For the Nth time, don't use local to save/restore file handles, especially the STD* handles. Follow the example in the open documentation for saving and restoring file handles.

A subprocess will inherit and use file descriptors 0, 1, and 2 from the parent process. local *STDOUT; hides away file descriptor 1 and prevents the process from being able to change it (and prevents Perl from doing its normal extra work when STDOUT is re-opened such that file descriptor 1 would be used).

Update: s/file handle (\d)/file descriptor $1/g, etc.

++tye, for accuracy, but --perl for correctness. The filenos for clib stdin, stdout and stderr should not be hardwired. I recall showing you, a few years ago, how to make stderr have file descriptor 3 on linux and still behave properly there.

I'm not very up-to-date on either linux or perl at the moment, but I'll check and see if my preferred conventions still hold.

After Compline,
Zaxo

how to make stderr have file descriptor 3 on linux and still behave properly there

When you fork exec, the only I/O units that the process keeps are the file descriptors. Whatever you have for file descriptor 0 will be stdin/STDIN after exec, no matter what you were using it for and no matter what conventions you set up for how you use it. Whatever your conventions are, they don't change how fork/exec work nor how the process initialization sets up stdin in C (which is what Perl uses to set up its STDIN).

So if you have conventions that make it so your code will use fd 3 for stderr when you've set it up for that, those have no impact on what the standard process initialization code will do when you fork/exec nor how the child process will set up stderr based on what fd 2 was in the parent process (which will also mean that your code would use fd 2 as stderr when the process is freshly initialized). So your conventions can't "fix" Perl so that, when somebody buries fd 1 with local(*STDOUT) Perl can somehow cause child processes to not use that buried fd 1 as their stdout.

- tye        

use strict;
use warnings;
use IPC::Open2 ;

my $out = '' ;
{  local *STDOUT ;
   open(STDOUT, ">", \$out);
   process();
}
print "\$out = \n", $out;

sub process {
   my $pid = open2( *Reader, *Writer, "/bin/date" );
   my @prog_out = <Reader> ;
   close Writer;
   close Reader;
   print "output from date:\n",join("|", @prog_out), "\n";
}
[download]

This will produce:

Thu Feb 28 08:08:13 CST 2008
$out = 
output from date:
[download]

which shows that date printed to the original STDOUT and that the print in the process subroutine printed to $out. If you don't localize STDOUT you get:

output from date:
Thu Feb 28 08:09:54 CST 2008

$out =
[download]

which shows that date emitted its output through the pipe.

Do you really need to localize STDOUT and STDERR? How about just using select to select a different default file handle? If you need to capture STDERR from your sub-process, how about using IPC::Open3?