Perl Cookbook

Opening and Closing File Descriptors by Number

Problem

You know which file descriptors you’d like to do I/O on, but Perl requires filehandles, not descriptor numbers.

Solution

To open the file descriptor, use the "<&=" or "<&" open modes or the IO::Handle module’s fdopen class method:

open(FH, "<&=$FDNUM");      # open FH to the descriptor itself
open(FH, "<&$FDNUM");       # open FH to a copy of the descriptor

use IO::Handle;

$fh->fdopen($FDNUM, "r");   # open file descriptor 3 for reading

To close one by number, either use the POSIX::close function or else first open it as we did previously.

Discussion

Occasionally you have a file descriptor but no filehandle. Perl’s I/O system uses filehandles instead of file descriptors, so you have to make a new filehandle for an already open file descriptor. The "<&", ">&", and "+<&" modes to open will do this for reading, writing, and updating respectively. Adding an equal sign to these to make "<&=", ">&=", and "+<&=" is more parsimonious of file descriptors and nearly always what you want to do. That’s because it used only a C-level fdopen function, not a dup2 system call.

If you have version 5.004 or better installed, you can use the IO::Handle object method. This is the same as:

use IO::Handle;
$fh = IO::Handle->new();

$fh->fdopen(3, "r");            # open fd 3 for reading

Closing a file descriptor by number is even rarer. The POSIX::close function does so directly. If your system doesn’t have a working POSIX library but does have a working syscall (and your sysadmin ...