.  $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
      13.  
    13.  # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
      14.  
    14.  # This is the UTF-8 encoding of the string in the
      15.  
    15.  # previous example
      16.  
    16.  
    17.  $foo = pack("ccxxcc",65,66,67,68);
      18.  
    18.  # foo eq "AB\0\0CD"
      19.  
    19.  
    20.  # NOTE: The examples above featuring "W" and "c" are true
      21.  
    21.  # only on ASCII and ASCII-derived systems such as ISO Latin 1
      22.  
    22.  # and UTF-8.  On EBCDIC systems, the first example would be
      23.  
    23.  #  $foo = pack("WWWW",193,194,195,196);
      24.  
    24.  
    25.  $foo = pack("s2",1,2);
      26.  
    26.  # "\001\000\002\000" on little-endian
      27.  
    27.  # "\000\001\000\002" on big-endian
      28.  
    28.  
    29.  $foo = pack("a4","abcd","x","y","z");
      30.  
    30.  # "abcd"
      31.  
    31.  
    32.  $foo = pack("aaaa","abcd","x","y","z");
      33.  
    33.  # "axyz"
      34.  
    34.  
    35.  $foo = pack("a14","abcdefg");
      36.  
    36.  # "abcdefg\0\0\0\0\0\0\0"
      37.  
    37.  
    38.  $foo = pack("i9pl", gmtime);
      39.  
    39.  # a real struct tm (on my system anyway)
      40.  
    40.  
    41.  $utmp_template = "Z8 Z8 Z16 L";
      42.  
    42.  $utmp = pack($utmp_template, @utmp1);
      43.  
    43.  # a struct utmp (BSDish)
      44.  
    44.  
    45.  @utmp2 = unpack($utmp_template, $utmp);
      46.  
    46.  # "@utmp1" eq "@utmp2"
      47.  
    47.  
    48.  sub bintodec {
      49.  
    49.  unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
      50.  
    50.  }
      51.  
    51.  
    52.  $foo = pack('sx2l', 12, 34);
      53.  
    53.  # short 12, two zero bytes padding, long 34
      54.  
    54.  $bar = pack('s@4l', 12, 34);
      55.  
    55.  # short 12, zero fill to position 4, long 34
      56.  
    56.  # $foo eq $bar
      57.  
    57.  $baz = pack('s.l', 12, 4, 34);
      58.  
    58.  # short 12, zero fill to position 4, long 34
      59.  
    59.  
    60.  $foo = pack('nN', 42, 4711);
      61.  
    61.  # pack big-endian 16- and 32-bit unsigned integers
      62.  
    62.  $foo = pack('S>L>', 42, 4711);
      63.  
    63.  # exactly the same
      64.  
    64.  $foo = pack('s<l<', -42, 4711);
      65.  
    65.  # pack little-endian 16- and 32-bit signed integers
      66.  
    66.  $foo = pack('(sl)<', -42, 4711);
      67.  
    67.  # exactly the same
      68.

    The same template may generally also be used in unpack().

  • package NAMESPACE
  • package NAMESPACE VERSION
  • package NAMESPACE BLOCK
  • package NAMESPACE VERSION BLOCK

    Declares the BLOCK or the rest of the compilation unit as being in thegiven namespace. The scope of the package declaration is either thesupplied code BLOCK or, in the absence of a BLOCK, from the declarationitself through the end of current scope (the enclosing block, file, oreval). That is, the forms without a BLOCK are operative through the endof the current scope, just like the my, state, and our operators.All unqualified dynamic identifiers in this scope will be in the givennamespace, except where overridden by another package declaration orwhen they're one of the special identifiers that qualify into main::,like STDOUT, ARGV, ENV, and the punctuation variables.

    A package statement affects dynamic variables only, including thoseyou've used local on, but not lexical variables, which are createdwith my, state, or our. Typically it would be the first declaration in a file included by require or use. You can switch into apackage in more than one place, since this only determines which default symbol table the compiler uses for the rest of that block. You can refer toidentifiers in other packages than the current one by prefixing the identifierwith the package name and a double colon, as in $SomePack::varor ThatPack::INPUT_HANDLE. If package name is omitted, the mainpackage as assumed. That is, $::sail is equivalent to$main::sail (as well as to $main'sail, still seen in ancientcode, mostly from Perl 4).

    If VERSION is provided, package sets the $VERSION variable in the givennamespace to a version object with the VERSION provided. VERSION must be a"strict" style version number as defined by the version module: a positivedecimal number (integer or decimal-fraction) without exponentiation or else adotted-decimal v-string with a leading 'v' character and at least threecomponents. You should set $VERSION only once per package.

    See Packages in perlmod for more information about packages, modules,and classes. See perlsub for other scoping issues.

  • __PACKAGE__

    A special token that returns the name of the package in which it occurs.

  • pipe READHANDLE,WRITEHANDLE

    Opens a pair of connected pipes like the corresponding system call.Note that if you set up a loop of piped processes, deadlock can occurunless you are very careful. In addition, note that Perl's pipes useIO buffering, so you may need to set $| to flush your WRITEHANDLEafter each command, depending on the application.

    See IPC::Open2, IPC::Open3, andBidirectional Communication with Another Process in perlipcfor examples of such things.

    On systems that support a close-on-exec flag on files, that flag is seton all newly opened file descriptors whose filenos are higher than the current value of $^F (by default 2 for STDERR). See $^F in perlvar.

  • pop ARRAY
  • pop EXPR
  • pop

    Pops and returns the last value of the array, shortening the array byone element.

    Returns the undefined value if the array is empty, although this may alsohappen at other times. If ARRAY is omitted, pops the @ARGV array in themain program, but the @_ array in subroutines, just like shift.

    Starting with Perl 5.14, pop can take a scalar EXPR, which must hold areference to an unblessed array. The argument will be dereferencedautomatically. This aspect of pop is considered highly experimental.The exact behaviour may change in a future version of Perl.

    To avoid confusing would-be users of your code who are running earlierversions of Perl with mysterious syntax errors, put this sort of thing atthe top of your file to signal that your code will work only on Perls ofa recent vintage:

    1.  use 5.014;# so push/pop/etc work on scalars (experimental)
      2.
  • pos SCALAR
  • pos

    Returns the offset of where the last m//g search left off for thevariable in question ($_ is used when the variable is notspecified). Note that 0 is a valid match offset. undef indicatesthat the search position is reset (usually due to match failure, butcan also be because no match has yet been run on the scalar).

    pos directly accesses the location used by the regexp engine tostore the offset, so assigning to pos will change that offset, andso will also influence the \G zero-width assertion in regularexpressions. Both of these effects take place for the next match, soyou can't affect the position with pos during the current match,such as in (?{pos() = 5}) or s//pos() = 5/e.

    Setting pos also resets the matched with zero-length flag, describedunder Repeated Patterns Matching a Zero-length Substring in perlre.

    Because a failed m//gc match doesn't reset the offset, the returnfrom pos won't change either in this case. See perlre andperlop.

  • print FILEHANDLE LIST
  • print FILEHANDLE
  • print LIST
  • print

    Prints a string or a list of strings. Returns true if successful.FILEHANDLE may be a scalar variable containing the name of or a referenceto the filehandle, thus introducing one level of indirection. (NOTE: IfFILEHANDLE is a variable and the next token is a term, it may bemisinterpreted as an operator unless you interpose a + or putparentheses around the arguments.) If FILEHANDLE is omitted, prints to thelast selected (see select) output handle. If LIST is omitted, prints$_ to the currently selected output handle. To use FILEHANDLE alone toprint the content of $_ to it, you must use a real filehandle likeFH, not an indirect one like $fh. To set the default output handleto something other than STDOUT, use the select operation.

    The current value of $, (if any) is printed between each LIST item. Thecurrent value of $\ (if any) is printed after the entire LIST has beenprinted. Because print takes a LIST, anything in the LIST is evaluated inlist context, including any subroutines whose return lists you pass toprint. Be careful not to follow the print keyword with a leftparenthesis unless you want the corresponding right parenthesis toterminate the arguments to the print; put parentheses around all arguments(or interpose a +, but that doesn't look as good).

    If you're storing handles in an array or hash, or in general wheneveryou're using any expression more complex than a bareword handle or a plain,unsubscripted scalar variable to retrieve it, you will have to use a blockreturning the filehandle value instead, in which case the LIST may not beomitted:

    1.  print { $files[$i] } "stuff\n";
      2.  
    2.  print { $OK ? STDOUT : STDERR } "stuff\n";
      3.

    Printing to a closed pipe or socket will generate a SIGPIPE signal. Seeperlipc for more on signal handling.

  • printf FILEHANDLE FORMAT, LIST
  • printf FILEHANDLE
  • printf FORMAT, LIST
  • printf

    Equivalent to print FILEHANDLE sprintf(FORMAT, LIST), except that $\(the output record separator) is not appended. The first argument of thelist will be interpreted as the printf format. Seesprintf for anexplanation of the format argument. If you omit the LIST, $_ is used;to use FILEHANDLE without a LIST, you must use a real filehandle likeFH, not an indirect one like $fh. If use locale (includinguse locale ':not_characters') is in effect andPOSIX::setlocale() has been called, the character used for the decimalseparator in formatted floating-point numbers is affected by the LC_NUMERIClocale setting. See perllocale and POSIX.

    Don't fall into the trap of using a printf when a simpleprint would do. The print is more efficient and lesserror prone.

  • prototype FUNCTION

    Returns the prototype of a function as a string (or undef if thefunction has no prototype). FUNCTION is a reference to, or the name of,the function whose prototype you want to retrieve.

    If FUNCTION is a string starting with CORE::, the rest is taken as aname for a Perl builtin. If the builtin is not overridable (such asqw//) or if its arguments cannot be adequately expressed by a prototype(such as system), prototype() returns undef, because the builtindoes not really behave like a Perl function. Otherwise, the stringdescribing the equivalent prototype is returned.

  • push ARRAY,LIST
  • push EXPR,LIST

    Treats ARRAY as a stack by appending the values of LIST to the end ofARRAY. The length of ARRAY increases by the length of LIST. Has the sameeffect as

    1.  for $value (LIST) {
      2.  
    2.  $ARRAY[++$#ARRAY] = $value;
      3.  
    3.  }
      4.

    but is more efficient. Returns the number of elements in the array followingthe completed push.

    Starting with Perl 5.14, push can take a scalar EXPR, which must hold areference to an unblessed array. The argument will be dereferencedautomatically. This aspect of push is considered highly experimental.The exact behaviour may change in a future version of Perl.

    To avoid confusing would-be users of your code who are running earlierversions of Perl with mysterious syntax errors, put this sort of thing atthe top of your file to signal that your code will work only on Perls ofa recent vintage:

    1.  use 5.014;# so push/pop/etc work on scalars (experimental)
      2.
  • q/STRING/
  • qq/STRING/
  • qw/STRING/
  • qx/STRING/

    Generalized quotes. See Quote-Like Operators in perlop.

  • qr/STRING/

    Regexp-like quote. See Regexp Quote-Like Operators in perlop.

  • quotemeta EXPR
  • quotemeta

    Returns the value of EXPR with all the ASCII non-"word"characters backslashed. (That is, all ASCII characters not matching/[A-Za-z_0-9]/ will be preceded by a backslash in thereturned string, regardless of any locale settings.)This is the internal function implementingthe \Q escape in double-quoted strings.(See below for the behavior on non-ASCII code points.)

    If EXPR is omitted, uses $_.

    quotemeta (and \Q ... \E) are useful when interpolating strings intoregular expressions, because by default an interpolated variable will beconsidered a mini-regular expression. For example:

    1.  my $sentence = 'The quick brown fox jumped over the lazy dog';
      2.  
    2.  my $substring = 'quick.*?fox';
      3.  
    3.  $sentence =~ s{$substring}{big bad wolf};
      4.

    Will cause $sentence to become 'The big bad wolf jumped over...'.

    On the other hand:

    1.  my $sentence = 'The quick brown fox jumped over the lazy dog';
      2.  
    2.  my $substring = 'quick.*?fox';
      3.  
    3.  $sentence =~ s{\Q$substring\E}{big bad wolf};
      4.

    Or:

    1.  my $sentence = 'The quick brown fox jumped over the lazy dog';
      2.  
    2.  my $substring = 'quick.*?fox';
      3.  
    3.  my $quoted_substring = quotemeta($substring);
      4.  
    4.  $sentence =~ s{$quoted_substring}{big bad wolf};
      5.

    Will both leave the sentence as is.Normally, when accepting literal stringinput from the user, quotemeta() or \Q must be used.

    In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encodedstrings, but not quoted in UTF-8 strings.

    Starting in Perl v5.16, Perl adopted a Unicode-defined strategy forquoting non-ASCII characters; the quoting of ASCII characters isunchanged.

    Also unchanged is the quoting of non-UTF-8 strings when outside thescope of a use feature 'unicode_strings', which is to quote allcharacters in the upper Latin1 range. This provides complete backwardscompatibility for old programs which do not use Unicode. (Note thatunicode_strings is automatically enabled within the scope of ause v5.12 or greater.)

    Within the scope of use locale, all non-ASCII Latin1 code pointsare quoted whether the string is encoded as UTF-8 or not. As mentionedabove, locale does not affect the quoting of ASCII-range characters.This protects against those locales where characters such as "|" areconsidered to be word characters.

    Otherwise, Perl quotes non-ASCII characters using an adaptation fromUnicode (see http://www.unicode.org/reports/tr31/.)The only code points that are quoted are those that have any of theUnicode properties: Pattern_Syntax, Pattern_White_Space, White_Space,Default_Ignorable_Code_Point, or General_Category=Control.

    Of these properties, the two important ones are Pattern_Syntax andPattern_White_Space. They have been set up by Unicode for exactly thispurpose of deciding which characters in a regular expression patternshould be quoted. No character that can be in an identifier has theseproperties.

    Perl promises, that if we ever add regular expression patternmetacharacters to the dozen already defined(\ | ( ) [ { ^ $ * + ? .), that we will only use ones that have thePattern_Syntax property. Perl also promises, that if we ever addcharacters that are considered to be white space in regular expressions(currently mostly affected by /x), they will all have thePattern_White_Space property.

    Unicode promises that the set of code points that have these twoproperties will never change, so something that is not quoted in v5.16will never need to be quoted in any future Perl release. (Not all thecode points that match Pattern_Syntax have actually had charactersassigned to them; so there is room to grow, but they are quotedwhether assigned or not. Perl, of course, would never use anunassigned code point as an actual metacharacter.)

    Quoting characters that have the other 3 properties is done to enhancethe readability of the regular expression and not because they actuallyneed to be quoted for regular expression purposes (characters with theWhite_Space property are likely to be indistinguishable on the page orscreen from those with the Pattern_White_Space property; and the othertwo properties contain non-printing characters).

  • rand EXPR
  • rand

    Returns a random fractional number greater than or equal to 0 and lessthan the value of EXPR. (EXPR should be positive.) If EXPR isomitted, the value 1 is used. Currently EXPR with the value 0 isalso special-cased as 1 (this was undocumented before Perl 5.8.0and is subject to change in future versions of Perl). Automatically callssrand unless srand has already been called. See also srand.

    Apply int() to the value returned by rand() if you want randomintegers instead of random fractional numbers. For example,

    1.  int(rand(10))
      2.

    returns a random integer between 0 and 9, inclusive.

    (Note: If your rand function consistently returns numbers that are toolarge or too small, then your version of Perl was probably compiledwith the wrong number of RANDBITS.)

    rand() is not cryptographically secure. You should not relyon it in security-sensitive situations. As of this writing, anumber of third-party CPAN modules offer random number generatorsintended by their authors to be cryptographically secure,including: Data::Entropy, Crypt::Random, Math::Random::Secure,and Math::TrulyRandom.

  • read FILEHANDLE,SCALAR,LENGTH,OFFSET
  • read FILEHANDLE,SCALAR,LENGTH

    Attempts to read LENGTH characters of data into variable SCALARfrom the specified FILEHANDLE. Returns the number of charactersactually read, 0 at end of file, or undef if there was an error (inthe latter case $! is also set). SCALAR will be grown or shrunk so that the last character actually read is the last character of thescalar after the read.

    An OFFSET may be specified to place the read data at some place in thestring other than the beginning. A negative OFFSET specifiesplacement at that many characters counting backwards from the end ofthe string. A positive OFFSET greater than the length of SCALARresults in the string being padded to the required size with "\0"bytes before the result of the read is appended.

    The call is implemented in terms of either Perl's or your system's nativefread(3) library function. To get a true read(2) system call, seesysread.

    Note the characters: depending on the status of the filehandle,either (8-bit) bytes or characters are read. By default, allfilehandles operate on bytes, but for example if the filehandle hasbeen opened with the :utf8 I/O layer (see open, and the openpragma, open), the I/O will operate on UTF8-encoded Unicodecharacters, not bytes. Similarly for the :encoding pragma:in that case pretty much any characters can be read.

  • readdir DIRHANDLE

    Returns the next directory entry for a directory opened by opendir.If used in list context, returns all the rest of the entries in thedirectory. If there are no more entries, returns the undefined value inscalar context and the empty list in list context.

    If you're planning to filetest the return values out of a readdir, you'dbetter prepend the directory in question. Otherwise, because we didn'tchdir there, it would have been testing the wrong file.

    1.  opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
      2.  
    2.  @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
      3.  
    3.  closedir $dh;
      4.

    As of Perl 5.11.2 you can use a bare readdir in a while loop,which will set $_ on every iteration.

    1.  opendir(my $dh, $some_dir) || die;
      2.  
    2.  while(readdir $dh) {
      3.  
    3.  print "$some_dir/$_\n";
      4.  
    4.  }
      5.  
    5.  closedir $dh;
      6.

    To avoid confusing would-be users of your code who are running earlierversions of Perl with mysterious failures, put this sort of thing at thetop of your file to signal that your code will work only on Perls of arecent vintage:

    1.  use 5.012; # so readdir assigns to $_ in a lone while test
      2.
  • readline EXPR
  • readline

    Reads from the filehandle whose typeglob is contained in EXPR (or from*ARGV if EXPR is not provided). In scalar context, each call reads andreturns the next line until end-of-file is reached, whereupon thesubsequent call returns undef. In list context, reads until end-of-fileis reached and returns a list of lines. Note that the notion of "line"used here is whatever you may have defined with $/ or$INPUT_RECORD_SEPARATOR). See $/ in perlvar.

    When $/ is set to undef, when readline is in scalarcontext (i.e., file slurp mode), and when an empty file is read, itreturns '' the first time, followed by undef subsequently.

    This is the internal function implementing the <EXPR>operator, but you can use it directly. The <EXPR>operator is discussed in more detail in I/O Operators in perlop.

