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Internals and C language interface

Perl's IO abstraction interface.

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NAME

perlapio - perl's IO abstraction interface.

SYNOPSIS

  1.  #define PERLIO_NOT_STDIO 0 /* For co-existence with stdio only */
    2.  
  2.  #include <perlio.h>   /* Usually via #include <perl.h> */
    3.  
  3.  
  4.  PerlIO *PerlIO_stdin(void);
    5.  
  5.  PerlIO *PerlIO_stdout(void);
    6.  
  6.  PerlIO *PerlIO_stderr(void);
    7.  
  7.  
  8.  PerlIO *PerlIO_open(const char *path,const char *mode);
    9.  
  9.  PerlIO *PerlIO_fdopen(int fd, const char *mode);
    10.  
  10.  PerlIO *PerlIO_reopen(const char *path, const char *mode, PerlIO *old);  /* deprecated */
    11.  
  11.  int PerlIO_close(PerlIO *f);
    12.  
  12.  
  13.  int PerlIO_stdoutf(const char *fmt,...)
    14.  
  14.  int PerlIO_puts(PerlIO *f,const char *string);
    15.  
  15.  int PerlIO_putc(PerlIO *f,int ch);
    16.  
  16.  int PerlIO_write(PerlIO *f,const void *buf,size_t numbytes);
    17.  
  17.  int PerlIO_printf(PerlIO *f, const char *fmt,...);
    18.  
  18.  int PerlIO_vprintf(PerlIO *f, const char *fmt, va_list args);
    19.  
  19.  int PerlIO_flush(PerlIO *f);
    20.  
  20.  
  21.  int PerlIO_eof(PerlIO *f);
    22.  
  22.  int PerlIO_error(PerlIO *f);
    23.  
  23.  void PerlIO_clearerr(PerlIO *f);
    24.  
  24.  
  25.  int PerlIO_getc(PerlIO *d);
    26.  
  26.  int PerlIO_ungetc(PerlIO *f,int ch);
    27.  
  27.  int PerlIO_read(PerlIO *f, void *buf, size_t numbytes);
    28.  
  28.  
  29.  int PerlIO_fileno(PerlIO *f);
    30.  
  30.  
  31.  void PerlIO_setlinebuf(PerlIO *f);
    32.  
  32.  
  33.  Off_t   PerlIO_tell(PerlIO *f);
    34.  
  34.  int PerlIO_seek(PerlIO *f, Off_t offset, int whence);
    35.  
  35.  void PerlIO_rewind(PerlIO *f);
    36.  
  36.  
  37.  int PerlIO_getpos(PerlIO *f, SV *save); /* prototype changed */
    38.  
  38.  int PerlIO_setpos(PerlIO *f, SV *saved);   /* prototype changed */
    39.  
  39.  
  40.  int PerlIO_fast_gets(PerlIO *f);
    41.  
  41.  int PerlIO_has_cntptr(PerlIO *f);
    42.  
  42.  int PerlIO_get_cnt(PerlIO *f);
    43.  
  43.  char   *PerlIO_get_ptr(PerlIO *f);
    44.  
  44.  void PerlIO_set_ptrcnt(PerlIO *f, char *ptr, int count);
    45.  
  45.  
  46.  int PerlIO_canset_cnt(PerlIO *f);  /* deprecated */
    47.  
  47.  void PerlIO_set_cnt(PerlIO *f, int count);  /* deprecated */
    48.  
  48.  
  49.  int PerlIO_has_base(PerlIO *f);
    50.  
  50.  char   *PerlIO_get_base(PerlIO *f);
    51.  
  51.  int PerlIO_get_bufsiz(PerlIO *f);
    52.  
  52.  
  53.  PerlIO *PerlIO_importFILE(FILE *stdio, const char *mode);
    54.  
  54.  FILE   *PerlIO_exportFILE(PerlIO *f, int flags);
    55.  
  55.  FILE   *PerlIO_findFILE(PerlIO *f);
    56.  
  56.  void PerlIO_releaseFILE(PerlIO *f,FILE *stdio);
    57.  
  57.  
  58.  int PerlIO_apply_layers(PerlIO *f, const char *mode, const char *layers);
    59.  
  59.  int PerlIO_binmode(PerlIO *f, int ptype, int imode, const char *layers);
    60.  
  60.  void PerlIO_debug(const char *fmt,...)
    61.

