/ 
POSIX-1003-1.02
River stage one • 2 direct dependents • 2 total dependents
/ POSIX::1003::FdIO

NAME

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

SYNOPSIS

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 $!;

DESCRIPTION

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.

FUNCTIONS

Overview

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

closefd($fd)

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().

creatfd($filename, $mode)

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.

dupfd($fd)

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.

openfd($filename, $flags, $mode)

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).

pipefd()

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

example:

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

seekfd($fd, $offset, $whence)

The $whence is a SEEK_* constant.

statfd($fd)

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.

Additional

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

rewindfd($fd)

Seek to the beginning of the file.

tellfd($fd)

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.

CONSTANTS

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

The constant names for this fdio module are inserted here during installation.

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().

SEE ALSO

This module is part of POSIX-1003 distribution version 1.02, built on November 10, 2020. Website: http://perl.overmeer.net/CPAN. The code is based on POSIX, which is released with Perl itself. See also POSIX::Util for additional functionality.

COPYRIGHTS

Copyrights 2011-2020 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://dev.perl.org/licenses/