    1.  $line = <STDIN>;
      2.  
    2.  $line = readline(*STDIN); # same thing
      3.

    If readline encounters an operating system error, $! will be setwith the corresponding error message. It can be helpful to check$! when you are reading from filehandles you don't trust, such as atty or a socket. The following example uses the operator form ofreadline and dies if the result is not defined.

    1.  while ( ! eof($fh) ) {
      2.  
    2.  defined( $_ = <$fh> ) or die "readline failed: $!";
      3.  
    3.  ...
      4.  
    4.  }
      5.

    Note that you have can't handle readline errors that way with theARGV filehandle. In that case, you have to open each element of@ARGV yourself since eof handles ARGV differently.

    1.  foreach my $arg (@ARGV) {
      2.  
    2.  open(my $fh, $arg) or warn "Can't open $arg: $!";
      3.  
    3.  
    4.  while ( ! eof($fh) ) {
      5.  
    5.  defined( $_ = <$fh> )
      6.  
    6.  or die "readline failed for $arg: $!";
      7.  
    7.  ...
      8.  
    8.  }
      9.  
    9.  }
      10.
  • readlink EXPR
  • readlink

    Returns the value of a symbolic link, if symbolic links areimplemented. If not, raises an exception. If there is a systemerror, returns the undefined value and sets $! (errno). If EXPR isomitted, uses $_.

    Portability issues: readlink in perlport.

  • readpipe EXPR
  • readpipe

    EXPR is executed as a system command.The collected standard output of the command is returned.In scalar context, it comes back as a single (potentiallymulti-line) string. In list context, returns a list of lines(however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).This is the internal function implementing the qx/EXPR/operator, but you can use it directly. The qx/EXPR/operator is discussed in more detail in I/O Operators in perlop.If EXPR is omitted, uses $_.

  • recv SOCKET,SCALAR,LENGTH,FLAGS

    Receives a message on a socket. Attempts to receive LENGTH charactersof data into variable SCALAR from the specified SOCKET filehandle.SCALAR will be grown or shrunk to the length actually read. Takes thesame flags as the system call of the same name. Returns the addressof the sender if SOCKET's protocol supports this; returns an emptystring otherwise. If there's an error, returns the undefined value.This call is actually implemented in terms of recvfrom(2) system call.See UDP: Message Passing in perlipc for examples.

    Note the characters: depending on the status of the socket, either(8-bit) bytes or characters are received. By default all socketsoperate on bytes, but for example if the socket has been changed usingbinmode() to operate with the :encoding(utf8) I/O layer (see theopen pragma, open), the I/O will operate on UTF8-encoded Unicodecharacters, not bytes. Similarly for the :encoding pragma: in thatcase pretty much any characters can be read.

  • redo LABEL
  • redo

    The redo command restarts the loop block without evaluating theconditional again. The continue block, if any, is not executed. Ifthe LABEL is omitted, the command refers to the innermost enclosingloop. Programs that want to lie to themselves about what was just input normally use this command:

    1.  # a simpleminded Pascal comment stripper
      2.  
    2.  # (warning: assumes no { or } in strings)
      3.  
    3.  LINE: while (<STDIN>) {
      4.  
    4.  while (s|({.*}.*){.*}|$1 |) {}
      5.  
    5.  s|{.*}| |;
      6.  
    6.  if (s|{.*| |) {
      7.  
    7.  $front = $_;
      8.  
    8.  while (<STDIN>) {
      9.  
    9.  if (/}/) {  # end of comment?
      10.  
    10.  s|^|$front\{|;
      11.  
    11.  redo LINE;
      12.  
    12.  }
      13.  
    13.  }
      14.  
    14.  }
      15.  
    15.  print;
      16.  
    16.  }
      17.

    redo cannot be used to retry a block that returns a value such aseval {}, sub {}, or do {}, and should not be used to exita grep() or map() operation.

    Note that a block by itself is semantically identical to a loopthat executes once. Thus redo inside such a block will effectivelyturn it into a looping construct.

    See also continue for an illustration of how last, next, andredo work.

  • ref EXPR
  • ref

    Returns a non-empty string if EXPR is a reference, the emptystring otherwise. If EXPRis not specified, $_ will be used. The value returned depends on thetype of thing the reference is a reference to.Builtin types include:

    1.  SCALAR
      2.  
    2.  ARRAY
      3.  
    3.  HASH
      4.  
    4.  CODE
      5.  
    5.  REF
      6.  
    6.  GLOB
      7.  
    7.  LVALUE
      8.  
    8.  FORMAT
      9.  
    9.  IO
      10.  
    10.  VSTRING
      11.  
    11.  Regexp
      12.

    If the referenced object has been blessed into a package, then that packagename is returned instead. You can think of ref as a typeof operator.

    1.  if (ref($r) eq "HASH") {
      2.  
    2.  print "r is a reference to a hash.\n";
      3.  
    3.  }
      4.  
    4.  unless (ref($r)) {
      5.  
    5.  print "r is not a reference at all.\n";
      6.  
    6.  }
      7.

    The return value LVALUE indicates a reference to an lvalue that is nota variable. You get this from taking the reference of function calls likepos() or substr(). VSTRING is returned if the reference pointsto a version string.

    The result Regexp indicates that the argument is a regular expressionresulting from qr//.

    See also perlref.

  • rename OLDNAME,NEWNAME

    Changes the name of a file; an existing file NEWNAME will beclobbered. Returns true for success, false otherwise.

    Behavior of this function varies wildly depending on your systemimplementation. For example, it will usually not work across file systemboundaries, even though the system mv command sometimes compensatesfor this. Other restrictions include whether it works on directories,open files, or pre-existing files. Check perlport and either therename(2) manpage or equivalent system documentation for details.

    For a platform independent move function look at the File::Copymodule.

    Portability issues: rename in perlport.

  • require VERSION
  • require EXPR
  • require

    Demands a version of Perl specified by VERSION, or demands some semanticsspecified by EXPR or by $_ if EXPR is not supplied.

    VERSION may be either a numeric argument such as 5.006, which will becompared to $], or a literal of the form v5.6.1, which will be comparedto $^V (aka $PERL_VERSION). An exception is raised ifVERSION is greater than the version of the current Perl interpreter.Compare with use, which can do a similar check at compile time.

    Specifying VERSION as a literal of the form v5.6.1 should generally beavoided, because it leads to misleading error messages under earlierversions of Perl that do not support this syntax. The equivalent numericversion should be used instead.

    1.  require v5.6.1; # run time version check
      2.  
    2.  require 5.6.1;  # ditto
      3.  
    3.  require 5.006_001;  # ditto; preferred for backwards compatibility
      4.

    Otherwise, require demands that a library file be included if ithasn't already been included. The file is included via the do-FILEmechanism, which is essentially just a variety of eval with thecaveat that lexical variables in the invoking script will be invisibleto the included code. Has semantics similar to the following subroutine:

    1.  sub require {
      2.  
    2.    my ($filename) = @_;
      3.  
    3.    if (exists $INC{$filename}) {
      4.  
    4.    return 1 if $INC{$filename};
      5.  
    5.    die "Compilation failed in require";
      6.  
    6.    }
      7.  
    7.    my ($realfilename,$result);
      8.  
    8.    ITER: {
      9.  
    9.    foreach $prefix (@INC) {
      10.  
    10.    $realfilename = "$prefix/$filename";
      11.  
    11.    if (-f $realfilename) {
      12.  
    12.    $INC{$filename} = $realfilename;
      13.  
    13.    $result = do $realfilename;
      14.  
    14.    last ITER;
      15.  
    15.    }
      16.  
    16.    }
      17.  
    17.    die "Can't find $filename in \@INC";
      18.  
    18.    }
      19.  
    19.    if ($@) {
      20.  
    20.    $INC{$filename} = undef;
      21.  
    21.    die $@;
      22.  
    22.    } elsif (!$result) {
      23.  
    23.    delete $INC{$filename};
      24.  
    24.    die "$filename did not return true value";
      25.  
    25.    } else {
      26.  
    26.    return $result;
      27.  
    27.    }
      28.  
    28.  }
      29.

    Note that the file will not be included twice under the same specifiedname.

    The file must return true as the last statement to indicatesuccessful execution of any initialization code, so it's customary toend such a file with 1; unless you're sure it'll return trueotherwise. But it's better just to put the 1;, in case you add morestatements.

    If EXPR is a bareword, the require assumes a ".pm" extension andreplaces "::" with "/" in the filename for you,to make it easy to load standard modules. This form of loading ofmodules does not risk altering your namespace.

    In other words, if you try this:

    1.  require Foo::Bar; # a splendid bareword
      2.

    The require function will actually look for the "Foo/Bar.pm" file in thedirectories specified in the @INC array.

    But if you try this:

    1.  $class = 'Foo::Bar';
      2.  
    2.  require $class;   # $class is not a bareword
      3.  
    3.  #or
      4.  
    4.  require "Foo::Bar";   # not a bareword because of the ""
      5.

    The require function will look for the "Foo::Bar" file in the @INC array andwill complain about not finding "Foo::Bar" there. In this case you can do:

    1.  eval "require $class";
      2.

    Now that you understand how require looks for files with abareword argument, there is a little extra functionality going on behindthe scenes. Before require looks for a ".pm" extension, it willfirst look for a similar filename with a ".pmc" extension. If this fileis found, it will be loaded in place of any file ending in a ".pm"extension.

    You can also insert hooks into the import facility by putting Perl codedirectly into the @INC array. There are three forms of hooks: subroutinereferences, array references, and blessed objects.

    Subroutine references are the simplest case. When the inclusion systemwalks through @INC and encounters a subroutine, this subroutine getscalled with two parameters, the first a reference to itself, and thesecond the name of the file to be included (e.g., "Foo/Bar.pm"). Thesubroutine should return either nothing or else a list of up to three values in the following order:

    1

    A filehandle, from which the file will be read.

    2

    A reference to a subroutine. If there is no filehandle (previous item),then this subroutine is expected to generate one line of source code percall, writing the line into $_ and returning 1, then finally at end offile returning 0. If there is a filehandle, then the subroutine will becalled to act as a simple source filter, with the line as read in $_.Again, return 1 for each valid line, and 0 after all lines have beenreturned.

    3

    Optional state for the subroutine. The state is passed in as $_[1]. Areference to the subroutine itself is passed in as $_[0].

    If an empty list, undef, or nothing that matches the first 3 values aboveis returned, then require looks at the remaining elements of @INC.Note that this filehandle must be a real filehandle (strictly a typeglobor reference to a typeglob, whether blessed or unblessed); tied filehandles will be ignored and processing will stop there.

    If the hook is an array reference, its first element must be a subroutinereference. This subroutine is called as above, but the first parameter isthe array reference. This lets you indirectly pass arguments tothe subroutine.

    In other words, you can write:

    1.  push @INC, \&my_sub;
      2.  
    2.  sub my_sub {
      3.  
    3.  my ($coderef, $filename) = @_;  # $coderef is \&my_sub
      4.  
    4.  ...
      5.  
    5.  }
      6.

    or:

    1.  push @INC, [ \&my_sub, $x, $y, ... ];
      2.  
    2.  sub my_sub {
      3.  
    3.  my ($arrayref, $filename) = @_;
      4.  
    4.  # Retrieve $x, $y, ...
      5.  
    5.  my @parameters = @$arrayref[1..$#$arrayref];
      6.  
    6.  ...
      7.  
    7.  }
      8.

    If the hook is an object, it must provide an INC method that will becalled as above, the first parameter being the object itself. (Note thatyou must fully qualify the sub's name, as unqualified INC is always forcedinto package main.) Here is a typical code layout:

    1.  # In Foo.pm
      2.  
    2.  package Foo;
      3.  
    3.  sub new { ... }
      4.  
    4.  sub Foo::INC {
      5.  
    5.  my ($self, $filename) = @_;
      6.  
    6.  ...
      7.  
    7.  }
      8.  
    8.  
    9.  # In the main program
      10.  
    10.  push @INC, Foo->new(...);
      11.

    These hooks are also permitted to set the %INC entrycorresponding to the files they have loaded. See %INC in perlvar.

    For a yet-more-powerful import facility, see use and perlmod.

  • reset EXPR
  • reset

    Generally used in a continue block at the end of a loop to clearvariables and reset ?? searches so that they work again. Theexpression is interpreted as a list of single characters (hyphensallowed for ranges). All variables and arrays beginning with one ofthose letters are reset to their pristine state. If the expression isomitted, one-match searches (?pattern?) are reset to match again. Only resets variables or searches in the current package. Always returns1. Examples:

    1.  reset 'X';  # reset all X variables
      2.  
    2.  reset 'a-z'; # reset lower case variables
      3.  
    3.  reset;  # just reset ?one-time? searches
      4.

    Resetting "A-Z" is not recommended because you'll wipe out your@ARGV and @INC arrays and your %ENV hash. Resets only packagevariables; lexical variables are unaffected, but they clean themselvesup on scope exit anyway, so you'll probably want to use them instead.See my.

  • return EXPR
  • return

    Returns from a subroutine, eval, or do FILE with the valuegiven in EXPR. Evaluation of EXPR may be in list, scalar, or voidcontext, depending on how the return value will be used, and the contextmay vary from one execution to the next (see wantarray). If no EXPRis given, returns an empty list in list context, the undefined value inscalar context, and (of course) nothing at all in void context.

    (In the absence of an explicit return, a subroutine, eval,or do FILE automatically returns the value of the last expressionevaluated.)

  • reverse LIST

    In list context, returns a list value consisting of the elementsof LIST in the opposite order. In scalar context, concatenates theelements of LIST and returns a string value with all charactersin the opposite order.

    1.  print join(", ", reverse "world", "Hello"); # Hello, world
      2.  
    2.  
    3.  print scalar reverse "dlrow ,", "olleH"; # Hello, world
      4.

    Used without arguments in scalar context, reverse() reverses $_.

    1.  $_ = "dlrow ,olleH";
      2.  
    2.  print reverse;  # No output, list context
      3.  
    3.  print scalar reverse;   # Hello, world
      4.

    Note that reversing an array to itself (as in @a = reverse @a) willpreserve non-existent elements whenever possible, i.e., for non magicalarrays or tied arrays with EXISTS and DELETE methods.

    This operator is also handy for inverting a hash, although there are somecaveats. If a value is duplicated in the original hash, only one of thosecan be represented as a key in the inverted hash. Also, this has tounwind one hash and build a whole new one, which may take some timeon a large hash, such as from a DBM file.

    1.  %by_name = reverse %by_address;  # Invert the hash
      2.
  • rewinddir DIRHANDLE

    Sets the current position to the beginning of the directory for thereaddir routine on DIRHANDLE.

    Portability issues: rewinddir in perlport.

  • rindex STR,SUBSTR,POSITION
  • rindex STR,SUBSTR

    Works just like index() except that it returns the position of the lastoccurrence of SUBSTR in STR. If POSITION is specified, returns thelast occurrence beginning at or before that position.

  • rmdir FILENAME
  • rmdir

    Deletes the directory specified by FILENAME if that directory isempty. If it succeeds it returns true; otherwise it returns false andsets $! (errno). If FILENAME is omitted, uses $_.

    To remove a directory tree recursively (rm -rf on Unix) look atthe rmtree function of the File::Path module.

  • s///

    The substitution operator. See Regexp Quote-Like Operators in perlop.

  • say FILEHANDLE LIST
  • say FILEHANDLE
  • say LIST
  • say

    Just like print, but implicitly appends a newline. say LIST issimply an abbreviation for { local $\ = "\n"; print LIST }. To useFILEHANDLE without a LIST to print the contents of $_ to it, you mustuse a real filehandle like FH, not an indirect one like $fh.

    This keyword is available only when the "say" featureis enabled, or when prefixed with CORE::; seefeature. Alternately, include a use v5.10 or later to the currentscope.

  • scalar EXPR

    Forces EXPR to be interpreted in scalar context and returns the valueof EXPR.

    1.  @counts = ( scalar @a, scalar @b, scalar @c );
      2.

    There is no equivalent operator to force an expression tobe interpolated in list context because in practice, this is neverneeded. If you really wanted to do so, however, you could usethe construction @{[ (some expression) ]}, but usually a simple(some expression) suffices.

    Because scalar is a unary operator, if you accidentally use aparenthesized list for the EXPR, this behaves as a scalar comma expression,evaluating all but the last element in void context and returning the finalelement evaluated in scalar context. This is seldom what you want.

    The following single statement:

    1.  print uc(scalar(&foo,$bar)),$baz;
      2.

    is the moral equivalent of these two:

    1.  &foo;
      2.  
    2.  print(uc($bar),$baz);
      3.

    See perlop for more details on unary operators and the comma operator.

  • seek FILEHANDLE,POSITION,WHENCE

    Sets FILEHANDLE's position, just like the fseek call of stdio.FILEHANDLE may be an expression whose value gives the name of thefilehandle. The values for WHENCE are 0 to set the new positionin bytes to POSITION; 1 to set it to the current position plusPOSITION; and 2 to set it to EOF plus POSITION, typicallynegative. For WHENCE you may use the constants SEEK_SET,SEEK_CUR, and SEEK_END (start of the file, current position, endof the file) from the Fcntl module. Returns 1 on success, falseotherwise.

    Note the in bytes: even if the filehandle has been set tooperate on characters (for example by using the :encoding(utf8) openlayer), tell() will return byte offsets, not character offsets(because implementing that would render seek() and tell() rather slow).

    If you want to position the file for sysread or syswrite, don't useseek, because buffering makes its effect on the file's read-write positionunpredictable and non-portable. Use sysseek instead.

    Due to the rules and rigors of ANSI C, on some systems you have to do aseek whenever you switch between reading and writing. Amongst otherthings, this may have the effect of calling stdio's clearerr(3).A WHENCE of 1 (SEEK_CUR) is useful for not moving the file position:

    1.  seek(TEST,0,1);
      2.

    This is also useful for applications emulating tail -f. Once you hitEOF on your read and then sleep for a while, you (probably) have to stick in adummy seek() to reset things. The seek doesn't change the position,but it does clear the end-of-file condition on the handle, so that thenext <FILE> makes Perl try again to read something. (We hope.)

    If that doesn't work (some I/O implementations are particularlycantankerous), you might need something like this:

    1.  for (;;) {
      2.  
    2.  for ($curpos = tell(FILE); $_ = <FILE>;
      3.  
    3.  $curpos = tell(FILE)) {
      4.  
    4.  # search for some stuff and put it into files
      5.  
    5.  }
      6.  
    6.  sleep($for_a_while);
      7.  
    7.  seek(FILE, $curpos, 0);
      8.  
    8.  }
      9.
  • seekdir DIRHANDLE,POS

    Sets the current position for the readdir routine on DIRHANDLE. POSmust be a value returned by telldir. seekdir also has the same caveatsabout possible directory compaction as the corresponding system libraryroutine.