DESCRIPTION

Perl's source code, and extensions that want maximum portability,should use the above functions instead of those defined in ANSI C'sstdio.h. The perl headers (in particular "perlio.h") will#define them to the I/O mechanism selected at Configure time.

The functions are modeled on those in stdio.h, but parameter orderhas been "tidied up a little".

PerlIO * takes the place of FILE *. Like FILE * it should betreated as opaque (it is probably safe to assume it is a pointer tosomething).

There are currently three implementations:

1.
USE_STDIO

All above are #define'd to stdio functions or are trivial wrapperfunctions which call stdio. In this case only PerlIO * is a FILE *.This has been the default implementation since the abstraction wasintroduced in perl5.003_02.

2.
USE_SFIO

A "legacy" implementation in terms of the "sfio" library. Used forsome specialist applications on Unix machines ("sfio" is not widelyported away from Unix). Most of above are #define'd to the sfiofunctions. PerlIO * is in this case Sfio_t *.

3.
USE_PERLIO

Introduced just after perl5.7.0, this is a re-implementation of theabove abstraction which allows perl more control over how IO is doneas it decouples IO from the way the operating system and C librarychoose to do things. For USE_PERLIO PerlIO * has an extra layer ofindirection - it is a pointer-to-a-pointer. This allows the PerlIO *to remain with a known value while swapping the implementation aroundunderneath at run time. In this case all the above are true (butvery simple) functions which call the underlying implementation.

This is the only implementation for which PerlIO_apply_layers()does anything "interesting".

The USE_PERLIO implementation is described in perliol.

Because "perlio.h" is a thin layer (for efficiency) the semantics ofthese functions are somewhat dependent on the underlying implementation.Where these variations are understood they are noted below.

Unless otherwise noted, functions return 0 on success, or a negativevalue (usually EOF which is usually -1) and set errno on error.

  • PerlIO_stdin(), PerlIO_stdout(), PerlIO_stderr()

    Use these rather than stdin, stdout, stderr. They are writtento look like "function calls" rather than variables because this makesit easier to make them function calls if platform cannot export datato loaded modules, or if (say) different "threads" might have differentvalues.

  • PerlIO_open(path, mode), PerlIO_fdopen(fd,mode)

    These correspond to fopen()/fdopen() and the arguments are the same.Return NULL and set errno if there is an error. There may be animplementation limit on the number of open handles, which may be lowerthan the limit on the number of open files - errno may not be setwhen NULL is returned if this limit is exceeded.

  • PerlIO_reopen(path,mode,f)

    While this currently exists in all three implementations perl itselfdoes not use it. As perl does not use it, it is not well tested.

    Perl prefers to dup the new low-level descriptor to the descriptorused by the existing PerlIO. This may become the behaviour of thisfunction in the future.

  • PerlIO_printf(f,fmt,...), PerlIO_vprintf(f,fmt,a)

    These are fprintf()/vfprintf() equivalents.

  • PerlIO_stdoutf(fmt,...)

    This is printf() equivalent. printf is #defined to this function,so it is (currently) legal to use printf(fmt,...) in perl sources.

  • PerlIO_read(f,buf,count), PerlIO_write(f,buf,count)

    These correspond functionally to fread() and fwrite() but thearguments and return values are different. The PerlIO_read() andPerlIO_write() signatures have been modeled on the more sane low levelread() and write() functions instead: The "file" argument is passedfirst, there is only one "count", and the return value can distinguishbetween error and EOF.

    Returns a byte count if successful (which may be zero orpositive), returns negative value and sets errno on error.Depending on implementation errno may be EINTR if operation wasinterrupted by a signal.

  • PerlIO_close(f)

    Depending on implementation errno may be EINTR if operation wasinterrupted by a signal.

