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Mark Overmeer
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POSIX::1003::FdIO - POSIX handling file descriptors


   is a POSIX::1003::Module


  use POSIX::1003::FdIO;

  $fd = openfd($fn, O_RDWR);
  defined $fd or die $!;   # $fd==0 is valid value! (STDIN)

  $fd = openfd($fn, O_WRONLY|O_TRUNC);
  $fd = openfd($fn, O_CREAT|O_WRONLY, 0640);

  my $buf;
  $bytes_read    = readfd($fd, $buf, BUFSIZ);
  $bytes_written = writefd($fd, $buf, 5);

  $off_t = seekfd($fd, 0, SEEK_SET);  # rewind!
  $fd2   = dupfd($fd);

  closefd($fd) or die $!;

  my ($r, $w) = pipefd();
  writefd($w, "hello", 5);
  readfd($r, $buf, 5);
  closefd($r) && closefd($w) or die $!;

  my $fh = fdopen($fd, 'w') or die $!;


Most people believe that the sys* commands in Perl Core are not capable of doing unbuffered IO. For those people, we have this module. But there is more in file-descriptor space, missing from Core.

The question whether sysread() or readfd() is meassurable faster cannot be answered.

The fcntl() command has its separate module POSIX::1003::Fcntl. Locking functions are locate there as well, because they are often implemented via fcntl.

See documentation in the base class.


See documentation in the base class.


See "METHODS" in POSIX::1003::Module


See "METHODS" in POSIX::1003::Module



Perl defaults to use file-handles, avoiding file descriptors. For that reason, the fread of POSIX is the read of Perl; that's confusing. But the POSIX standard is confused as well: some function names which start with an f are actually for file-descriptors, other for file-handles!

The POSIX module, distributed with Perl, makes you write CORE::read() and POSIX::read() explicitly. However, POSIX::read() is the same as CORE::sysread()!

To avoid conflicts with function names in Perl core, and the confusion that the POSIX created, all exported function names provided by this module contain 'fd' in their name.

    POSIX    Perl-Core POSIX.pm  POSIX::1003::FdIO
 FH fseek     seek
 FD lseek     sysseek   lseek    seekfd
 FH fopen     open
 FD open      sysopen            openfd   # sysopen is clumpsy
 FD fdopen    open               fdopen   # IO::Handle->new_from_fd
 FH fclose    close
 FD close     close     close    closefd
 FH fread     read
 FD read      sysread   read     readfd
 FH fwrite    print
 FD write     syswrite  write    writefd
 FH           pipe,open                   # buffered unless $|=0
 FD pipe                pipe     pipefd
 FH stat      stat
 FD fstat               fstat    statfd
 FN lstat     lstat
 FH ftell     tell
 FD                              tellfd   # tell on fd not in POSIX
 FH rewind              rewind
 FD                              rewindfd # rewind on fd not in POSIX
 FD creat               creat    creatfd
 FD dup                          dupfd
 FD fcntl     fcntl              (many)   # see ::Fcntl
 FD flock     flock              flockfd  # see ::Fcntl
 FD lockf                        lockf    # see ::Fcntl
 FN truncate  truncate
 FD ftruncate                    truncfd

Works on: FH=file handle, FD=file descriptor, FN=file name

Standard POSIX


Always check the return code: undef on error, cause in $!. closefd $fd or die $!;

There is no sysclose() in core, because sysopen() does unbuffered IO via its perl-style file-handle: when you open with CORE::sysopen(), you must close with CORE::close().


Implemented via openfd(), which is true by definition of POSIX.

dup2fd(FD, NEWFD)

Copy file-descriptor FD to an explicit NEWFD number. When already in use, the file at NEWFD will be closed first. Returns undef on failure.


Copy the file-descriptor FD into the lowest-numbered unused descriptor. The new fd is returned, undef on failure.

fdopen(FD, MODE)

Converts a FD into an (buffered) FH. You probably want to set binmode after this. MODE can be Perl-like '<', '>', '>>', or POSIX standard 'r', 'w', 'a'. POSIX modes 'r+', 'w+', and 'a+' can probably not be supported.


Returned is an integer file descriptor (FD). Returns undef on failure (and '0' is a valid FD!)

FLAGS are composed from the O_* constants defined by this module (import tag :mode) The MODE field combines S_I* constants defined by POSIX::1003::FS (import tag :stat).


Returns the reader and writer file descriptors. See also POSIX::1003::Fcntl::setfd_pipe_size()


  my ($r, $w) = pipefd;
  writefd($w, "hello", 5 );
  readfd($r, $buf, 5 );
readfd(FD, SCALAR, [LENGTH])

Read the maximum of LENGTH bytes from FD into the SCALAR. Returned is the actual number of bytes read. The value -1 tells you there is an error, reported in $!

Be warned that a returned value smaller than LENGTH does not mean that the FD has nothing more to offer: the end is reached only when 0 (zero) is returned. Therefore, this reading is quite inconvenient. You may want to use POSIX::Util subroutine readfd_all


The WHENCE is a SEEK_* constant.


Request file administration information about an open file. It returns the same list of values as stat on filenames.

truncfd(FD, [LENGTH])

[0.96] Shorten the file to the LENGTH (defaults to 0). The file offset (your pointer in the file) is not changed, so you may need to seekfd() as well. Behavior is undefined when LENGTH is larger than the file size.

The POSIX name for this function is ftruncate.

writefd(FD, BYTES, [LENGTH])

Attempt to write the first LENGTH bytes of STRING to FD. Returned is the number of bytes actually written. You have an error only when -1 is returned.


Zillions of Perl programs reimplement these functions. Let's simplify code.


Seek to the beginning of the file.


Reports the location in the file. This call does not exist (not in POSIX, nor on other UNIXes), however is a logical counterpart of the tell() on filenames.


The following constants are exported, shown here with the values discovered during installation of this module.

  BUFSIZ         8192
  EOF            -1
  MAX_INPUT      255
  O_ACCMODE      3
  O_APPEND       1024
  O_ASYNC        8192
  O_CLOEXEC      524288
  O_CREAT        64
  O_DIRECT       16384
  O_DIRECTORY    65536
  O_DSYNC        4096
  O_EXCL         128
  O_FSYNC        1052672
  O_NDELAY       2048
  O_NOATIME      262144
  O_NOCTTY       256
  O_NOFOLLOW     131072
  O_NONBLOCK     2048
  O_RDONLY       0
  O_RDWR         2
  O_RSYNC        1052672
  O_SYNC         1052672
  O_TRUNC        512
  O_WRONLY       1
  PIPE_BUF       4096
  SEEK_CUR       1
  SEEK_DATA      3
  SEEK_END       2
  SEEK_HOLE      4
  SEEK_SET       0
  SSIZE_MAX      9223372036854775807

You can limit the import to the SEEK_* constants by explicitly using the :seek import tag. Use the :mode for all O_* constants, to be used with openfd().


This module is part of POSIX-1003 distribution version 0.98, built on December 03, 2013. Website: http://perl.overmeer.net. The code is based on POSIX, which is released with Perl itself. See also POSIX::Util for additional functionality.


Copyrights 2011-2013 on the perl code and the related documentation by [Mark Overmeer]. For other contributors see ChangeLog.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. See http://www.perl.com/perl/misc/Artistic.html