  • select FILEHANDLE
  • select

    Returns the currently selected filehandle. If FILEHANDLE is supplied,sets the new current default filehandle for output. This has twoeffects: first, a write or a print without a filehandle default to this FILEHANDLE. Second, references to variables related tooutput will refer to this output channel.

    For example, to set the top-of-form format for more than oneoutput channel, you might do the following:

    1.  select(REPORT1);
      2.  
    2.  $^ = 'report1_top';
      3.  
    3.  select(REPORT2);
      4.  
    4.  $^ = 'report2_top';
      5.

    FILEHANDLE may be an expression whose value gives the name of theactual filehandle. Thus:

    1.  $oldfh = select(STDERR); $| = 1; select($oldfh);
      2.

    Some programmers may prefer to think of filehandles as objects withmethods, preferring to write the last example as:

    1.  use IO::Handle;
      2.  
    2.  STDERR->autoflush(1);
      3.

    Portability issues: select in perlport.

  • select RBITS,WBITS,EBITS,TIMEOUT

    This calls the select(2) syscall with the bit masks specified, whichcan be constructed using fileno and vec, along these lines:

    1.  $rin = $win = $ein = '';
      2.  
    2.  vec($rin, fileno(STDIN),  1) = 1;
      3.  
    3.  vec($win, fileno(STDOUT), 1) = 1;
      4.  
    4.  $ein = $rin | $win;
      5.

    If you want to select on many filehandles, you may wish to write asubroutine like this:

    1.  sub fhbits {
      2.  
    2.  my @fhlist = @_;
      3.  
    3.  my $bits = "";
      4.  
    4.  for my $fh (@fhlist) {
      5.  
    5.  vec($bits, fileno($fh), 1) = 1;
      6.  
    6.  }
      7.  
    7.  return $bits;
      8.  
    8.  }
      9.  
    9.  $rin = fhbits(*STDIN, *TTY, *MYSOCK);
      10.

    The usual idiom is:

    1.  ($nfound,$timeleft) =
      2.  
    2.   select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
      3.

    or to block until something becomes ready just do this

    1.  $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
      2.

    Most systems do not bother to return anything useful in $timeleft, socalling select() in scalar context just returns $nfound.

    Any of the bit masks can also be undef. The timeout, if specified, isin seconds, which may be fractional. Note: not all implementations arecapable of returning the $timeleft. If not, they always return$timeleft equal to the supplied $timeout.

    You can effect a sleep of 250 milliseconds this way:

    1.  select(undef, undef, undef, 0.25);
      2.

    Note that whether select gets restarted after signals (say, SIGALRM)is implementation-dependent. See also perlport for notes on theportability of select.

    On error, select behaves just like select(2): it returns-1 and sets $!.

    On some Unixes, select(2) may report a socket file descriptor as "ready forreading" even when no data is available, and thus any subsequent readwould block. This can be avoided if you always use O_NONBLOCK on thesocket. See select(2) and fcntl(2) for further details.

    The standard IO::Select module provides a user-friendlier interfaceto select, mostly because it does all the bit-mask work for you.

    WARNING: One should not attempt to mix buffered I/O (like reador <FH>) with select, except as permitted by POSIX, and eventhen only on POSIX systems. You have to use sysread instead.

    Portability issues: select in perlport.

  • semctl ID,SEMNUM,CMD,ARG

    Calls the System V IPC function semctl(2). You'll probably have to say

    1.  use IPC::SysV;
      2.

    first to get the correct constant definitions. If CMD is IPC_STAT orGETALL, then ARG must be a variable that will hold the returnedsemid_ds structure or semaphore value array. Returns like ioctl:the undefined value for error, "0 but true" for zero, or the actualreturn value otherwise. The ARG must consist of a vector of nativeshort integers, which may be created with pack("s!",(0)x$nsem).See also SysV IPC in perlipc, IPC::SysV, IPC::Semaphoredocumentation.

    Portability issues: semctl in perlport.

  • semget KEY,NSEMS,FLAGS

    Calls the System V IPC function semget(2). Returns the semaphore id, orthe undefined value on error. See alsoSysV IPC in perlipc, IPC::SysV, IPC::SysV::Semaphoredocumentation.

    Portability issues: semget in perlport.

  • semop KEY,OPSTRING

    Calls the System V IPC function semop(2) for semaphore operationssuch as signalling and waiting. OPSTRING must be a packed array ofsemop structures. Each semop structure can be generated withpack("s!3", $semnum, $semop, $semflag). The length of OPSTRING implies the number of semaphore operations. Returns true ifsuccessful, false on error. As an example, thefollowing code waits on semaphore $semnum of semaphore id $semid:

    1.  $semop = pack("s!3", $semnum, -1, 0);
      2.  
    2.  die "Semaphore trouble: $!\n" unless semop($semid, $semop);
      3.

    To signal the semaphore, replace -1 with 1. See alsoSysV IPC in perlipc, IPC::SysV, and IPC::SysV::Semaphoredocumentation.

    Portability issues: semop in perlport.

  • send SOCKET,MSG,FLAGS,TO
  • send SOCKET,MSG,FLAGS

    Sends a message on a socket. Attempts to send the scalar MSG to the SOCKETfilehandle. Takes the same flags as the system call of the same name. Onunconnected sockets, you must specify a destination to send to, in whichcase it does a sendto(2) syscall. Returns the number of characters sent,or the undefined value on error. The sendmsg(2) syscall is currentlyunimplemented. See UDP: Message Passing in perlipc for examples.

    Note the characters: depending on the status of the socket, either(8-bit) bytes or characters are sent. By default all sockets operateon bytes, but for example if the socket has been changed usingbinmode() to operate with the :encoding(utf8) I/O layer (seeopen, or the open pragma, open), the I/O will operate on UTF-8encoded Unicode characters, not bytes. Similarly for the :encodingpragma: in that case pretty much any characters can be sent.

  • setpgrp PID,PGRP

    Sets the current process group for the specified PID, 0 for the currentprocess. Raises an exception when used on a machine that doesn'timplement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,it defaults to 0,0. Note that the BSD 4.2 version of setpgrp does notaccept any arguments, so only setpgrp(0,0) is portable. See alsoPOSIX::setsid().

    Portability issues: setpgrp in perlport.

  • setpriority WHICH,WHO,PRIORITY

    Sets the current priority for a process, a process group, or a user.(See setpriority(2).) Raises an exception when used on a machinethat doesn't implement setpriority(2).

    Portability issues: setpriority in perlport.

  • setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL

    Sets the socket option requested. Returns undef on error.Use integer constants provided by the Socket module forLEVEL and OPNAME. Values for LEVEL can also be obtained fromgetprotobyname. OPTVAL might either be a packed string or an integer.An integer OPTVAL is shorthand for pack("i", OPTVAL).

    An example disabling Nagle's algorithm on a socket:

    1.  use Socket qw(IPPROTO_TCP TCP_NODELAY);
      2.  
    2.  setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
      3.

    Portability issues: setsockopt in perlport.

  • shift ARRAY
  • shift EXPR
  • shift

    Shifts the first value of the array off and returns it, shortening thearray by 1 and moving everything down. If there are no elements in thearray, returns the undefined value. If ARRAY is omitted, shifts the@_ array within the lexical scope of subroutines and formats, and the@ARGV array outside a subroutine and also within the lexical scopesestablished by the eval STRING, BEGIN {}, INIT {}, CHECK {},UNITCHECK {}, and END {} constructs.

    Starting with Perl 5.14, shift can take a scalar EXPR, which must hold areference to an unblessed array. The argument will be dereferencedautomatically. This aspect of shift is considered highly experimental.The exact behaviour may change in a future version of Perl.

    To avoid confusing would-be users of your code who are running earlierversions of Perl with mysterious syntax errors, put this sort of thing atthe top of your file to signal that your code will work only on Perls ofa recent vintage:

    1.  use 5.014;# so push/pop/etc work on scalars (experimental)
      2.

    See also unshift, push, and pop. shift and unshift do thesame thing to the left end of an array that pop and push do to theright end.

  • shmctl ID,CMD,ARG

    Calls the System V IPC function shmctl. You'll probably have to say

    1.  use IPC::SysV;
      2.

    first to get the correct constant definitions. If CMD is IPC_STAT,then ARG must be a variable that will hold the returned shmid_dsstructure. Returns like ioctl: undef for error; "0 buttrue" for zero; and the actual return value otherwise.See also SysV IPC in perlipc and IPC::SysV documentation.

    Portability issues: shmctl in perlport.

  • shmget KEY,SIZE,FLAGS

    Calls the System V IPC function shmget. Returns the shared memorysegment id, or undef on error.See also SysV IPC in perlipc and IPC::SysV documentation.

    Portability issues: shmget in perlport.

  • shmread ID,VAR,POS,SIZE
  • shmwrite ID,STRING,POS,SIZE

    Reads or writes the System V shared memory segment ID starting atposition POS for size SIZE by attaching to it, copying in/out, anddetaching from it. When reading, VAR must be a variable that willhold the data read. When writing, if STRING is too long, only SIZEbytes are used; if STRING is too short, nulls are written to fill outSIZE bytes. Return true if successful, false on error.shmread() taints the variable. See also SysV IPC in perlipc,IPC::SysV, and the IPC::Shareable module from CPAN.

    Portability issues: shmread in perlport and shmwrite in perlport.

  • shutdown SOCKET,HOW

    Shuts down a socket connection in the manner indicated by HOW, whichhas the same interpretation as in the syscall of the same name.

    1.  shutdown(SOCKET, 0); # I/we have stopped reading data
      2.  
    2.  shutdown(SOCKET, 1); # I/we have stopped writing data
      3.  
    3.  shutdown(SOCKET, 2); # I/we have stopped using this socket
      4.

    This is useful with sockets when you want to tell the otherside you're done writing but not done reading, or vice versa.It's also a more insistent form of close because it alsodisables the file descriptor in any forked copies in otherprocesses.

    Returns 1 for success; on error, returns undef ifthe first argument is not a valid filehandle, or returns 0 and sets$! for any other failure.

  • sin EXPR
  • sin

    Returns the sine of EXPR (expressed in radians). If EXPR is omitted,returns sine of $_.

    For the inverse sine operation, you may use the Math::Trig::asinfunction, or use this relation:

    1.  sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
      2.
  • sleep EXPR
  • sleep

    Causes the script to sleep for (integer) EXPR seconds, or forever if no argument is given. Returns the integer number of seconds actually slept.

    May be interrupted if the process receives a signal such as SIGALRM.

    1.  eval {
      2.  
    2.  local $SIG{ALARM} = sub { die "Alarm!\n" };
      3.  
    3.  sleep;
      4.  
    4.  };
      5.  
    5.  die $@ unless $@ eq "Alarm!\n";
      6.

    You probably cannot mix alarm and sleep calls, because sleepis often implemented using alarm.

    On some older systems, it may sleep up to a full second less than whatyou requested, depending on how it counts seconds. Most modern systemsalways sleep the full amount. They may appear to sleep longer than that,however, because your process might not be scheduled right away in abusy multitasking system.

    For delays of finer granularity than one second, the Time::HiRes module(from CPAN, and starting from Perl 5.8 part of the standarddistribution) provides usleep(). You may also use Perl's four-argumentversion of select() leaving the first three arguments undefined, or youmight be able to use the syscall interface to access setitimer(2) ifyour system supports it. See perlfaq8 for details.

    See also the POSIX module's pause function.

  • socket SOCKET,DOMAIN,TYPE,PROTOCOL

    Opens a socket of the specified kind and attaches it to filehandleSOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as forthe syscall of the same name. You should use Socket firstto get the proper definitions imported. See the examples inSockets: Client/Server Communication in perlipc.

    On systems that support a close-on-exec flag on files, the flag willbe set for the newly opened file descriptor, as determined by thevalue of $^F. See $^F in perlvar.

  • socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL

    Creates an unnamed pair of sockets in the specified domain, of thespecified type. DOMAIN, TYPE, and PROTOCOL are specified the same asfor the syscall of the same name. If unimplemented, raises an exception.Returns true if successful.

    On systems that support a close-on-exec flag on files, the flag willbe set for the newly opened file descriptors, as determined by the valueof $^F. See $^F in perlvar.

    Some systems defined pipe in terms of socketpair, in which a callto pipe(Rdr, Wtr) is essentially:

    1.  use Socket;
      2.  
    2.  socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
      3.  
    3.  shutdown(Rdr, 1); # no more writing for reader
      4.  
    4.  shutdown(Wtr, 0); # no more reading for writer
      5.

    See perlipc for an example of socketpair use. Perl 5.8 and later willemulate socketpair using IP sockets to localhost if your system implementssockets but not socketpair.

    Portability issues: socketpair in perlport.

  • sort SUBNAME LIST
  • sort BLOCK LIST
  • sort LIST

    In list context, this sorts the LIST and returns the sorted list value.In scalar context, the behaviour of sort() is undefined.

    If SUBNAME or BLOCK is omitted, sorts in standard string comparisonorder. If SUBNAME is specified, it gives the name of a subroutinethat returns an integer less than, equal to, or greater than 0,depending on how the elements of the list are to be ordered. (The <=> and cmp operators are extremely useful in such routines.)SUBNAME may be a scalar variable name (unsubscripted), in which casethe value provides the name of (or a reference to) the actualsubroutine to use. In place of a SUBNAME, you can provide a BLOCK asan anonymous, in-line sort subroutine.

    If the subroutine's prototype is ($$), the elements to be compared arepassed by reference in @_, as for a normal subroutine. This is slowerthan unprototyped subroutines, where the elements to be compared are passedinto the subroutine as the package global variables $a and $b (see examplebelow). Note that in the latter case, it is usually highly counter-productiveto declare $a and $b as lexicals.

    If the subroutine is an XSUB, the elements to be compared are pushed on tothe stack, the way arguments are usually passed to XSUBs. $a and $b arenot set.

    The values to be compared are always passed by reference and should notbe modified.

    You also cannot exit out of the sort block or subroutine using any of theloop control operators described in perlsyn or with goto.

    When use locale (but not use locale 'not_characters') is ineffect, sort LIST sorts LIST according to thecurrent collation locale. See perllocale.

    sort() returns aliases into the original list, much as a for loop's indexvariable aliases the list elements. That is, modifying an element of alist returned by sort() (for example, in a foreach, map or grep)actually modifies the element in the original list. This is usuallysomething to be avoided when writing clear code.

    Perl 5.6 and earlier used a quicksort algorithm to implement sort.That algorithm was not stable, so could go quadratic. (A stable sortpreserves the input order of elements that compare equal. Althoughquicksort's run time is O(NlogN) when averaged over all arrays oflength N, the time can be O(N**2), quadratic behavior, for someinputs.) In 5.7, the quicksort implementation was replaced witha stable mergesort algorithm whose worst-case behavior is O(NlogN).But benchmarks indicated that for some inputs, on some platforms,the original quicksort was faster. 5.8 has a sort pragma forlimited control of the sort. Its rather blunt control of theunderlying algorithm may not persist into future Perls, but theability to characterize the input or output in implementationindependent ways quite probably will. See the sort pragma.

    Examples:

    1.  # sort lexically
      2.  
    2.  @articles = sort @files;
      3.  
    3.  
      4.  
    4.  # same thing, but with explicit sort routine
      5.  
    5.  @articles = sort {$a cmp $b} @files;
      6.  
    6.  
      7.  
    7.  # now case-insensitively
      8.  
    8.  @articles = sort {fc($a) cmp fc($b)} @files;
      9.  
    9.  
      10.  
    10.  # same thing in reversed order
      11.  
    11.  @articles = sort {$b cmp $a} @files;
      12.  
    12.  
      13.  
    13.  # sort numerically ascending
      14.  
    14.  @articles = sort {$a <=> $b} @files;
      15.  
    15.  
      16.  
    16.  # sort numerically descending
      17.  
    17.  @articles = sort {$b <=> $a} @files;
      18.  
    18.  
      19.  
    19.  # this sorts the %age hash by value instead of key
      20.  
    20.  # using an in-line function
      21.  
    21.  @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
      22.  
    22.  
      23.  
    23.  # sort using explicit subroutine name
      24.  
    24.  sub byage {
      25.  
    25.  $age{$a} <=> $age{$b};  # presuming numeric
      26.  
    26.  }
      27.  
    27.  @sortedclass = sort byage @class;
      28.  
    28.  
      29.  
    29.  sub backwards { $b cmp $a }
      30.  
    30.  @harry  = qw(dog cat x Cain Abel);
      31.  
    31.  @george = qw(gone chased yz Punished Axed);
      32.  
    32.  print sort @harry;
      33.  
    33.  # prints AbelCaincatdogx
      34.  
    34.  print sort backwards @harry;
      35.  
    35.  # prints xdogcatCainAbel
      36.  
    36.  print sort @george, 'to', @harry;
      37.  
    37.  # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
      38.  
    38.  
    39.  # inefficiently sort by descending numeric compare using
      40.  
    40.  # the first integer after the first = sign, or the
      41.  
    41.  # whole record case-insensitively otherwise
      42.  
    42.  
    43.  my @new = sort {
      44.  
    44.  ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
      45.  
    45.  ||
      46.  
    46.  fc($a)  cmp  fc($b)
      47.  
    47.  } @old;
      48.  
    48.  
    49.  # same thing, but much more efficiently;
      50.  
    50.  # we'll build auxiliary indices instead
      51.  
    51.  # for speed
      52.  
    52.  my @nums = @caps = ();
      53.  
    53.  for (@old) {
      54.  
    54.  push @nums, ( /=(\d+)/ ? $1 : undef );
      55.  
    55.  push @caps, fc($_);
      56.  
    56.  }
      57.  
    57.  
    58.  my @new = @old[ sort {
      59.  
    59.    $nums[$b] <=> $nums[$a]
      60.  
    60.  ||
      61.  
    61.    $caps[$a] cmp $caps[$b]
      62.  
    62.  } 0..$#old
      63.  
    63.   ];
      64.  
    64.  
    65.  # same thing, but without any temps
      66.  
    66.  @new = map { $_->[0] }
      67.  
    67.    sort { $b->[1] <=> $a->[1]
      68.  
    68.    ||
      69.  
    69.   $a->[2] cmp $b->[2]
      70.  
    70.    } map { [$_, /=(\d+)/, fc($_)] } @old;
      71.  
    71.  
    72.  # using a prototype allows you to use any comparison subroutine
      73.  
    73.  # as a sort subroutine (including other package's subroutines)
      74.  
    74.  package other;
      75.  
    75.  sub backwards ($$) { $_[1] cmp $_[0]; }  # $a and $b are not set here
      76.  
    76.  
      77.  
    77.  package main;
      78.  
    78.  @new = sort other::backwards @old;
      79.  
    79.  
      80.  
    80.  # guarantee stability, regardless of algorithm
      81.  
    81.  use sort 'stable';
      82.  
    82.  @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
      83.  
    83.  
      84.  
    84.  # force use of mergesort (not portable outside Perl 5.8)
      85.  
    85.  use sort '_mergesort';  # note discouraging _
      86.  
    86.  @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
      87.