  • PerlIO_puts(f,s), PerlIO_putc(f,c)

    These correspond to fputs() and fputc().Note that arguments have been revised to have "file" first.

  • PerlIO_ungetc(f,c)

    This corresponds to ungetc(). Note that arguments have been revisedto have "file" first. Arranges that next read operation will returnthe byte c. Despite the implied "character" in the name onlyvalues in the range 0..0xFF are defined. Returns the byte c onsuccess or -1 (EOF) on error. The number of bytes that can be"pushed back" may vary, only 1 character is certain, and then only ifit is the last character that was read from the handle.

  • PerlIO_getc(f)

    This corresponds to getc().Despite the c in the name only byte range 0..0xFF is supported.Returns the character read or -1 (EOF) on error.

  • PerlIO_eof(f)

    This corresponds to feof(). Returns a true/false indication ofwhether the handle is at end of file. For terminal devices this mayor may not be "sticky" depending on the implementation. The flag iscleared by PerlIO_seek(), or PerlIO_rewind().

  • PerlIO_error(f)

    This corresponds to ferror(). Returns a true/false indication ofwhether there has been an IO error on the handle.

  • PerlIO_fileno(f)

    This corresponds to fileno(), note that on some platforms, the meaningof "fileno" may not match Unix. Returns -1 if the handle has no opendescriptor associated with it.

  • PerlIO_clearerr(f)

    This corresponds to clearerr(), i.e., clears 'error' and (usually)'eof' flags for the "stream". Does not return a value.

  • PerlIO_flush(f)

    This corresponds to fflush(). Sends any buffered write data to theunderlying file. If called with NULL this may flush all openstreams (or core dump with some USE_STDIO implementations). Callingon a handle open for read only, or on which last operation was a readof some kind may lead to undefined behaviour on some USE_STDIOimplementations. The USE_PERLIO (layers) implementation tries tobehave better: it flushes all open streams when passed NULL, andattempts to retain data on read streams either in the buffer or byseeking the handle to the current logical position.

  • PerlIO_seek(f,offset,whence)

    This corresponds to fseek(). Sends buffered write data to theunderlying file, or discards any buffered read data, then positionsthe file descriptor as specified by offset and whence (sic).This is the correct thing to do when switching between read and writeon the same handle (see issues with PerlIO_flush() above). Offset isof type Off_t which is a perl Configure value which may not be sameas stdio's off_t.

  • PerlIO_tell(f)

    This corresponds to ftell(). Returns the current file position, or(Off_t) -1 on error. May just return value system "knows" withoutmaking a system call or checking the underlying file descriptor (souse on shared file descriptors is not safe without aPerlIO_seek()). Return value is of type Off_t which is a perlConfigure value which may not be same as stdio's off_t.

  • PerlIO_getpos(f,p), PerlIO_setpos(f,p)

    These correspond (loosely) to fgetpos() and fsetpos(). Rather thanstdio's Fpos_t they expect a "Perl Scalar Value" to be passed. What isstored there should be considered opaque. The layout of the data mayvary from handle to handle. When not using stdio or if platform doesnot have the stdio calls then they are implemented in terms ofPerlIO_tell() and PerlIO_seek().

  • PerlIO_rewind(f)

    This corresponds to rewind(). It is usually defined as being

    1.  PerlIO_seek(f,(Off_t)0L, SEEK_SET);
      2.  
    2.  PerlIO_clearerr(f);
      3.
  • PerlIO_tmpfile()

    This corresponds to tmpfile(), i.e., returns an anonymous PerlIO orNULL on error. The system will attempt to automatically delete thefile when closed. On Unix the file is usually unlink-ed just afterit is created so it does not matter how it gets closed. On othersystems the file may only be deleted if closed via PerlIO_close()and/or the program exits via exit. Depending on the implementationthere may be "race conditions" which allow other processes access tothe file, though in general it will be safer in this regard thanad. hoc. schemes.

  • PerlIO_setlinebuf(f)

    This corresponds to setlinebuf(). Does not return a value. Whatconstitutes a "line" is implementation dependent but usually meansthat writing "\n" flushes the buffer. What happens with things like"this\nthat" is uncertain. (Perl core uses it only when "dumping";it has nothing to do with $| auto-flush.)

