Daftar Isi

• 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:

   select(REPORT1);
   
   $^ = 'report1_top';
   
   select(REPORT2);
   
   $^ = 'report2_top';
   

  FILEHANDLE may be an expression whose value gives the name of theactual filehandle. Thus:

   $oldfh = select(STDERR); $| = 1; select($oldfh);
   

  Some programmers may prefer to think of filehandles as objects withmethods, preferring to write the last example as:

   use IO::Handle;
   
   STDERR->autoflush(1);
   

  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:

   $rin = $win = $ein = '';
   
   vec($rin, fileno(STDIN),  1) = 1;
   
   vec($win, fileno(STDOUT), 1) = 1;
   
   $ein = $rin | $win;
   

  If you want to select on many filehandles, you may wish to write asubroutine like this:

   sub fhbits {
   
   my @fhlist = @_;
   
   my $bits = "";
   
   for my $fh (@fhlist) {
   
   vec($bits, fileno($fh), 1) = 1;
   
   }
   
   return $bits;
   
   }
   
   $rin = fhbits(*STDIN, *TTY, *MYSOCK);
   

  The usual idiom is:

   ($nfound,$timeleft) =
   
    select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
   

  or to block until something becomes ready just do this

   $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
   

  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:

   select(undef, undef, undef, 0.25);
   

  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.