    Warning: syntactical care is required when sorting the list returned froma function. If you want to sort the list returned by the function callfind_records(@key), you can use:

    1.  @contact = sort { $a cmp $b } find_records @key;
      2.  
    2.  @contact = sort +find_records(@key);
      3.  
    3.  @contact = sort &find_records(@key);
      4.  
    4.  @contact = sort(find_records(@key));
      5.

    If instead you want to sort the array @key with the comparison routinefind_records() then you can use:

    1.  @contact = sort { find_records() } @key;
      2.  
    2.  @contact = sort find_records(@key);
      3.  
    3.  @contact = sort(find_records @key);
      4.  
    4.  @contact = sort(find_records (@key));
      5.

    If you're using strict, you must not declare $aand $b as lexicals. They are package globals. That meansthat if you're in the main package and type

    1.  @articles = sort {$b <=> $a} @files;
      2.

    then $a and $b are $main::a and $main::b (or $::a and $::b),but if you're in the FooPack package, it's the same as typing

    1.  @articles = sort {$FooPack::b <=> $FooPack::a} @files;
      2.

    The comparison function is required to behave. If it returnsinconsistent results (sometimes saying $x[1] is less than $x[2] andsometimes saying the opposite, for example) the results are notwell-defined.

    Because <=> returns undef when either operand is NaN(not-a-number), be careful when sorting with acomparison function like $a <=> $b any lists that might contain aNaN. The following example takes advantage that NaN != NaN toeliminate any NaNs from the input list.

    1.  @result = sort { $a <=> $b } grep { $_ == $_ } @input;
      2.
  • splice ARRAY or EXPR,OFFSET,LENGTH,LIST
  • splice ARRAY or EXPR,OFFSET,LENGTH
  • splice ARRAY or EXPR,OFFSET
  • splice ARRAY or EXPR

    Removes the elements designated by OFFSET and LENGTH from an array, andreplaces them with the elements of LIST, if any. In list context,returns the elements removed from the array. In scalar context,returns the last element removed, or undef if no elements areremoved. The array grows or shrinks as necessary.If OFFSET is negative then it starts that far from the end of the array.If LENGTH is omitted, removes everything from OFFSET onward.If LENGTH is negative, removes the elements from OFFSET onwardexcept for -LENGTH elements at the end of the array.If both OFFSET and LENGTH are omitted, removes everything. If OFFSET ispast the end of the array, Perl issues a warning, and splices at theend of the array.

    The following equivalences hold (assuming $#a >= $i )

    1.  push(@a,$x,$y)  splice(@a,@a,0,$x,$y)
      2.  
    2.  pop(@a) splice(@a,-1)
      3.  
    3.  shift(@a)   splice(@a,0,1)
      4.  
    4.  unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)
      5.  
    5.  $a[$i] = $y splice(@a,$i,1,$y)
      6.

    Example, assuming array lengths are passed before arrays:

    1.  sub aeq {  # compare two list values
      2.  
    2.  my(@a) = splice(@_,0,shift);
      3.  
    3.  my(@b) = splice(@_,0,shift);
      4.  
    4.  return 0 unless @a == @b;  # same len?
      5.  
    5.  while (@a) {
      6.  
    6.  return 0 if pop(@a) ne pop(@b);
      7.  
    7.  }
      8.  
    8.  return 1;
      9.  
    9.  }
      10.  
    10.  if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
      11.

    Starting with Perl 5.14, splice can take scalar EXPR, which must hold areference to an unblessed array. The argument will be dereferencedautomatically. This aspect of splice is considered highly experimental.The exact behaviour may change in a future version of Perl.

    To avoid confusing would-be users of your code who are running earlierversions of Perl with mysterious syntax errors, put this sort of thing atthe top of your file to signal that your code will work only on Perls ofa recent vintage:

    1.  use 5.014;# so push/pop/etc work on scalars (experimental)
      2.
  • split /PATTERN/,EXPR,LIMIT
  • split /PATTERN/,EXPR
  • split /PATTERN/
  • split

    Splits the string EXPR into a list of strings and returns thelist in list context, or the size of the list in scalar context.

    If only PATTERN is given, EXPR defaults to $_.

    Anything in EXPR that matches PATTERN is taken to be a separatorthat separates the EXPR into substrings (called "fields") thatdo not include the separator. Note that a separator may belonger than one character or even have no characters at all (theempty string, which is a zero-width match).

    The PATTERN need not be constant; an expression may be usedto specify a pattern that varies at runtime.

    If PATTERN matches the empty string, the EXPR is split at the matchposition (between characters). As an example, the following:

    1.  print join(':', split('b', 'abc')), "\n";
      2.

    uses the 'b' in 'abc' as a separator to produce the output 'a:c'.However, this:

    1.  print join(':', split('', 'abc')), "\n";
      2.

    uses empty string matches as separators to produce the output'a:b:c'; thus, the empty string may be used to split EXPR into alist of its component characters.

    As a special case for split, the empty pattern given inmatch operator syntax (//) specifically matches the empty string, which is contrary to its usualinterpretation as the last successful match.

    If PATTERN is /^/, then it is treated as if it used themultiline modifier (/^/m), since itisn't much use otherwise.

    As another special case, split emulates the default behavior of thecommand line tool awk when the PATTERN is either omitted or a literalstring composed of a single space character (such as ' ' or"\x20", but not e.g. / /). In this case, any leadingwhitespace in EXPR is removed before splitting occurs, and the PATTERN isinstead treated as if it were /\s+/; in particular, this means thatany contiguous whitespace (not just a single space character) is used asa separator. However, this special treatment can be avoided by specifyingthe pattern / / instead of the string " ", thereby allowingonly a single space character to be a separator.

    If omitted, PATTERN defaults to a single space, " ", triggeringthe previously described awk emulation.

    If LIMIT is specified and positive, it represents the maximum numberof fields into which the EXPR may be split; in other words, LIMIT isone greater than the maximum number of times EXPR may be split. Thus,the LIMIT value 1 means that EXPR may be split a maximum of zerotimes, producing a maximum of one field (namely, the entire value ofEXPR). For instance:

    1.  print join(':', split(//, 'abc', 1)), "\n";
      2.

    produces the output 'abc', and this:

    1.  print join(':', split(//, 'abc', 2)), "\n";
      2.

    produces the output 'a:bc', and each of these:

    1.  print join(':', split(//, 'abc', 3)), "\n";
      2.  
    2.  print join(':', split(//, 'abc', 4)), "\n";
      3.

    produces the output 'a:b:c'.

    If LIMIT is negative, it is treated as if it were instead arbitrarilylarge; as many fields as possible are produced.

    If LIMIT is omitted (or, equivalently, zero), then it is usuallytreated as if it were instead negative but with the exception thattrailing empty fields are stripped (empty leading fields are alwayspreserved); if all fields are empty, then all fields are considered tobe trailing (and are thus stripped in this case). Thus, the following:

    1.  print join(':', split(',', 'a,b,c,,,')), "\n";
      2.

    produces the output 'a:b:c', but the following:

    1.  print join(':', split(',', 'a,b,c,,,', -1)), "\n";
      2.

    produces the output 'a:b:c:::'.

    In time-critical applications, it is worthwhile to avoid splittinginto more fields than necessary. Thus, when assigning to a list,if LIMIT is omitted (or zero), then LIMIT is treated as though itwere one larger than the number of variables in the list; for thefollowing, LIMIT is implicitly 4:

    1.  ($login, $passwd, $remainder) = split(/:/);
      2.

    Note that splitting an EXPR that evaluates to the empty string alwaysproduces zero fields, regardless of the LIMIT specified.

    An empty leading field is produced when there is a positive-widthmatch at the beginning of EXPR. For instance:

    1.  print join(':', split(/ /, ' abc')), "\n";
      2.

    produces the output ':abc'. However, a zero-width match at thebeginning of EXPR never produces an empty field, so that:

    1.  print join(':', split(//, ' abc'));
      2.

    produces the output ' :a:b:c' (rather than ': :a:b:c').

    An empty trailing field, on the other hand, is produced when there is amatch at the end of EXPR, regardless of the length of the match(of course, unless a non-zero LIMIT is given explicitly, such fields areremoved, as in the last example). Thus:

    1.  print join(':', split(//, ' abc', -1)), "\n";
      2.

    produces the output ' :a:b:c:'.

    If the PATTERN containscapturing groups,then for each separator, an additional field is produced for each substringcaptured by a group (in the order in which the groups are specified,as per backreferences); if any group does notmatch, then it captures the undef value instead of a substring. Also,note that any such additional field is produced whenever there is aseparator (that is, whenever a split occurs), and such an additional fielddoes not count towards the LIMIT. Consider the following expressionsevaluated in list context (each returned list is provided in the associatedcomment):

    1.  split(/-|,/, "1-10,20", 3)
      2.  
    2.  # ('1', '10', '20')
      3.  
    3.  
    4.  split(/(-|,)/, "1-10,20", 3)
      5.  
    5.  # ('1', '-', '10', ',', '20')
      6.  
    6.  
    7.  split(/-|(,)/, "1-10,20", 3)
      8.  
    8.  # ('1', undef, '10', ',', '20')
      9.  
    9.  
    10.  split(/(-)|,/, "1-10,20", 3)
      11.  
    11.  # ('1', '-', '10', undef, '20')
      12.  
    12.  
    13.  split(/(-)|(,)/, "1-10,20", 3)
      14.  
    14.  # ('1', '-', undef, '10', undef, ',', '20')
      15.
  • sprintf FORMAT, LIST

    Returns a string formatted by the usual printf conventions of the Clibrary function sprintf. See below for more detailsand see sprintf(3) or printf(3) on your system for an explanation ofthe general principles.

    For example:

    1.  # Format number with up to 8 leading zeroes
      2.  
    2.  $result = sprintf("%08d", $number);
      3.  
    3.  
    4.  # Round number to 3 digits after decimal point
      5.  
    5.  $rounded = sprintf("%.3f", $number);
      6.

    Perl does its own sprintf formatting: it emulates the Cfunction sprintf(3), but doesn't use it except for floating-pointnumbers, and even then only standard modifiers are allowed. Non-standard extensions in your local sprintf(3) are therefore unavailable from Perl.

    Unlike printf, sprintf does not do what you probably mean when youpass it an array as your first argument. The array is given scalar context,and instead of using the 0th element of the array as the format, Perl willuse the count of elements in the array as the format, which is almost neveruseful.

    Perl's sprintf permits the following universally-known conversions:

    1.    %% a percent sign
      2.  
    2.    %c a character with the given number
      3.  
    3.    %s a string
      4.  
    4.    %d a signed integer, in decimal
      5.  
    5.    %u an unsigned integer, in decimal
      6.  
    6.    %o an unsigned integer, in octal
      7.  
    7.    %x an unsigned integer, in hexadecimal
      8.  
    8.    %e a floating-point number, in scientific notation
      9.  
    9.    %f a floating-point number, in fixed decimal notation
      10.  
    10.    %g a floating-point number, in %e or %f notation
      11.

    In addition, Perl permits the following widely-supported conversions:

    1.    %X like %x, but using upper-case letters
      2.  
    2.    %E like %e, but using an upper-case "E"
      3.  
    3.    %G like %g, but with an upper-case "E" (if applicable)
      4.  
    4.    %b an unsigned integer, in binary
      5.  
    5.    %B like %b, but using an upper-case "B" with the # flag
      6.  
    6.    %p a pointer (outputs the Perl value's address in hexadecimal)
      7.  
    7.    %n special: *stores* the number of characters output so far
      8.  
    8.  into the next argument in the parameter list
      9.

    Finally, for backward (and we do mean "backward") compatibility, Perlpermits these unnecessary but widely-supported conversions:

    1.    %i a synonym for %d
      2.  
    2.    %D a synonym for %ld
      3.  
    3.    %U a synonym for %lu
      4.  
    4.    %O a synonym for %lo
      5.  
    5.    %F a synonym for %f
      6.

    Note that the number of exponent digits in the scientific notation producedby %e, %E, %g and %G for numbers with the modulus of theexponent less than 100 is system-dependent: it may be three or less(zero-padded as necessary). In other words, 1.23 times ten to the99th may be either "1.23e99" or "1.23e099".

    Between the % and the format letter, you may specify severaladditional attributes controlling the interpretation of the format.In order, these are:

    • format parameter index

      An explicit format parameter index, such as 2$. By default sprintfwill format the next unused argument in the list, but this allows youto take the arguments out of order:

      1.   printf '%2$d %1$d', 12, 34;  # prints "34 12"
        2.  
      2.   printf '%3$d %d %1$d', 1, 2, 3;  # prints "3 1 1"
        3.
    • flags

      one or more of:

      1.    space   prefix non-negative number with a space
        2.  
      2.    +   prefix non-negative number with a plus sign
        3.  
      3.    -   left-justify within the field
        4.  
      4.    0   use zeros, not spaces, to right-justify
        5.  
      5.    #   ensure the leading "0" for any octal,
        6.  
      6.    prefix non-zero hexadecimal with "0x" or "0X",
        7.  
      7.    prefix non-zero binary with "0b" or "0B"
        8.

      For example:

      1.   printf '<% d>',  12;   # prints "< 12>"
        2.  
      2.   printf '<%+d>',  12;   # prints "<+12>"
        3.  
      3.   printf '<%6s>',  12;   # prints "< 12>"
        4.  
      4.   printf '<%-6s>', 12;   # prints "<12 >"
        5.  
      5.   printf '<%06s>', 12;   # prints "<000012>"
        6.  
      6.   printf '<%#o>',  12;   # prints "<014>"
        7.  
      7.   printf '<%#x>',  12;   # prints "<0xc>"
        8.  
      8.   printf '<%#X>',  12;   # prints "<0XC>"
        9.  
      9.   printf '<%#b>',  12;   # prints "<0b1100>"
        10.  
      10.   printf '<%#B>',  12;   # prints "<0B1100>"
        11.

      When a space and a plus sign are given as the flags at once,a plus sign is used to prefix a positive number.

      1.   printf '<%+ d>', 12;   # prints "<+12>"
        2.  
      2.   printf '<% +d>', 12;   # prints "<+12>"
        3.

      When the # flag and a precision are given in the %o conversion,the precision is incremented if it's necessary for the leading "0".

      1.   printf '<%#.5o>', 012;  # prints "<00012>"
        2.  
      2.   printf '<%#.5o>', 012345;   # prints "<012345>"
        3.  
      3.   printf '<%#.0o>', 0; # prints "<0>"
        4.
    • vector flag

      This flag tells Perl to interpret the supplied string as a vector ofintegers, one for each character in the string. Perl applies the format toeach integer in turn, then joins the resulting strings with a separator (adot . by default). This can be useful for displaying ordinal values ofcharacters in arbitrary strings:

      1.   printf "%vd", "AB\x{100}";   # prints "65.66.256"
        2.  
      2.   printf "version is v%vd\n", $^V; # Perl's version
        3.

      Put an asterisk * before the v to override the string touse to separate the numbers:

      1.   printf "address is %*vX\n", ":", $addr;   # IPv6 address
        2.  
      2.   printf "bits are %0*v8b\n", " ", $bits;   # random bitstring
        3.

      You can also explicitly specify the argument number to use forthe join string using something like *2$v; for example:

      1.   printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":";   # 3 IPv6 addresses
        2.
    • (minimum) width

      Arguments are usually formatted to be only as wide as required todisplay the given value. You can override the width by puttinga number here, or get the width from the next argument (with *)or from a specified argument (e.g., with *2$):

      1.   printf "<%s>", "a";   # prints "<a>"
        2.  
      2.   printf "<%6s>", "a";  # prints "< a>"
        3.  
      3.   printf "<%*s>", 6, "a";   # prints "< a>"
        4.  
      4.   printf "<%*2$s>", "a", 6; # prints "< a>"
        5.  
      5.   printf "<%2s>", "long";   # prints "<long>" (does not truncate)
        6.

      If a field width obtained through * is negative, it has the sameeffect as the - flag: left-justification.

    • precision, or maximum width

      You can specify a precision (for numeric conversions) or a maximumwidth (for string conversions) by specifying a . followed by a number.For floating-point formats except g and G, this specifieshow many places right of the decimal point to show (the default being 6).For example:

      1.   # these examples are subject to system-specific variation
        2.  
      2.   printf '<%f>', 1; # prints "<1.000000>"
        3.  
      3.   printf '<%.1f>', 1;  # prints "<1.0>"
        4.  
      4.   printf '<%.0f>', 1;  # prints "<1>"
        5.  
      5.   printf '<%e>', 10;   # prints "<1.000000e+01>"
        6.  
      6.   printf '<%.1e>', 10; # prints "<1.0e+01>"
        7.

      For "g" and "G", this specifies the maximum number of digits to show,including those prior to the decimal point and those after it; for example:

      1.   # These examples are subject to system-specific variation.
        2.  
      2.   printf '<%g>', 1; # prints "<1>"
        3.  
      3.   printf '<%.10g>', 1; # prints "<1>"
        4.  
      4.   printf '<%g>', 100;  # prints "<100>"
        5.  
      5.   printf '<%.1g>', 100; # prints "<1e+02>"
        6.  
      6.   printf '<%.2g>', 100.01; # prints "<1e+02>"
        7.  
      7.   printf '<%.5g>', 100.01; # prints "<100.01>"
        8.  
      8.   printf '<%.4g>', 100.01; # prints "<100>"
        9.

      For integer conversions, specifying a precision implies that theoutput of the number itself should be zero-padded to this width,where the 0 flag is ignored:

      1.   printf '<%.6d>', 1;  # prints "<000001>"
        2.  
      2.   printf '<%+.6d>', 1; # prints "<+000001>"
        3.  
      3.   printf '<%-10.6d>', 1;   # prints "<000001 >"
        4.  
      4.   printf '<%10.6d>', 1; # prints "< 000001>"
        5.  
      5.   printf '<%010.6d>', 1;   # prints "< 000001>"
        6.  
      6.   printf '<%+10.6d>', 1;   # prints "<   +000001>"
        7.  
      7.  
      8.   printf '<%.6x>', 1;  # prints "<000001>"
        9.  
      9.   printf '<%#.6x>', 1; # prints "<0x000001>"
        10.  
      10.   printf '<%-10.6x>', 1;   # prints "<000001 >"
        11.  
      11.   printf '<%10.6x>', 1; # prints "< 000001>"
        12.  
      12.   printf '<%010.6x>', 1;   # prints "< 000001>"
        13.  
      13.   printf '<%#10.6x>', 1;   # prints "<  0x000001>"
        14.