Co-existence with stdio

There is outline support for co-existence of PerlIO with stdio.Obviously if PerlIO is implemented in terms of stdio there is noproblem. However in other cases then mechanisms must exist to create aFILE * which can be passed to library code which is going to use stdiocalls.

The first step is to add this line:

  1.    #define PERLIO_NOT_STDIO 0
    2.

before including any perl header files. (This will probably becomethe default at some point). That prevents "perlio.h" from attemptingto #define stdio functions onto PerlIO functions.

XS code is probably better using "typemap" if it expects FILE *arguments. The standard typemap will be adjusted to comprehend anychanges in this area.

  • PerlIO_importFILE(f,mode)

    Used to get a PerlIO * from a FILE *.

    The mode argument should be a string as would be passed tofopen/PerlIO_open. If it is NULL then - for legacy support - the codewill (depending upon the platform and the implementation) eitherattempt to empirically determine the mode in which f is open, oruse "r+" to indicate a read/write stream.

    Once called the FILE * should ONLY be closed by callingPerlIO_close() on the returned PerlIO *.

    The PerlIO is set to textmode. Use PerlIO_binmode if this isnot the desired mode.

    This is not the reverse of PerlIO_exportFILE().

  • PerlIO_exportFILE(f,mode)

    Given a PerlIO * create a 'native' FILE * suitable for passing to codeexpecting to be compiled and linked with ANSI C stdio.h. The modeargument should be a string as would be passed to fopen/PerlIO_open.If it is NULL then - for legacy support - the FILE * is opened in samemode as the PerlIO *.

    The fact that such a FILE * has been 'exported' is recorded, (normallyby pushing a new :stdio "layer" onto the PerlIO *), which may affectfuture PerlIO operations on the original PerlIO *. You should notcall fclose() on the file unless you call PerlIO_releaseFILE()to disassociate it from the PerlIO *. (Do not use PerlIO_importFILE()for doing the disassociation.)

    Calling this function repeatedly will create a FILE * on each call(and will push an :stdio layer each time as well).

  • PerlIO_releaseFILE(p,f)

    Calling PerlIO_releaseFILE informs PerlIO that all use of FILE * iscomplete. It is removed from the list of 'exported' FILE *s, and theassociated PerlIO * should revert to its original behaviour.

    Use this to disassociate a file from a PerlIO * that was associatedusing PerlIO_exportFILE().

  • PerlIO_findFILE(f)

    Returns a native FILE * used by a stdio layer. If there is none, itwill create one with PerlIO_exportFILE. In either case the FILE *should be considered as belonging to PerlIO subsystem and shouldonly be closed by calling PerlIO_close().

"Fast gets" Functions

In addition to standard-like API defined so far above there is an"implementation" interface which allows perl to get at internals ofPerlIO. The following calls correspond to the various FILE_xxx macrosdetermined by Configure - or their equivalent in otherimplementations. This section is really of interest to only thoseconcerned with detailed perl-core behaviour, implementing a PerlIOmapping or writing code which can make use of the "read ahead" thathas been done by the IO system in the same way perl does. Note thatany code that uses these interfaces must be prepared to do things thetraditional way if a handle does not support them.

  • PerlIO_fast_gets(f)

    Returns true if implementation has all the interfaces required toallow perl's sv_gets to "bypass" normal IO mechanism. This canvary from handle to handle.

    1.   PerlIO_fast_gets(f) = PerlIO_has_cntptr(f) && \
      2.  
    2.  PerlIO_canset_cnt(f) && \
      3.  
    3.  'Can set pointer into buffer'
      4.
  • PerlIO_has_cntptr(f)

    Implementation can return pointer to current position in the "buffer"and a count of bytes available in the buffer. Do not use this - usePerlIO_fast_gets.

  • PerlIO_get_cnt(f)

    Return count of readable bytes in the buffer. Zero or negative returnmeans no more bytes available.

