Re: Passing a File Descriptor to a New Process on Windows

[Corion]

There is IO::FDPass, which implements the appropriate Windows system calls.

[tye]

For the sake of clarity, IO::FDPass doesn't implement letting a child process inherit an open file descriptor. It implements passing an open file descriptor through a Unix-domain socket (and similar functionality on Windows).

It also doesn't work with cygwin Perl.

To pass an open file descriptor to a child process in Windows Perl you actually have to pass an open low-level Windows file handle to the child and you have to create the child with something other than fork.

The core module Win32API::File includes the required functionality for looking up the low-level handle so you can inform the child where to find it and for opening a Perl file handle so you can read from and/or write to it.

There are several ways to spawn the child process, including: Win32::Process::Create(), system(1,...) (see perlport), system("start ...").

Unfortunately, although I've done this successfully before and have helped others do it at PerlMonks, none of us has actually posted working examples of this as far as I can tell.

But it wasn't particularly hard. You do the steps the same as on Unix except:

• You set the "INHERIT" flag via Win32API::File::SetHandleInformation().
• You get the low-level handle via FdGetOsFHandle().
• You don't use fork() (see above).
• You "open" the handle via OsFHandleOpen().

[hardburn]

Thanks, tye, have this all figured out now.

fd_pass.pl

#!/usr/bin/perl
use v5.14;
use warnings;
use Win32API::File 'FdGetOsFHandle';


my $FILE = shift or die "Need file to read\n";


say "Opening file";
open( my $in, '<', $FILE ) or die "Can't open '$FILE': $!\n";

# Get the real FD from Windows
my $fd = FdGetOsFHandle( fileno($in) );

say "Spawning child process";
my $pid = system(1, 'perl', './fd_get.pl', $fd )
    or die "Could not spawn child process: $!\n";

wait;

fd_get.pl

#!/usr/bin/perl
use v5.14;
use warnings;
use Win32API::File 'OsFHandleOpen';

my $FD = shift or die "Need file descriptor\n";
OsFHandleOpen( *IN, $FD, 'r' )
    or die "Could not open file descriptor '$FD': $!\n";


while(<IN>) {
    chomp;
    say "Got in child: $_";
}

close IN;