      For string conversions, specifying a precision truncates the stringto fit the specified width:

      1.   printf '<%.5s>', "truncated";   # prints "<trunc>"
        2.  
      2.   printf '<%10.5s>', "truncated"; # prints "< trunc>"
        3.

      You can also get the precision from the next argument using .*:

      1.   printf '<%.6x>', 1;   # prints "<000001>"
        2.  
      2.   printf '<%.*x>', 6, 1; # prints "<000001>"
        3.

      If a precision obtained through * is negative, it countsas having no precision at all.

      1.   printf '<%.*s>',  7, "string";   # prints "<string>"
        2.  
      2.   printf '<%.*s>',  3, "string";   # prints "<str>"
        3.  
      3.   printf '<%.*s>',  0, "string";   # prints "<>"
        4.  
      4.   printf '<%.*s>', -1, "string";   # prints "<string>"
        5.  
      5.  
      6.   printf '<%.*d>',  1, 0;   # prints "<0>"
        7.  
      7.   printf '<%.*d>',  0, 0;   # prints "<>"
        8.  
      8.   printf '<%.*d>', -1, 0;   # prints "<0>"
        9.

      You cannot currently get the precision from a specified number,but it is intended that this will be possible in the future, forexample using .*2$:

      1.   printf "<%.*2$x>", 1, 6;   # INVALID, but in future will print "<000001>"
        2.
    • size

      For numeric conversions, you can specify the size to interpret thenumber as using l, h, V, q, L, or ll. For integerconversions (d u o x X b i D U O), numbers are usually assumed to bewhatever the default integer size is on your platform (usually 32 or 64bits), but you can override this to use instead one of the standard C types,as supported by the compiler used to build Perl:

      1.    hh  interpret integer as C type "char" or "unsigned char"
        2.  
      2.    on Perl 5.14 or later
        3.  
      3.    h   interpret integer as C type "short" or "unsigned short"
        4.  
      4.    j   interpret integer as C type "intmax_t" on Perl 5.14 
        5.  
      5.    or later, and only with a C99 compiler (unportable)
        6.  
      6.    l   interpret integer as C type "long" or "unsigned long"
        7.  
      7.    q, L, or ll interpret integer as C type "long long", "unsigned long long",
        8.  
      8.    or "quad" (typically 64-bit integers)
        9.  
      9.    t   interpret integer as C type "ptrdiff_t" on Perl 5.14 or later
        10.  
      10.    z   interpret integer as C type "size_t" on Perl 5.14 or later
        11.

      As of 5.14, none of these raises an exception if they are not supported onyour platform. However, if warnings are enabled, a warning of theprintf warning class is issued on an unsupported conversion flag. Should you instead prefer an exception, do this:

      1.  use warnings FATAL => "printf";
        2.

      If you would like to know about a version dependency before youstart running the program, put something like this at its top:

      1.  use 5.014;  # for hh/j/t/z/ printf modifiers
        2.

      You can find out whether your Perl supports quads via Config:

      1.  use Config;
        2.  
      2.  if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
        3.  
      3.  print "Nice quads!\n";
        4.  
      4.  }
        5.

      For floating-point conversions (e f g E F G), numbers are usually assumedto be the default floating-point size on your platform (double or long double),but you can force "long double" with q, L, or ll if yourplatform supports them. You can find out whether your Perl supports longdoubles via Config:

      1.  use Config;
        2.  
      2.  print "long doubles\n" if $Config{d_longdbl} eq "define";
        3.

      You can find out whether Perl considers "long double" to be the defaultfloating-point size to use on your platform via Config:

      1.  use Config;
        2.  
      2.  if ($Config{uselongdouble} eq "define") {
        3.  
      3. print "long doubles by default\n";
        4.  
      4.  }
        5.

      It can also be that long doubles and doubles are the same thing:

      1.  use Config;
        2.  
      2.  ($Config{doublesize} == $Config{longdblsize}) &&
        3.  
      3.  print "doubles are long doubles\n";
        4.

      The size specifier V has no effect for Perl code, but is supported forcompatibility with XS code. It means "use the standard size for a Perlinteger or floating-point number", which is the default.

    • order of arguments

      Normally, sprintf() takes the next unused argument as the value toformat for each format specification. If the format specificationuses * to require additional arguments, these are consumed fromthe argument list in the order they appear in the formatspecification before the value to format. Where an argument isspecified by an explicit index, this does not affect the normalorder for the arguments, even when the explicitly specified indexwould have been the next argument.

      So:

      1.  printf "<%*.*s>", $a, $b, $c;
        2.

      uses $a for the width, $b for the precision, and $cas the value to format; while:

      1.   printf "<%*1$.*s>", $a, $b;
        2.

      would use $a for the width and precision, and $b as thevalue to format.

      Here are some more examples; be aware that when using an explicitindex, the $ may need escaping:

      1.   printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
        2.  
      2.   printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
        3.  
      3.   printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
        4.  
      4.   printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
        5.

    If use locale (including use locale 'not_characters') is in effectand POSIX::setlocale() has been called,the character used for the decimal separator in formatted floating-pointnumbers is affected by the LC_NUMERIC locale. See perllocaleand POSIX.

  • sqrt EXPR
  • sqrt

    Return the positive square root of EXPR. If EXPR is omitted, uses$_. Works only for non-negative operands unless you'veloaded the Math::Complex module.

    1.  use Math::Complex;
      2.  
    2.  print sqrt(-4); # prints 2i
      3.
  • srand EXPR
  • srand

    Sets and returns the random number seed for the rand operator.

    The point of the function is to "seed" the rand function so that randcan produce a different sequence each time you run your program. Whencalled with a parameter, srand uses that for the seed; otherwise it(semi-)randomly chooses a seed. In either case, starting with Perl 5.14,it returns the seed. To signal that your code will work only on Perlsof a recent vintage:

    1.  use 5.014;# so srand returns the seed
      2.

    If srand() is not called explicitly, it is called implicitly without aparameter at the first use of the rand operator. However, this was not trueof versions of Perl before 5.004, so if your script will run under olderPerl versions, it should call srand; otherwise most programs won't callsrand() at all.

    But there are a few situations in recent Perls where programs are likely towant to call srand. One is for generating predictable results generally fortesting or debugging. There, you use srand($seed), with the same $seedeach time. Another case is that you may want to call srand()after a fork() to avoid child processes sharing the same seed value as theparent (and consequently each other).

    Do not call srand() (i.e., without an argument) more than once perprocess. The internal state of the random number generator shouldcontain more entropy than can be provided by any seed, so callingsrand() again actually loses randomness.

    Most implementations of srand take an integer and will silentlytruncate decimal numbers. This means srand(42) will usuallyproduce the same results as srand(42.1). To be safe, always passsrand an integer.

    In versions of Perl prior to 5.004 the default seed was just thecurrent time. This isn't a particularly good seed, so many oldprograms supply their own seed value (often time ^ $$ or time ^($$ + ($$ << 15))), but that isn't necessary any more.

    Frequently called programs (like CGI scripts) that simply use

    1.  time ^ $$
      2.

    for a seed can fall prey to the mathematical property that

    1.  a^b == (a+1)^(b+1)
      2.

    one-third of the time. So don't do that.

    A typical use of the returned seed is for a test program which has too manycombinations to test comprehensively in the time available to it each run. Itcan test a random subset each time, and should there be a failure, log the seedused for that run so that it can later be used to reproduce the same results.

    rand() is not cryptographically secure. You should not relyon it in security-sensitive situations. As of this writing, anumber of third-party CPAN modules offer random number generatorsintended by their authors to be cryptographically secure,including: Data::Entropy, Crypt::Random, Math::Random::Secure,and Math::TrulyRandom.

  • stat FILEHANDLE
  • stat EXPR
  • stat DIRHANDLE
  • stat

    Returns a 13-element list giving the status info for a file, eitherthe file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is omitted, it stats $_ (not _!). Returns the empty list if stat fails. Typicallyused as follows:

    1.  ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
      2.  
    2.    $atime,$mtime,$ctime,$blksize,$blocks)
      3.  
    3.    = stat($filename);
      4.

    Not all fields are supported on all filesystem types. Here are themeanings of the fields:

    1.   0 dev  device number of filesystem
      2.  
    2.   1 ino  inode number
      3.  
    3.   2 mode file mode  (type and permissions)
      4.  
    4.   3 nlink number of (hard) links to the file
      5.  
    5.   4 uid  numeric user ID of file's owner
      6.  
    6.   5 gid  numeric group ID of file's owner
      7.  
    7.   6 rdev the device identifier (special files only)
      8.  
    8.   7 size total size of file, in bytes
      9.  
    9.   8 atime last access time in seconds since the epoch
      10.  
    10.   9 mtime last modify time in seconds since the epoch
      11.  
    11.  10 ctime inode change time in seconds since the epoch (*)
      12.  
    12.  11 blksize  preferred block size for file system I/O
      13.  
    13.  12 blocks   actual number of blocks allocated
      14.

    (The epoch was at 00:00 January 1, 1970 GMT.)

    (*) Not all fields are supported on all filesystem types. Notably, thectime field is non-portable. In particular, you cannot expect it to be a"creation time"; see Files and Filesystems in perlport for details.

    If stat is passed the special filehandle consisting of an underline, nostat is done, but the current contents of the stat structure from thelast stat, lstat, or filetest are returned. Example:

    1.  if (-x $file && (($d) = stat(_)) && $d < 0) {
      2.  
    2.  print "$file is executable NFS file\n";
      3.  
    3.  }
      4.

    (This works on machines only for which the device number is negativeunder NFS.)

    Because the mode contains both the file type and its permissions, youshould mask off the file type portion and (s)printf using a "%o"if you want to see the real permissions.

    1.  $mode = (stat($filename))[2];
      2.  
    2.  printf "Permissions are %04o\n", $mode & 07777;
      3.

    In scalar context, stat returns a boolean value indicating successor failure, and, if successful, sets the information associated withthe special filehandle _.

    The File::stat module provides a convenient, by-name access mechanism:

    1.  use File::stat;
      2.  
    2.  $sb = stat($filename);
      3.  
    3.  printf "File is %s, size is %s, perm %04o, mtime %s\n",
      4.  
    4.    $filename, $sb->size, $sb->mode & 07777,
      5.  
    5.    scalar localtime $sb->mtime;
      6.

    You can import symbolic mode constants (S_IF*) and functions(S_IS*) from the Fcntl module:

    1.  use Fcntl ':mode';
      2.  
    2.  
    3.  $mode = (stat($filename))[2];
      4.  
    4.  
    5.  $user_rwx  = ($mode & S_IRWXU) >> 6;
      6.  
    6.  $group_read = ($mode & S_IRGRP) >> 3;
      7.  
    7.  $other_execute =  $mode & S_IXOTH;
      8.  
    8.  
    9.  printf "Permissions are %04o\n", S_IMODE($mode), "\n";
      10.  
    10.  
    11.  $is_setuid =  $mode & S_ISUID;
      12.  
    12.  $is_directory  =  S_ISDIR($mode);
      13.

    You could write the last two using the -u and -d operators.Commonly available S_IF* constants are:

    1.  # Permissions: read, write, execute, for user, group, others.
      2.  
    2.  
    3.  S_IRWXU S_IRUSR S_IWUSR S_IXUSR
      4.  
    4.  S_IRWXG S_IRGRP S_IWGRP S_IXGRP
      5.  
    5.  S_IRWXO S_IROTH S_IWOTH S_IXOTH
      6.  
    6.  
    7.  # Setuid/Setgid/Stickiness/SaveText.
      8.  
    8.  # Note that the exact meaning of these is system-dependent.
      9.  
    9.  
    10.  S_ISUID S_ISGID S_ISVTX S_ISTXT
      11.  
    11.  
    12.  # File types.  Not all are necessarily available on
      13.  
    13.  # your system.
      14.  
    14.  
    15.  S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
      16.  
    16.  S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
      17.  
    17.  
    18.  # The following are compatibility aliases for S_IRUSR,
      19.  
    19.  # S_IWUSR, and S_IXUSR.
      20.  
    20.  
    21.  S_IREAD S_IWRITE S_IEXEC
      22.

    and the S_IF* functions are

    1.  S_IMODE($mode) the part of $mode containing the permission
      2.  
    2.   bits and the setuid/setgid/sticky bits
      3.  
    3.  
    4.  S_IFMT($mode) the part of $mode containing the file type
      5.  
    5.   which can be bit-anded with (for example)
      6.  
    6.   S_IFREG or with the following functions
      7.  
    7.  
    8.  # The operators -f, -d, -l, -b, -c, -p, and -S.
      9.  
    9.  
    10.  S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
      11.  
    11.  S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
      12.  
    12.  
    13.  # No direct -X operator counterpart, but for the first one
      14.  
    14.  # the -g operator is often equivalent.  The ENFMT stands for
      15.  
    15.  # record flocking enforcement, a platform-dependent feature.
      16.  
    16.  
    17.  S_ISENFMT($mode) S_ISWHT($mode)
      18.

    See your native chmod(2) and stat(2) documentation for more detailsabout the S_* constants. To get status info for a symbolic linkinstead of the target file behind the link, use the lstat function.

    Portability issues: stat in perlport.

  • state EXPR
  • state TYPE EXPR
  • state EXPR : ATTRS
  • state TYPE EXPR : ATTRS

    state declares a lexically scoped variable, just like my.However, those variables will never be reinitialized, contrary tolexical variables that are reinitialized each time their enclosing blockis entered.See Persistent Private Variables in perlsub for details.

    state variables are enabled only when the use feature "state" pragma is in effect, unless the keyword is written as CORE::state.See also feature.

  • study SCALAR
  • study

    Takes extra time to study SCALAR ($_ if unspecified) in anticipation ofdoing many pattern matches on the string before it is next modified.This may or may not save time, depending on the nature and number ofpatterns you are searching and the distribution of characterfrequencies in the string to be searched; you probably want to comparerun times with and without it to see which is faster. Those loopsthat scan for many short constant strings (including the constantparts of more complex patterns) will benefit most.(The way study works is this: a linked list of everycharacter in the string to be searched is made, so we know, forexample, where all the 'k' characters are. From each search string,the rarest character is selected, based on some static frequency tablesconstructed from some C programs and English text. Only those placesthat contain this "rarest" character are examined.)

    For example, here is a loop that inserts index producing entriesbefore any line containing a certain pattern:

    1.  while (<>) {
      2.  
    2.  study;
      3.  
    3.  print ".IX foo\n" if /\bfoo\b/;
      4.  
    4.  print ".IX bar\n" if /\bbar\b/;
      5.  
    5.  print ".IX blurfl\n" if /\bblurfl\b/;
      6.  
    6.  # ...
      7.  
    7.  print;
      8.  
    8.  }
      9.

    In searching for /\bfoo\b/, only locations in $_ that contain fwill be looked at, because f is rarer than o. In general, this isa big win except in pathological cases. The only question is whetherit saves you more time than it took to build the linked list in thefirst place.

    Note that if you have to look for strings that you don't know tillruntime, you can build an entire loop as a string and eval that toavoid recompiling all your patterns all the time. Together withundefining $/ to input entire files as one record, this can be quitefast, often faster than specialized programs like fgrep(1). The followingscans a list of files (@files) for a list of words (@words), and printsout the names of those files that contain a match:

    1.  $search = 'while (<>) { study;';
      2.  
    2.  foreach $word (@words) {
      3.  
    3.  $search .= "++\$seen{\$ARGV} if /\b$word\b/;\n";
      4.  
    4.  }
      5.  
    5.  $search .= "}";
      6.  
    6.  @ARGV = @files;
      7.  
    7.  undef $/;
      8.  
    8.  eval $search; # this screams
      9.  
    9.  $/ = "\n"; # put back to normal input delimiter
      10.  
    10.  foreach $file (sort keys(%seen)) {
      11.  
    11.  print $file, "\n";
      12.  
    12.  }
      13.
  • sub NAME BLOCK
  • sub NAME (PROTO) BLOCK
  • sub NAME : ATTRS BLOCK
  • sub NAME (PROTO) : ATTRS BLOCK

    This is subroutine definition, not a real function per se. Without aBLOCK it's just a forward declaration. Without a NAME, it's an anonymousfunction declaration, so does return a value: the CODE ref of the closurejust created.

    See perlsub and perlref for details about subroutines andreferences; see attributes and Attribute::Handlers for moreinformation about attributes.

  • __SUB__

    A special token that returns the a reference to the current subroutine, orundef outside of a subroutine.

    This token is only available under use v5.16 or the "current_sub"feature. See feature.

  • substr EXPR,OFFSET,LENGTH,REPLACEMENT
  • substr EXPR,OFFSET,LENGTH
  • substr EXPR,OFFSET

    Extracts a substring out of EXPR and returns it. First character is atoffset zero. If OFFSET is negative, startsthat far back from the end of the string. If LENGTH is omitted, returnseverything through the end of the string. If LENGTH is negative, leaves thatmany characters off the end of the string.

    1.  my $s = "The black cat climbed the green tree";
      2.  
    2.  my $color  = substr $s, 4, 5;  # black
      3.  
    3.  my $middle = substr $s, 4, -11; # black cat climbed the
      4.  
    4.  my $end = substr $s, 14; # climbed the green tree
      5.  
    5.  my $tail   = substr $s, -4; # tree
      6.  
    6.  my $z  = substr $s, -4, 2; # tr
      7.

    You can use the substr() function as an lvalue, in which case EXPRmust itself be an lvalue. If you assign something shorter than LENGTH,the string will shrink, and if you assign something longer than LENGTH,the string will grow to accommodate it. To keep the string the samelength, you may need to pad or chop your value using sprintf.

    If OFFSET and LENGTH specify a substring that is partly outside thestring, only the part within the string is returned. If the substringis beyond either end of the string, substr() returns the undefinedvalue and produces a warning. When used as an lvalue, specifying asubstring that is entirely outside the string raises an exception.Here's an example showing the behavior for boundary cases:

    1.  my $name = 'fred';
      2.  
    2.  substr($name, 4) = 'dy'; # $name is now 'freddy'
      3.  
    3.  my $null = substr $name, 6, 2;   # returns "" (no warning)
      4.  
    4.  my $oops = substr $name, 7;  # returns undef, with warning
      5.  
    5.  substr($name, 7) = 'gap'; # raises an exception
      6.

    An alternative to using substr() as an lvalue is to specify thereplacement string as the 4th argument. This allows you to replaceparts of the EXPR and return what was there before in one operation,just as you can with splice().