  • PerlIO_get_ptr(f)

    Return pointer to next readable byte in buffer, accessing via thepointer (dereferencing) is only safe if PerlIO_get_cnt() has returneda positive value. Only positive offsets up to value returned byPerlIO_get_cnt() are allowed.

  • PerlIO_set_ptrcnt(f,p,c)

    Set pointer into buffer, and a count of bytes still in thebuffer. Should be used only to set pointer to within range implied byprevious calls to PerlIO_get_ptr and PerlIO_get_cnt. The twovalues must be consistent with each other (implementation may onlyuse one or the other or may require both).

  • PerlIO_canset_cnt(f)

    Implementation can adjust its idea of number of bytes in the buffer.Do not use this - use PerlIO_fast_gets.

  • PerlIO_set_cnt(f,c)

    Obscure - set count of bytes in the buffer. Deprecated. Only usableif PerlIO_canset_cnt() returns true. Currently used in only doio.c toforce count less than -1 to -1. Perhaps should be PerlIO_set_empty orsimilar. This call may actually do nothing if "count" is deduced frompointer and a "limit". Do not use this - use PerlIO_set_ptrcnt().

  • PerlIO_has_base(f)

    Returns true if implementation has a buffer, and can return pointerto whole buffer and its size. Used by perl for -T / -B tests.Other uses would be very obscure...

  • PerlIO_get_base(f)

    Return start of buffer. Access only positive offsets in the bufferup to the value returned by PerlIO_get_bufsiz().

  • PerlIO_get_bufsiz(f)

    Return the total number of bytes in the buffer, this is neither thenumber that can be read, nor the amount of memory allocated to thebuffer. Rather it is what the operating system and/or implementationhappened to read() (or whatever) last time IO was requested.

Other Functions

  • PerlIO_apply_layers(f,mode,layers)

    The new interface to the USE_PERLIO implementation. The layers ":crlf"and ":raw" are only ones allowed for other implementations and thoseare silently ignored. (As of perl5.8 ":raw" is deprecated.) UsePerlIO_binmode() below for the portable case.

  • PerlIO_binmode(f,ptype,imode,layers)

    The hook used by perl's binmode operator.ptype is perl's character for the kind of IO:

    • '<' read
    • '>' write
    • '+' read/write

    imode is O_BINARY or O_TEXT.

    layers is a string of layers to apply, only ":crlf" makes sense inthe non USE_PERLIO case. (As of perl5.8 ":raw" is deprecated in favourof passing NULL.)

    Portable cases are:

    1.  PerlIO_binmode(f,ptype,O_BINARY,NULL);
      2.  
    2. and
      3.  
    3.  PerlIO_binmode(f,ptype,O_TEXT,":crlf");
      4.

    On Unix these calls probably have no effect whatsoever. Elsewherethey alter "\n" to CR,LF translation and possibly cause a special text"end of file" indicator to be written or honoured on read. The effectof making the call after doing any IO to the handle depends on theimplementation. (It may be ignored, affect any data which is alreadybuffered as well, or only apply to subsequent data.)

  • PerlIO_debug(fmt,...)

    PerlIO_debug is a printf()-like function which can be used fordebugging. No return value. Its main use is inside PerlIO where usingreal printf, warn() etc. would recursively call PerlIO and be aproblem.

    PerlIO_debug writes to the file named by $ENV{'PERLIO_DEBUG'} typicaluse might be

    1.   Bourne shells (sh, ksh, bash, zsh, ash, ...):
      2.  
    2.    PERLIO_DEBUG=/dev/tty ./perl somescript some args
      3.  
    3.  
    4.   Csh/Tcsh:
      5.  
    5.    setenv PERLIO_DEBUG /dev/tty
      6.  
    6.    ./perl somescript some args
      7.  
    7.  
    8.   If you have the "env" utility:
      9.  
    9.    env PERLIO_DEBUG=/dev/tty ./perl somescript some args
      10.  
    10.  
    11.   Win32:
      12.  
    12.    set PERLIO_DEBUG=CON
      13.  
    13.    perl somescript some args
      14.

    If $ENV{'PERLIO_DEBUG'} is not set PerlIO_debug() is a no-op.

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