    1.  my $s = "The black cat climbed the green tree";
      2.  
    2.  my $z = substr $s, 14, 7, "jumped from"; # climbed
      3.  
    3.  # $s is now "The black cat jumped from the green tree"
      4.

    Note that the lvalue returned by the three-argument version of substr() acts asa 'magic bullet'; each time it is assigned to, it remembers which partof the original string is being modified; for example:

    1.  $x = '1234';
      2.  
    2.  for (substr($x,1,2)) {
      3.  
    3.  $_ = 'a';   print $x,"\n"; # prints 1a4
      4.  
    4.  $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
      5.  
    5.  $x = '56789';
      6.  
    6.  $_ = 'pq';  print $x,"\n"; # prints 5pq9
      7.  
    7.  }
      8.

    With negative offsets, it remembers its position from the end of the stringwhen the target string is modified:

    1.  $x = '1234';
      2.  
    2.  for (substr($x, -3, 2)) {
      3.  
    3.  $_ = 'a';   print $x,"\n"; # prints 1a4, as above
      4.  
    4.  $x = 'abcdefg';
      5.  
    5.  print $_,"\n"; # prints f
      6.  
    6.  }
      7.

    Prior to Perl version 5.10, the result of using an lvalue multiple times wasunspecified. Prior to 5.16, the result with negative offsets wasunspecified.

  • symlink OLDFILE,NEWFILE

    Creates a new filename symbolically linked to the old filename.Returns 1 for success, 0 otherwise. On systems that don't supportsymbolic links, raises an exception. To check for that,use eval:

    1.  $symlink_exists = eval { symlink("",""); 1 };
      2.

    Portability issues: symlink in perlport.

  • syscall NUMBER, LIST

    Calls the system call specified as the first element of the list,passing the remaining elements as arguments to the system call. Ifunimplemented, raises an exception. The arguments are interpretedas follows: if a given argument is numeric, the argument is passed asan int. If not, the pointer to the string value is passed. You areresponsible to make sure a string is pre-extended long enough toreceive any result that might be written into a string. You can't use astring literal (or other read-only string) as an argument to syscallbecause Perl has to assume that any string pointer might be writtenthrough. If yourinteger arguments are not literals and have never been interpreted in anumeric context, you may need to add 0 to them to force them to looklike numbers. This emulates the syswrite function (or vice versa):

    1.  require 'syscall.ph'; # may need to run h2ph
      2.  
    2.  $s = "hi there\n";
      3.  
    3.  syscall(&SYS_write, fileno(STDOUT), $s, length $s);
      4.

    Note that Perl supports passing of up to only 14 arguments to your syscall,which in practice should (usually) suffice.

    Syscall returns whatever value returned by the system call it calls.If the system call fails, syscall returns -1 and sets $! (errno).Note that some system calls can legitimately return -1. The properway to handle such calls is to assign $!=0 before the call, thencheck the value of $! if syscall returns -1.

    There's a problem with syscall(&SYS_pipe): it returns the filenumber of the read end of the pipe it creates, but there is no wayto retrieve the file number of the other end. You can avoid thisproblem by using pipe instead.

    Portability issues: syscall in perlport.

  • sysopen FILEHANDLE,FILENAME,MODE
  • sysopen FILEHANDLE,FILENAME,MODE,PERMS

    Opens the file whose filename is given by FILENAME, and associates it withFILEHANDLE. If FILEHANDLE is an expression, its value is used as the realfilehandle wanted; an undefined scalar will be suitably autovivified. Thisfunction calls the underlying operating system's open(2) function with theparameters FILENAME, MODE, and PERMS.

    The possible values and flag bits of the MODE parameter aresystem-dependent; they are available via the standard module Fcntl. Seethe documentation of your operating system's open(2) syscall to seewhich values and flag bits are available. You may combine several flagsusing the |-operator.

    Some of the most common values are O_RDONLY for opening the file inread-only mode, O_WRONLY for opening the file in write-only mode,and O_RDWR for opening the file in read-write mode.

    For historical reasons, some values work on almost every systemsupported by Perl: 0 means read-only, 1 means write-only, and 2means read/write. We know that these values do not work underOS/390 & VM/ESA Unix and on the Macintosh; you probably don't want touse them in new code.

    If the file named by FILENAME does not exist and the open call createsit (typically because MODE includes the O_CREAT flag), then the value ofPERMS specifies the permissions of the newly created file. If you omitthe PERMS argument to sysopen, Perl uses the octal value 0666.These permission values need to be in octal, and are modified by yourprocess's current umask.

    In many systems the O_EXCL flag is available for opening files inexclusive mode. This is not locking: exclusiveness means here thatif the file already exists, sysopen() fails. O_EXCL may not workon network filesystems, and has no effect unless the O_CREAT flagis set as well. Setting O_CREAT|O_EXCL prevents the file frombeing opened if it is a symbolic link. It does not protect againstsymbolic links in the file's path.

    Sometimes you may want to truncate an already-existing file. Thiscan be done using the O_TRUNC flag. The behavior ofO_TRUNC with O_RDONLY is undefined.

    You should seldom if ever use 0644 as argument to sysopen, becausethat takes away the user's option to have a more permissive umask.Better to omit it. See the perlfunc(1) entry on umask for moreon this.

    Note that sysopen depends on the fdopen() C library function.On many Unix systems, fdopen() is known to fail when file descriptorsexceed a certain value, typically 255. If you need more filedescriptors than that, consider rebuilding Perl to use the sfiolibrary, or perhaps using the POSIX::open() function.

    See perlopentut for a kinder, gentler explanation of opening files.

    Portability issues: sysopen in perlport.

  • sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
  • sysread FILEHANDLE,SCALAR,LENGTH

    Attempts to read LENGTH bytes of data into variable SCALAR from thespecified FILEHANDLE, using the read(2). It bypassesbuffered IO, so mixing this with other kinds of reads, print,write, seek, tell, or eof can cause confusion because theperlio or stdio layers usually buffers data. Returns the number ofbytes actually read, 0 at end of file, or undef if there was anerror (in the latter case $! is also set). SCALAR will be grown orshrunk so that the last byte actually read is the last byte of thescalar after the read.

    An OFFSET may be specified to place the read data at some place in thestring other than the beginning. A negative OFFSET specifiesplacement at that many characters counting backwards from the end ofthe string. A positive OFFSET greater than the length of SCALARresults in the string being padded to the required size with "\0"bytes before the result of the read is appended.

    There is no syseof() function, which is ok, since eof() doesn't workwell on device files (like ttys) anyway. Use sysread() and checkfor a return value for 0 to decide whether you're done.

    Note that if the filehandle has been marked as :utf8 Unicodecharacters are read instead of bytes (the LENGTH, OFFSET, and thereturn value of sysread() are in Unicode characters).The :encoding(...) layer implicitly introduces the :utf8 layer.See binmode, open, and the open pragma, open.

  • sysseek FILEHANDLE,POSITION,WHENCE

    Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE maybe an expression whose value gives the name of the filehandle. The valuesfor WHENCE are 0 to set the new position to POSITION; 1 to set the itto the current position plus POSITION; and 2 to set it to EOF plusPOSITION, typically negative.

    Note the in bytes: even if the filehandle has been set to operateon characters (for example by using the :encoding(utf8) I/O layer),tell() will return byte offsets, not character offsets (becauseimplementing that would render sysseek() unacceptably slow).

    sysseek() bypasses normal buffered IO, so mixing it with reads otherthan sysread (for example <> or read()) print, write,seek, tell, or eof may cause confusion.

    For WHENCE, you may also use the constants SEEK_SET, SEEK_CUR,and SEEK_END (start of the file, current position, end of the file)from the Fcntl module. Use of the constants is also more portablethan relying on 0, 1, and 2. For example to define a "systell" function:

    1.  use Fcntl 'SEEK_CUR';
      2.  
    2.  sub systell { sysseek($_[0], 0, SEEK_CUR) }
      3.

    Returns the new position, or the undefined value on failure. A positionof zero is returned as the string "0 but true"; thus sysseek returnstrue on success and false on failure, yet you can still easily determinethe new position.

  • system LIST
  • system PROGRAM LIST

    Does exactly the same thing as exec LIST, except that a fork isdone first and the parent process waits for the child process toexit. Note that argument processing varies depending on thenumber of arguments. If there is more than one argument in LIST,or if LIST is an array with more than one value, starts the programgiven by the first element of the list with arguments given by therest of the list. If there is only one scalar argument, the argumentis checked for shell metacharacters, and if there are any, theentire argument is passed to the system's command shell for parsing(this is /bin/sh -c on Unix platforms, but varies on otherplatforms). If there are no shell metacharacters in the argument,it is split into words and passed directly to execvp, which ismore efficient.

    Beginning with v5.6.0, Perl will attempt to flush all files opened foroutput before any operation that may do a fork, but this may not besupported on some platforms (see perlport). To be safe, you may needto set $| ($AUTOFLUSH in English) or call the autoflush() methodof IO::Handle on any open handles.

    The return value is the exit status of the program as returned by thewait call. To get the actual exit value, shift right by eight (seebelow). See also exec. This is not what you want to use to capturethe output from a command; for that you should use merely backticks orqx//, as described in `STRING` in perlop. Return value of -1indicates a failure to start the program or an error of the wait(2) systemcall (inspect $! for the reason).

    If you'd like to make system (and many other bits of Perl) die on error,have a look at the autodie pragma.

    Like exec, system allows you to lie to a program about its name ifyou use the system PROGRAM LIST syntax. Again, see exec.

    Since SIGINT and SIGQUIT are ignored during the execution ofsystem, if you expect your program to terminate on receipt of thesesignals you will need to arrange to do so yourself based on the returnvalue.

    1.  @args = ("command", "arg1", "arg2");
      2.  
    2.  system(@args) == 0
      3.  
    3.  or die "system @args failed: $?"
      4.

    If you'd like to manually inspect system's failure, you can check allpossible failure modes by inspecting $? like this:

    1.  if ($? == -1) {
      2.  
    2.  print "failed to execute: $!\n";
      3.  
    3.  }
      4.  
    4.  elsif ($? & 127) {
      5.  
    5.  printf "child died with signal %d, %s coredump\n",
      6.  
    6.  ($? & 127),  ($? & 128) ? 'with' : 'without';
      7.  
    7.  }
      8.  
    8.  else {
      9.  
    9.  printf "child exited with value %d\n", $? >> 8;
      10.  
    10.  }
      11.

    Alternatively, you may inspect the value of ${^CHILD_ERROR_NATIVE}with the W*() calls from the POSIX module.

    When system's arguments are executed indirectly by the shell, results and return codes are subject to its quirks.See `STRING` in perlop and exec for details.

    Since system does a fork and wait it may affect a SIGCHLDhandler. See perlipc for details.

    Portability issues: system in perlport.

  • syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
  • syswrite FILEHANDLE,SCALAR,LENGTH
  • syswrite FILEHANDLE,SCALAR

    Attempts to write LENGTH bytes of data from variable SCALAR to thespecified FILEHANDLE, using write(2). If LENGTH isnot specified, writes whole SCALAR. It bypasses buffered IO, somixing this with reads (other than sysread()), print, write,seek, tell, or eof may cause confusion because the perlio andstdio layers usually buffer data. Returns the number of bytesactually written, or undef if there was an error (in this case theerrno variable $! is also set). If the LENGTH is greater than thedata available in the SCALAR after the OFFSET, only as much data as isavailable will be written.

    An OFFSET may be specified to write the data from some part of thestring other than the beginning. A negative OFFSET specifies writingthat many characters counting backwards from the end of the string.If SCALAR is of length zero, you can only use an OFFSET of 0.

    WARNING: If the filehandle is marked :utf8, Unicode charactersencoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, andreturn value of syswrite() are in (UTF8-encoded Unicode) characters.The :encoding(...) layer implicitly introduces the :utf8 layer.Alternately, if the handle is not marked with an encoding but youattempt to write characters with code points over 255, raises an exception.See binmode, open, and the open pragma, open.

  • tell FILEHANDLE
  • tell

    Returns the current position in bytes for FILEHANDLE, or -1 onerror. FILEHANDLE may be an expression whose value gives the name ofthe actual filehandle. If FILEHANDLE is omitted, assumes the filelast read.

    Note the in bytes: even if the filehandle has been set tooperate on characters (for example by using the :encoding(utf8) openlayer), tell() will return byte offsets, not character offsets (becausethat would render seek() and tell() rather slow).

    The return value of tell() for the standard streams like the STDINdepends on the operating system: it may return -1 or something else.tell() on pipes, fifos, and sockets usually returns -1.

    There is no systell function. Use sysseek(FH, 0, 1) for that.

    Do not use tell() (or other buffered I/O operations) on a filehandlethat has been manipulated by sysread(), syswrite(), or sysseek().Those functions ignore the buffering, while tell() does not.

  • telldir DIRHANDLE

    Returns the current position of the readdir routines on DIRHANDLE.Value may be given to seekdir to access a particular location in adirectory. telldir has the same caveats about possible directorycompaction as the corresponding system library routine.

  • tie VARIABLE,CLASSNAME,LIST

    This function binds a variable to a package class that will provide theimplementation for the variable. VARIABLE is the name of the variableto be enchanted. CLASSNAME is the name of a class implementing objectsof correct type. Any additional arguments are passed to the newmethod of the class (meaning TIESCALAR, TIEHANDLE, TIEARRAY,or TIEHASH). Typically these are arguments such as might be passedto the dbm_open() function of C. The object returned by the newmethod is also returned by the tie function, which would be usefulif you want to access other methods in CLASSNAME.

    Note that functions such as keys and values may return huge listswhen used on large objects, like DBM files. You may prefer to use theeach function to iterate over such. Example:

    1.  # print out history file offsets
      2.  
    2.  use NDBM_File;
      3.  
    3.  tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
      4.  
    4.  while (($key,$val) = each %HIST) {
      5.  
    5.  print $key, ' = ', unpack('L',$val), "\n";
      6.  
    6.  }
      7.  
    7.  untie(%HIST);
      8.

    A class implementing a hash should have the following methods:

    1.  TIEHASH classname, LIST
      2.  
    2.  FETCH this, key
      3.  
    3.  STORE this, key, value
      4.  
    4.  DELETE this, key
      5.  
    5.  CLEAR this
      6.  
    6.  EXISTS this, key
      7.  
    7.  FIRSTKEY this
      8.  
    8.  NEXTKEY this, lastkey
      9.  
    9.  SCALAR this
      10.  
    10.  DESTROY this
      11.  
    11.  UNTIE this
      12.

    A class implementing an ordinary array should have the following methods:

    1.  TIEARRAY classname, LIST
      2.  
    2.  FETCH this, key
      3.  
    3.  STORE this, key, value
      4.  
    4.  FETCHSIZE this
      5.  
    5.  STORESIZE this, count
      6.  
    6.  CLEAR this
      7.  
    7.  PUSH this, LIST
      8.  
    8.  POP this
      9.  
    9.  SHIFT this
      10.  
    10.  UNSHIFT this, LIST
      11.  
    11.  SPLICE this, offset, length, LIST
      12.  
    12.  EXTEND this, count
      13.  
    13.  DESTROY this
      14.  
    14.  UNTIE this
      15.

    A class implementing a filehandle should have the following methods:

    1.  TIEHANDLE classname, LIST
      2.  
    2.  READ this, scalar, length, offset
      3.  
    3.  READLINE this
      4.  
    4.  GETC this
      5.  
    5.  WRITE this, scalar, length, offset
      6.  
    6.  PRINT this, LIST
      7.  
    7.  PRINTF this, format, LIST
      8.  
    8.  BINMODE this
      9.  
    9.  EOF this
      10.  
    10.  FILENO this
      11.  
    11.  SEEK this, position, whence
      12.  
    12.  TELL this
      13.  
    13.  OPEN this, mode, LIST
      14.  
    14.  CLOSE this
      15.  
    15.  DESTROY this
      16.  
    16.  UNTIE this
      17.

    A class implementing a scalar should have the following methods:

    1.  TIESCALAR classname, LIST
      2.  
    2.  FETCH this,
      3.  
    3.  STORE this, value
      4.  
    4.  DESTROY this
      5.  
    5.  UNTIE this
      6.

    Not all methods indicated above need be implemented. See perltie,Tie::Hash, Tie::Array, Tie::Scalar, and Tie::Handle.

    Unlike dbmopen, the tie function will not use or require a modulefor you; you need to do that explicitly yourself. See DB_Fileor the Config module for interesting tie implementations.

    For further details see perltie, tied VARIABLE.

  • tied VARIABLE

    Returns a reference to the object underlying VARIABLE (the same valuethat was originally returned by the tie call that bound the variableto a package.) Returns the undefined value if VARIABLE isn't tied to apackage.

  • time

    Returns the number of non-leap seconds since whatever time the systemconsiders to be the epoch, suitable for feeding to gmtime andlocaltime. On most systems the epoch is 00:00:00 UTC, January 1, 1970;a prominent exception being Mac OS Classic which uses 00:00:00, January 1,1904 in the current local time zone for its epoch.

    For measuring time in better granularity than one second, use theTime::HiRes module from Perl 5.8 onwards (or from CPAN before then), or,if you have gettimeofday(2), you may be able to use the syscallinterface of Perl. See perlfaq8 for details.

    For date and time processing look at the many related modules on CPAN.For a comprehensive date and time representation look at theDateTime module.

  • times

    Returns a four-element list giving the user and system times inseconds for this process and any exited children of this process.

    1.  ($user,$system,$cuser,$csystem) = times;
      2.

    In scalar context, times returns $user.

    Children's times are only included for terminated children.

    Portability issues: times in perlport.

  • tr///

    The transliteration operator. Same as y///. SeeQuote and Quote-like Operators in perlop.

  • truncate FILEHANDLE,LENGTH
  • truncate EXPR,LENGTH

    Truncates the file opened on FILEHANDLE, or named by EXPR, to thespecified length. Raises an exception if truncate isn't implementedon your system. Returns true if successful, undef on error.

    The behavior is undefined if LENGTH is greater than the length of thefile.

    The position in the file of FILEHANDLE is left unchanged. You may want tocall seek before writing to the file.

    Portability issues: truncate in perlport.

  • uc EXPR
  • uc

    Returns an uppercased version of EXPR. This is the internal functionimplementing the \U escape in double-quoted strings.It does not attempt to do titlecase mapping on initial letters. Seeucfirst for that.

    If EXPR is omitted, uses $_.

    This function behaves the same way under various pragma, such as in a locale,as lc does.

  • ucfirst EXPR
  • ucfirst

    Returns the value of EXPR with the first character in uppercase(titlecase in Unicode). This is the internal function implementingthe \u escape in double-quoted strings.

    If EXPR is omitted, uses $_.

    This function behaves the same way under various pragma, such as in a locale,as lc does.

  • umask EXPR
  • umask

    Sets the umask for the process to EXPR and returns the previous value.If EXPR is omitted, merely returns the current umask.

    The Unix permission rwxr-x--- is represented as three sets of threebits, or three octal digits: 0750 (the leading 0 indicates octaland isn't one of the digits). The umask value is such a numberrepresenting disabled permissions bits. The permission (or "mode")values you pass mkdir or sysopen are modified by your umask, soeven if you tell sysopen to create a file with permissions 0777,if your umask is 0022, then the file will actually be created withpermissions 0755. If your umask were 0027 (group can'twrite; others can't read, write, or execute), then passingsysopen 0666 would create a file with mode 0640 (because 0666 &~ 027 is 0640).

    Here's some advice: supply a creation mode of 0666 for regularfiles (in sysopen) and one of 0777 for directories (inmkdir) and executable files. This gives users the freedom ofchoice: if they want protected files, they might choose process umasksof 022, 027, or even the particularly antisocial mask of 077.Programs should rarely if ever make policy decisions better left tothe user. The exception to this is when writing files that should bekept private: mail files, web browser cookies, .rhosts files, andso on.

    If umask(2) is not implemented on your system and you are trying torestrict access for yourself (i.e., (EXPR & 0700) > 0), raises an exception. If umask(2) is not implemented and you arenot trying to restrict access for yourself, returns undef.

    Remember that a umask is a number, usually given in octal; it is not astring of octal digits. See also oct, if all you have is a string.

    Portability issues: umask in perlport.

  • undef EXPR
  • undef

    Undefines the value of EXPR, which must be an lvalue. Use only on ascalar value, an array (using @), a hash (using %), a subroutine(using &), or a typeglob (using *). Saying undef $hash{$key}will probably not do what you expect on most predefined variables orDBM list values, so don't do that; see delete. Always returns theundefined value. You can omit the EXPR, in which case nothing isundefined, but you still get an undefined value that you could, forinstance, return from a subroutine, assign to a variable, or pass as aparameter. Examples:

    1.  undef $foo;
      2.  
    2.  undef $bar{'blurfl'};  # Compare to: delete $bar{'blurfl'};
      3.  
    3.  undef @ary;
      4.  
    4.  undef %hash;
      5.  
    5.  undef &mysub;
      6.  
    6.  undef *xyz;   # destroys $xyz, @xyz, %xyz, &xyz, etc.
      7.  
    7.  return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
      8.  
    8.  select undef, undef, undef, 0.25;
      9.  
    9.  ($a, $b, undef, $c) = &foo;   # Ignore third value returned
      10.

    Note that this is a unary operator, not a list operator.

  • unlink LIST
  • unlink

    Deletes a list of files. On success, it returns the number of filesit successfully deleted. On failure, it returns false and sets $!(errno):

    1.  my $unlinked = unlink 'a', 'b', 'c';
      2.  
    2.  unlink @goners;
      3.  
    3.  unlink glob "*.bak";
      4.

    On error, unlink will not tell you which files it could not remove.If you want to know which files you could not remove, try them oneat a time:

    1.  foreach my $file ( @goners ) {
      2.  
    2.  unlink $file or warn "Could not unlink $file: $!";
      3.  
    3.  }
      4.

    Note: unlink will not attempt to delete directories unless you aresuperuser and the -U flag is supplied to Perl. Even if theseconditions are met, be warned that unlinking a directory can inflictdamage on your filesystem. Finally, using unlink on directories isnot supported on many operating systems. Use rmdir instead.

    If LIST is omitted, unlink uses $_.

  • unpack TEMPLATE,EXPR
  • unpack TEMPLATE

    unpack does the reverse of pack: it takes a stringand expands it out into a list of values.(In scalar context, it returns merely the first value produced.)

    If EXPR is omitted, unpacks the $_ string.See perlpacktut for an introduction to this function.

    The string is broken into chunks described by the TEMPLATE. Each chunkis converted separately to a value. Typically, either the string is a resultof pack, or the characters of the string represent a C structure of somekind.

    The TEMPLATE has the same format as in the pack function.Here's a subroutine that does substring:

    1.  sub substr {
      2.  
    2.  my($what,$where,$howmuch) = @_;
      3.  
    3.  unpack("x$where a$howmuch", $what);
      4.  
    4.  }
      5.

    and then there's

    1.  sub ordinal { unpack("W",$_[0]); } # same as ord()
      2.

    In addition to fields allowed in pack(), you may prefix a field witha %<number> to indicate thatyou want a <number>-bit checksum of the items instead of the itemsthemselves. Default is a 16-bit checksum. Checksum is calculated bysumming numeric values of expanded values (for string fields the sum oford($char) is taken; for bit fields the sum of zeroes and ones).

    For example, the followingcomputes the same number as the System V sum program:

    1.  $checksum = do {
      2.  
    2.  local $/;  # slurp!
      3.  
    3.  unpack("%32W*",<>) % 65535;
      4.  
    4.  };
      5.

    The following efficiently counts the number of set bits in a bit vector:

    1.  $setbits = unpack("%32b*", $selectmask);
      2.

    The p and P formats should be used with care. Since Perlhas no way of checking whether the value passed to unpack()corresponds to a valid memory location, passing a pointer value that'snot known to be valid is likely to have disastrous consequences.

    If there are more pack codes or if the repeat count of a field or a groupis larger than what the remainder of the input string allows, the resultis not well defined: the repeat count may be decreased, orunpack() may produce empty strings or zeros, or it may raise an exception.If the input string is longer than one described by the TEMPLATE,the remainder of that input string is ignored.

    See pack for more examples and notes.

  • unshift ARRAY,LIST
  • unshift EXPR,LIST

    Does the opposite of a shift. Or the opposite of a push,depending on how you look at it. Prepends list to the front of thearray and returns the new number of elements in the array.

    1.  unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
      2.

    Note the LIST is prepended whole, not one element at a time, so theprepended elements stay in the same order. Use reverse to do thereverse.

    Starting with Perl 5.14, unshift can take a scalar EXPR, which must holda reference to an unblessed array. The argument will be dereferencedautomatically. This aspect of unshift is considered highlyexperimental. The exact behaviour may change in a future version of Perl.

    To avoid confusing would-be users of your code who are running earlierversions of Perl with mysterious syntax errors, put this sort of thing atthe top of your file to signal that your code will work only on Perls ofa recent vintage:

    1.  use 5.014;# so push/pop/etc work on scalars (experimental)
      2.
  • untie VARIABLE

    Breaks the binding between a variable and a package.(See tie.)Has no effect if the variable is not tied.

  • use Module VERSION LIST
  • use Module VERSION
  • use Module LIST
  • use Module
  • use VERSION

    Imports some semantics into the current package from the named module,generally by aliasing certain subroutine or variable names into yourpackage. It is exactly equivalent to

    1.  BEGIN { require Module; Module->import( LIST ); }
      2.

    except that Module must be a bareword.The importation can be made conditional; see if.

    In the peculiar use VERSION form, VERSION may be either a positivedecimal fraction such as 5.006, which will be compared to $], or a v-stringof the form v5.6.1, which will be compared to $^V (aka $PERL_VERSION). Anexception is raised if VERSION is greater than the version of thecurrent Perl interpreter; Perl will not attempt to parse the rest of thefile. Compare with require, which can do a similar check at run time.Symmetrically, no VERSION allows you to specify that you want a versionof Perl older than the specified one.

    Specifying VERSION as a literal of the form v5.6.1 should generally beavoided, because it leads to misleading error messages under earlierversions of Perl (that is, prior to 5.6.0) that do not support thissyntax. The equivalent numeric version should be used instead.

    1.  use v5.6.1; # compile time version check
      2.  
    2.  use 5.6.1;  # ditto
      3.  
    3.  use 5.006_001;  # ditto; preferred for backwards compatibility
      4.

    This is often useful if you need to check the current Perl version beforeuseing library modules that won't work with older versions of Perl.(We try not to do this more than we have to.)

    use VERSION also enables all features available in the requestedversion as defined by the feature pragma, disabling any featuresnot in the requested version's feature bundle. See feature.Similarly, if the specified Perl version is greater than or equal to5.11.0, strictures are enabled lexically aswith use strict. Any explicit use ofuse strict or no strict overrides use VERSION, even if it comesbefore it. In both cases, the feature.pm and strict.pm files arenot actually loaded.

    The BEGIN forces the require and import to happen at compile time. Therequire makes sure the module is loaded into memory if it hasn't beenyet. The import is not a builtin; it's just an ordinary static methodcall into the Module package to tell the module to import the list offeatures back into the current package. The module can implement itsimport method any way it likes, though most modules just choose toderive their import method via inheritance from the Exporter class thatis defined in the Exporter module. See Exporter. If no importmethod can be found then the call is skipped, even if there is an AUTOLOADmethod.

    If you do not want to call the package's import method (for instance,to stop your namespace from being altered), explicitly supply the empty list:

    1.  use Module ();
      2.

    That is exactly equivalent to

    1.  BEGIN { require Module }
      2.

    If the VERSION argument is present between Module and LIST, then theuse will call the VERSION method in class Module with the givenversion as an argument. The default VERSION method, inherited fromthe UNIVERSAL class, croaks if the given version is larger than thevalue of the variable $Module::VERSION.

    Again, there is a distinction between omitting LIST (import calledwith no arguments) and an explicit empty LIST () (import notcalled). Note that there is no comma after VERSION!

    Because this is a wide-open interface, pragmas (compiler directives)are also implemented this way. Currently implemented pragmas are:

    1.  use constant;
      2.  
    2.  use diagnostics;
      3.  
    3.  use integer;
      4.  
    4.  use sigtrap  qw(SEGV BUS);
      5.  
    5.  use strict   qw(subs vars refs);
      6.  
    6.  use subs qw(afunc blurfl);
      7.  
    7.  use warnings qw(all);
      8.  
    8.  use sort qw(stable _quicksort _mergesort);
      9.

    Some of these pseudo-modules import semantics into the currentblock scope (like strict or integer, unlike ordinary modules,which import symbols into the current package (which are effectivethrough the end of the file).

    Because use takes effect at compile time, it doesn't respect theordinary flow control of the code being compiled. In particular, puttinga use inside the false branch of a conditional doesn't prevent itfrom being processed. If a module or pragma only needs to be loaded conditionally, this can be done using the if pragma:

    1.  use if $] < 5.008, "utf8";
      2.  
    2.  use if WANT_WARNINGS, warnings => qw(all);
      3.

    There's a corresponding no declaration that unimports meanings importedby use, i.e., it calls unimport Module LIST instead of import.It behaves just as import does with VERSION, an omitted or empty LIST, or no unimport method being found.

    1.  no integer;
      2.  
    2.  no strict 'refs';
      3.  
    3.  no warnings;
      4.

    Care should be taken when using the no VERSION form of no. It isonly meant to be used to assert that the running Perl is of a earlierversion than its argument and not to undo the feature-enabling side effectsof use VERSION.

    See perlmodlib for a list of standard modules and pragmas. See perlrunfor the -M and -m command-line options to Perl that give usefunctionality from the command-line.

  • utime LIST

    Changes the access and modification times on each file of a list offiles. The first two elements of the list must be the NUMERIC accessand modification times, in that order. Returns the number of filessuccessfully changed. The inode change time of each file is setto the current time. For example, this code has the same effect as theUnix touch(1) command when the files already exist and belong tothe user running the program:

    1.  #!/usr/bin/perl
      2.  
    2.  $atime = $mtime = time;
      3.  
    3.  utime $atime, $mtime, @ARGV;
      4.

    Since Perl 5.7.2, if the first two elements of the list are undef, the utime(2) syscall from your C library is called with a null secondargument. On most systems, this will set the file's access andmodification times to the current time (i.e., equivalent to the exampleabove) and will work even on files you don't own provided you have writepermission:

    1.  for $file (@ARGV) {
      2.  
    2. utime(undef, undef, $file) 
      3.  
    3.  || warn "couldn't touch $file: $!";
      4.  
    4.  }
      5.

    Under NFS this will use the time of the NFS server, not the time ofthe local machine. If there is a time synchronization problem, theNFS server and local machine will have different times. The Unixtouch(1) command will in fact normally use this form instead of theone shown in the first example.

    Passing only one of the first two elements as undef isequivalent to passing a 0 and will not have the effect described when both are undef. This also triggers anuninitialized warning.

    On systems that support futimes(2), you may pass filehandles among thefiles. On systems that don't support futimes(2), passing filehandles raisesan exception. Filehandles must be passed as globs or glob references to berecognized; barewords are considered filenames.

    Portability issues: utime in perlport.

  • values HASH
  • values ARRAY
  • values EXPR

    In list context, returns a list consisting of all the values of the namedhash. In Perl 5.12 or later only, will also return a list of the values ofan array; prior to that release, attempting to use an array argument willproduce a syntax error. In scalar context, returns the number of values.

    When called on a hash, the values are returned in an apparently randomorder. The actual random order is subject to change in future versions ofPerl, but it is guaranteed to be the same order as either the keys oreach function would produce on the same (unmodified) hash. Since Perl5.8.1 the ordering is different even between different runs of Perl forsecurity reasons (see Algorithmic Complexity Attacks in perlsec).

    As a side effect, calling values() resets the HASH or ARRAY's internaliterator, see each. (In particular, calling values() in void contextresets the iterator with no other overhead. Apart from resetting theiterator, values @array in list context is the same as plain @array.(We recommend that you use void context keys @array for this, butreasoned that taking values @array out would require moredocumentation than leaving it in.)

    Note that the values are not copied, which means modifying them willmodify the contents of the hash:

    1.  for (values %hash)  { s/foo/bar/g }   # modifies %hash values
      2.  
    2.  for (@hash{keys %hash}) { s/foo/bar/g }   # same
      3.

    Starting with Perl 5.14, values can take a scalar EXPR, which must holda reference to an unblessed hash or array. The argument will bedereferenced automatically. This aspect of values is considered highlyexperimental. The exact behaviour may change in a future version of Perl.

    1.  for (values $hashref) { ... }
      2.  
    2.  for (values $obj->get_arrayref) { ... }
      3.

    To avoid confusing would-be users of your code who are running earlierversions of Perl with mysterious syntax errors, put this sort of thing atthe top of your file to signal that your code will work only on Perls ofa recent vintage:

    1.  use 5.012;# so keys/values/each work on arrays
      2.  
    2.  use 5.014;# so keys/values/each work on scalars (experimental)
      3.

    See also keys, each, and sort.

  • vec EXPR,OFFSET,BITS

    Treats the string in EXPR as a bit vector made up of elements ofwidth BITS and returns the value of the element specified by OFFSETas an unsigned integer. BITS therefore specifies the number of bitsthat are reserved for each element in the bit vector. This mustbe a power of two from 1 to 32 (or 64, if your platform supportsthat).

    If BITS is 8, "elements" coincide with bytes of the input string.

    If BITS is 16 or more, bytes of the input string are grouped into chunksof size BITS/8, and each group is converted to a number as withpack()/unpack() with big-endian formats n/N (and analogouslyfor BITS==64). See pack for details.

    If bits is 4 or less, the string is broken into bytes, then the bitsof each byte are broken into 8/BITS groups. Bits of a byte arenumbered in a little-endian-ish way, as in 0x01, 0x02,0x04, 0x08, 0x10, 0x20, 0x40, 0x80. For example,breaking the single input byte chr(0x36) into two groups gives a list(0x6, 0x3); breaking it into 4 groups gives (0x2, 0x1, 0x3, 0x0).

    vec may also be assigned to, in which case parentheses are neededto give the expression the correct precedence as in

    1.  vec($image, $max_x * $x + $y, 8) = 3;
      2.

    If the selected element is outside the string, the value 0 is returned.If an element off the end of the string is written to, Perl will firstextend the string with sufficiently many zero bytes. It is an errorto try to write off the beginning of the string (i.e., negative OFFSET).

    If the string happens to be encoded as UTF-8 internally (and thus hasthe UTF8 flag set), this is ignored by vec, and it operates on theinternal byte string, not the conceptual character string, even if youonly have characters with values less than 256.

    Strings created with vec can also be manipulated with the logicaloperators |, &, ^, and ~. These operators will assume a bitvector operation is desired when both operands are strings.See Bitwise String Operators in perlop.

    The following code will build up an ASCII string saying 'PerlPerlPerl'.The comments show the string after each step. Note that this code worksin the same way on big-endian or little-endian machines.

    1.  my $foo = '';
      2.  
    2.  vec($foo,  0, 32) = 0x5065726C; # 'Perl'
      3.  
    3.  
    4.  # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
      5.  
    5.  print vec($foo, 0, 8);  # prints 80 == 0x50 == ord('P')
      6.  
    6.  
    7.  vec($foo,  2, 16) = 0x5065; # 'PerlPe'
      8.  
    8.  vec($foo,  3, 16) = 0x726C; # 'PerlPerl'
      9.  
    9.  vec($foo,  8,  8) = 0x50;   # 'PerlPerlP'
      10.  
    10.  vec($foo,  9,  8) = 0x65;   # 'PerlPerlPe'
      11.  
    11.  vec($foo, 20,  4) = 2;  # 'PerlPerlPe'   . "\x02"
      12.  
    12.  vec($foo, 21,  4) = 7;  # 'PerlPerlPer'
      13.  
    13.    # 'r' is "\x72"
      14.  
    14.  vec($foo, 45,  2) = 3;  # 'PerlPerlPer'  . "\x0c"
      15.  
    15.  vec($foo, 93,  1) = 1;  # 'PerlPerlPer'  . "\x2c"
      16.  
    16.  vec($foo, 94,  1) = 1;  # 'PerlPerlPerl'
      17.  
    17.    # 'l' is "\x6c"
      18.

    To transform a bit vector into a string or list of 0's and 1's, use these:

    1.  $bits = unpack("b*", $vector);
      2.  
    2.  @bits = split(//, unpack("b*", $vector));
      3.

    If you know the exact length in bits, it can be used in place of the *.

    Here is an example to illustrate how the bits actually fall in place:

    1.  #!/usr/bin/perl -wl
      2.  
    2.  
    3.  print <<'EOT';
      4.  
    4.   0 1 2 3
      5.  
    5.    unpack("V",$_) 01234567890123456789012345678901
      6.  
    6.  ------------------------------------------------------------------
      7.  
    7.  EOT
      8.  
    8.  
    9.  for $w (0..3) {
      10.  
    10.  $width = 2**$w;
      11.  
    11.  for ($shift=0; $shift < $width; ++$shift) {
      12.  
    12.  for ($off=0; $off < 32/$width; ++$off) {
      13.  
    13.  $str = pack("B*", "0"x32);
      14.  
    14.  $bits = (1<<$shift);
      15.  
    15.  vec($str, $off, $width) = $bits;
      16.  
    16.  $res = unpack("b*",$str);
      17.  
    17.  $val = unpack("V", $str);
      18.  
    18.  write;
      19.  
    19.  }
      20.  
    20.  }
      21.  
    21.  }
      22.  
    22.  
    23.  format STDOUT =
      24.  
    24.  vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
      25.  
    25.  $off, $width, $bits, $val, $res
      26.  
    26.  .
      27.  
    27.  __END__
      28.

    Regardless of the machine architecture on which it runs, the example above should print the following table:

    1.   0 1 2 3
      2.  
    2.    unpack("V",$_) 01234567890123456789012345678901
      3.  
    3.  ------------------------------------------------------------------
      4.  
    4.  vec($_, 0, 1) = 1   ==  1 10000000000000000000000000000000
      5.  
    5.  vec($_, 1, 1) = 1   ==  2 01000000000000000000000000000000
      6.  
    6.  vec($_, 2, 1) = 1   ==  4 00100000000000000000000000000000
      7.  
    7.  vec($_, 3, 1) = 1   ==  8 00010000000000000000000000000000
      8.  
    8.  vec($_, 4, 1) = 1   == 16 00001000000000000000000000000000
      9.  
    9.  vec($_, 5, 1) = 1   == 32 00000100000000000000000000000000
      10.  
    10.  vec($_, 6, 1) = 1   == 64 00000010000000000000000000000000
      11.  
    11.  vec($_, 7, 1) = 1   == 128 00000001000000000000000000000000
      12.  
    12.  vec($_, 8, 1) = 1   == 256 00000000100000000000000000000000
      13.  
    13.  vec($_, 9, 1) = 1   == 512 00000000010000000000000000000000
      14.  
    14.  vec($_,10, 1) = 1   ==   1024 00000000001000000000000000000000
      15.  
    15.  vec($_,11, 1) = 1   ==   2048 00000000000100000000000000000000
      16.  
    16.  vec($_,12, 1) = 1   ==   4096 00000000000010000000000000000000
      17.  
    17.  vec($_,13, 1) = 1   ==   8192 00000000000001000000000000000000
      18.  
    18.  vec($_,14, 1) = 1   ==  16384 00000000000000100000000000000000
      19.  
    19.  vec($_,15, 1) = 1   ==  32768 00000000000000010000000000000000
      20.  
    20.  vec($_,16, 1) = 1   ==  65536 00000000000000001000000000000000
      21.  
    21.  vec($_,17, 1) = 1   == 131072 00000000000000000100000000000000
      22.  
    22.  vec($_,18, 1) = 1   == 262144 00000000000000000010000000000000
      23.  
    23.  vec($_,19, 1) = 1   == 524288 00000000000000000001000000000000
      24.  
    24.  vec($_,20, 1) = 1   == 1048576 00000000000000000000100000000000
      25.  
    25.  vec($_,21, 1) = 1   == 2097152 00000000000000000000010000000000
      26.  
    26.  vec($_,22, 1) = 1   == 4194304 00000000000000000000001000000000
      27.  
    27.  vec($_,23, 1) = 1   == 8388608 00000000000000000000000100000000
      28.  
    28.  vec($_,24, 1) = 1   ==   16777216 00000000000000000000000010000000
      29.  
    29.  vec($_,25, 1) = 1   ==   33554432 00000000000000000000000001000000
      30.  
    30.  vec($_,26, 1) = 1   ==   67108864 00000000000000000000000000100000
      31.  
    31.  vec($_,27, 1) = 1   ==  134217728 00000000000000000000000000010000
      32.  
    32.  vec($_,28, 1) = 1   ==  268435456 00000000000000000000000000001000
      33.  
    33.  vec($_,29, 1) = 1   ==  536870912 00000000000000000000000000000100
      34.  
    34.  vec($_,30, 1) = 1   == 1073741824 00000000000000000000000000000010
      35.  
    35.  vec($_,31, 1) = 1   == 2147483648 00000000000000000000000000000001
      36.  
    36.  vec($_, 0, 2) = 1   ==  1 10000000000000000000000000000000
      37.  
    37.  vec($_, 1, 2) = 1   ==  4 00100000000000000000000000000000
      38.  
    38.  vec($_, 2, 2) = 1   == 16 00001000000000000000000000000000
      39.  
    39.  vec($_, 3, 2) = 1   == 64 00000010000000000000000000000000
      40.  
    40.  vec($_, 4, 2) = 1   == 256 00000000100000000000000000000000
      41.  
    41.  vec($_, 5, 2) = 1   ==   1024 00000000001000000000000000000000
      42.  
    42.  vec($_, 6, 2) = 1   ==   4096 00000000000010000000000000000000
      43.  
    43.  vec($_, 7, 2) = 1   ==  16384 00000000000000100000000000000000
      44.  
    44.  vec($_, 8, 2) = 1   ==  65536 00000000000000001000000000000000
      45.  
    45.  vec($_, 9, 2) = 1   == 262144 00000000000000000010000000000000
      46.  
    46.  vec($_,10, 2) = 1   == 1048576 00000000000000000000100000000000
      47.  
    47.  vec($_,11, 2) = 1   == 4194304 00000000000000000000001000000000
      48.  
    48.  vec($_,12, 2) = 1   ==   16777216 00000000000000000000000010000000
      49.  
    49.  vec($_,13, 2) = 1   ==   67108864 00000000000000000000000000100000
      50.  
    50.  vec($_,14, 2) = 1   ==  268435456 00000000000000000000000000001000
      51.  
    51.  vec($_,15, 2) = 1   == 1073741824 00000000000000000000000000000010
      52.  
    52.  vec($_, 0, 2) = 2   ==  2 01000000000000000000000000000000
      53.  
    53.  vec($_, 1, 2) = 2   ==  8 00010000000000000000000000000000
      54.  
    54.  vec($_, 2, 2) = 2   == 32 00000100000000000000000000000000
      55.  
    55.  vec($_, 3, 2) = 2   == 128 00000001000000000000000000000000
      56.  
    56.  vec($_, 4, 2) = 2   == 512 00000000010000000000000000000000
      57.  
    57.  vec($_, 5, 2) = 2   ==   2048 00000000000100000000000000000000
      58.  
    58.  vec($_, 6, 2) = 2   ==   8192 00000000000001000000000000000000
      59.  
    59.  vec($_, 7, 2) = 2   ==  32768 00000000000000010000000000000000
      60.  
    60.  vec($_, 8, 2) = 2   == 131072 00000000000000000100000000000000
      61.  
    61.  vec($_, 9, 2) = 2   == 524288 00000000000000000001000000000000
      62.  
    62.  vec($_,10, 2) = 2   == 2097152 00000000000000000000010000000000
      63.  
    63.  vec($_,11, 2) = 2   == 8388608 00000000000000000000000100000000
      64.  
    64.  vec($_,12, 2) = 2   ==   33554432 00000000000000000000000001000000
      65.  
    65.  vec($_,13, 2) = 2   ==  134217728 00000000000000000000000000010000
      66.  
    66.  vec($_,14, 2) = 2   ==  536870912 00000000000000000000000000000100
      67.  
    67.  vec($_,15, 2) = 2   == 2147483648 00000000000000000000000000000001
      68.  
    68.  vec($_, 0, 4) = 1   ==  1 10000000000000000000000000000000
      69.  
    69.  vec($_, 1, 4) = 1   == 16 00001000000000000000000000000000
      70.  
    70.  vec($_, 2, 4) = 1   == 256 00000000100000000000000000000000
      71.  
    71.  vec($_, 3, 4) = 1   ==   4096 00000000000010000000000000000000
      72.  
    72.  vec($_, 4, 4) = 1   ==  65536 00000000000000001000000000000000
      73.  
    73.  vec($_, 5, 4) = 1   == 1048576 00000000000000000000100000000000
      74.  
    74.  vec($_, 6, 4) = 1   ==   16777216 00000000000000000000000010000000
      75.  
    75.  vec($_, 7, 4) = 1   ==  268435456 00000000000000000000000000001000
      76.  
    76.  vec($_, 0, 4) = 2   ==  2 01000000000000000000000000000000
      77.  
    77.  vec($_, 1, 4) = 2   == 32 00000100000000000000000000000000
      78.  
    78.  vec($_, 2, 4) = 2   == 512 00000000010000000000000000000000
      79.  
    79.  vec($_, 3, 4) = 2   ==   8192 00000000000001000000000000000000
      80.  
    80.  vec($_, 4, 4) = 2   == 131072 00000000000000000100000000000000
      81.  
    81.  vec($_, 5, 4) = 2   == 2097152 00000000000000000000010000000000
      82.  
    82.  vec($_, 6, 4) = 2   ==   33554432 00000000000000000000000001000000
      83.  
    83.  vec($_, 7, 4) = 2   ==  536870912 00000000000000000000000000000100
      84.  
    84.  vec($_, 0, 4) = 4   ==  4 00100000000000000000000000000000
      85.  
    85.  vec($_, 1, 4) = 4   == 64 00000010000000000000000000000000
      86.  
    86.  vec($_, 2, 4) = 4   ==   1024 00000000001000000000000000000000
      87.  
    87.  vec($_, 3, 4) = 4   ==  16384 00000000000000100000000000000000
      88.  
    88.  vec($_, 4, 4) = 4   == 262144 00000000000000000010000000000000
      89.  
    89.  vec($_, 5, 4) = 4   == 4194304 00000000000000000000001000000000
      90.  
    90.  vec($_, 6, 4) = 4   ==   67108864 00000000000000000000000000100000
      91.  
    91.  vec($_, 7, 4) = 4   == 1073741824 00000000000000000000000000000010
      92.  
    92.  vec($_, 0, 4) = 8   ==  8 00010000000000000000000000000000
      93.  
    93.  vec($_, 1, 4) = 8   == 128 00000001000000000000000000000000
      94.  
    94.  vec($_, 2, 4) = 8   ==   2048 00000000000100000000000000000000
      95.  
    95.  vec($_, 3, 4) = 8   ==  32768 00000000000000010000000000000000
      96.  
    96.  vec($_, 4, 4) = 8   == 524288 00000000000000000001000000000000
      97.  
    97.  vec($_, 5, 4) = 8   == 8388608 00000000000000000000000100000000
      98.  
    98.  vec($_, 6, 4) = 8   ==  134217728 00000000000000000000000000010000
      99.  
    99.  vec($_, 7, 4) = 8   == 2147483648 00000000000000000000000000000001
      100.  
    100.  vec($_, 0, 8) = 1   ==  1 10000000000000000000000000000000
      101.  
    101.  vec($_, 1, 8) = 1   == 256 00000000100000000000000000000000
      102.  
    102.  vec($_, 2, 8) = 1   ==  65536 00000000000000001000000000000000
      103.  
    103.  vec($_, 3, 8) = 1   ==   16777216 00000000000000000000000010000000
      104.  
    104.  vec($_, 0, 8) = 2   ==  2 01000000000000000000000000000000
      105.  
    105.  vec($_, 1, 8) = 2   == 512 00000000010000000000000000000000
      106.  
    106.  vec($_, 2, 8) = 2   == 131072 00000000000000000100000000000000
      107.  
    107.  vec($_, 3, 8) = 2   ==   33554432 00000000000000000000000001000000
      108.  
    108.  vec($_, 0, 8) = 4   ==  4 00100000000000000000000000000000
      109.  
    109.  vec($_, 1, 8) = 4   ==   1024 00000000001000000000000000000000
      110.  
    110.  vec($_, 2, 8) = 4   == 262144 00000000000000000010000000000000
      111.  
    111.  vec($_, 3, 8) = 4   ==   67108864 00000000000000000000000000100000
      112.  
    112.  vec($_, 0, 8) = 8   ==  8 00010000000000000000000000000000
      113.  
    113.  vec($_, 1, 8) = 8   ==   2048 00000000000100000000000000000000
      114.  
    114.  vec($_, 2, 8) = 8   == 524288 00000000000000000001000000000000
      115.  
    115.  vec($_, 3, 8) = 8   ==  134217728 00000000000000000000000000010000
      116.  
    116.  vec($_, 0, 8) = 16  == 16 00001000000000000000000000000000
      117.  
    117.  vec($_, 1, 8) = 16  ==   4096 00000000000010000000000000000000
      118.  
    118.  vec($_, 2, 8) = 16  == 1048576 00000000000000000000100000000000
      119.  
    119.  vec($_, 3, 8) = 16  ==  268435456 00000000000000000000000000001000
      120.  
    120.  vec($_, 0, 8) = 32  == 32 00000100000000000000000000000000
      121.  
    121.  vec($_, 1, 8) = 32  ==   8192 00000000000001000000000000000000
      122.  
    122.  vec($_, 2, 8) = 32  == 2097152 00000000000000000000010000000000
      123.  
    123.  vec($_, 3, 8) = 32  ==  536870912 00000000000000000000000000000100
      124.  
    124.  vec($_, 0, 8) = 64  == 64 00000010000000000000000000000000
      125.  
    125.  vec($_, 1, 8) = 64  ==  16384 00000000000000100000000000000000
      126.  
    126.  vec($_, 2, 8) = 64  == 4194304 00000000000000000000001000000000
      127.  
    127.  vec($_, 3, 8) = 64  == 1073741824 00000000000000000000000000000010
      128.  
    128.  vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
      129.  
    129.  vec($_, 1, 8) = 128 ==  32768 00000000000000010000000000000000
      130.  
    130.  vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
      131.  
    131.  vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
      132.
  • wait

    Behaves like wait(2) on your system: it waits for a childprocess to terminate and returns the pid of the deceased process, or-1 if there are no child processes. The status is returned in $?and ${^CHILD_ERROR_NATIVE}.Note that a return value of -1 could mean that child processes arebeing automatically reaped, as described in perlipc.

    If you use wait in your handler for $SIG{CHLD} it may accidentally for thechild created by qx() or system(). See perlipc for details.

    Portability issues: wait in perlport.

  • waitpid PID,FLAGS

    Waits for a particular child process to terminate and returns the pid ofthe deceased process, or -1 if there is no such child process. On somesystems, a value of 0 indicates that there are processes still running.The status is returned in $? and ${^CHILD_ERROR_NATIVE}. If you say

    1.  use POSIX ":sys_wait_h";
      2.  
    2.  #...
      3.  
    3.  do {
      4.  
    4.  $kid = waitpid(-1, WNOHANG);
      5.  
    5.  } while $kid > 0;
      6.

    then you can do a non-blocking wait for all pending zombie processes.Non-blocking wait is available on machines supporting either thewaitpid(2) or wait4(2) syscalls. However, waiting for a particularpid with FLAGS of 0 is implemented everywhere. (Perl emulates thesystem call by remembering the status values of processes that haveexited but have not been harvested by the Perl script yet.)

    Note that on some systems, a return value of -1 could mean that childprocesses are being automatically reaped. See perlipc for details,and for other examples.

    Portability issues: waitpid in perlport.

  • wantarray

    Returns true if the context of the currently executing subroutine oreval is looking for a list value. Returns false if the context islooking for a scalar. Returns the undefined value if the context islooking for no value (void context).

    1.  return unless defined wantarray; # don't bother doing more
      2.  
    2.  my @a = complex_calculation();
      3.  
    3.  return wantarray ? @a : "@a";
      4.

    wantarray()'s result is unspecified in the top level of a file,in a BEGIN, UNITCHECK, CHECK, INIT or END block, orin a DESTROY method.

    This function should have been named wantlist() instead.

  • warn LIST

    Prints the value of LIST to STDERR. If the last element of LIST doesnot end in a newline, it appends the same file/line number text as diedoes.

    If the output is empty and $@ already contains a value (typically from aprevious eval) that value is used after appending "\t...caught"to $@. This is useful for staying almost, but not entirely similar todie.

    If $@ is empty then the string "Warning: Something's wrong" is used.

    No message is printed if there is a $SIG{__WARN__} handlerinstalled. It is the handler's responsibility to deal with the messageas it sees fit (like, for instance, converting it into a die). Mosthandlers must therefore arrange to actually display thewarnings that they are not prepared to deal with, by calling warnagain in the handler. Note that this is quite safe and will notproduce an endless loop, since __WARN__ hooks are not called frominside one.

    You will find this behavior is slightly different from that of$SIG{__DIE__} handlers (which don't suppress the error text, but caninstead call die again to change it).

    Using a __WARN__ handler provides a powerful way to silence allwarnings (even the so-called mandatory ones). An example:

    1.  # wipe out *all* compile-time warnings
      2.  
    2.  BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
      3.  
    3.  my $foo = 10;
      4.  
    4.  my $foo = 20;  # no warning about duplicate my $foo,
      5.  
    5.    # but hey, you asked for it!
      6.  
    6.  # no compile-time or run-time warnings before here
      7.  
    7.  $DOWARN = 1;
      8.  
    8.  
    9.  # run-time warnings enabled after here
      10.  
    10.  warn "\$foo is alive and $foo!"; # does show up
      11.

    See perlvar for details on setting %SIG entries and for moreexamples. See the Carp module for other kinds of warnings using itscarp() and cluck() functions.

  • write FILEHANDLE
  • write EXPR
  • write

    Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,using the format associated with that file. By default the format fora file is the one having the same name as the filehandle, but theformat for the current output channel (see the select function) may be setexplicitly by assigning the name of the format to the $~ variable.

    Top of form processing is handled automatically: if there is insufficientroom on the current page for the formatted record, the page is advanced bywriting a form feed, a special top-of-page format is used to format the newpage header before the record is written. By default, the top-of-pageformat is the name of the filehandle with "_TOP" appended. This would be aproblem with autovivified filehandles, but it may be dynamically set to theformat of your choice by assigning the name to the $^ variable whilethat filehandle is selected. The number of lines remaining on the currentpage is in variable $-, which can be set to 0 to force a new page.

    If FILEHANDLE is unspecified, output goes to the current default outputchannel, which starts out as STDOUT but may be changed by theselect operator. If the FILEHANDLE is an EXPR, then the expressionis evaluated and the resulting string is used to look up the name ofthe FILEHANDLE at run time. For more on formats, see perlform.

    Note that write is not the opposite of read. Unfortunately.

  • y///

    The transliteration operator. Same as tr///. SeeQuote and Quote-like Operators in perlop.

Non-function Keywords by Cross-reference

perldata

perlmod

perlobj

perlop

  • and
  • cmp
  • eq
  • ge
  • gt
  • if
  • le
  • lt
  • ne
  • not
  • or
  • x
  • xor

    These operators are documented in perlop.

perlsub

perlsyn

 
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