Question

I have a program which is calling another program and processing the child’s output, ie:

my $pid = open($handle, "$commandPath $options |");

Now I’ve tried a couple different ways to read from the handle without blocking with little or no success.

I found related questions:

• perl-win32-how-to-do-a-non-blocking-read-of-a-filehandle-from-another-process
• why-does-my-perl-sysread-block-when-reading-from-a-socket

But they suffer from the problems:

• ioctl consistently crashes perl
• sysread blocks on 0 bytes (a common occurrence)

I’m not sure how to go about solving this problem.

Answer 1

Pipes are not as functional on Windows as they are on Unix-y systems. You can’t use the 4-argument select on them and the default capacity is miniscule.

You are better off trying a socket or file based workaround.

$pid = fork();
if (defined($pid) && $pid == 0) {
    exit system("$commandPath $options > $someTemporaryFile");
}
open($handle, "<$someTemporaryFile");

Now you have a couple more cans of worms to deal with — running waitpid periodically to check when the background process has stopped creating output, calling seek $handle,0,1 to clear the eof condition after you read from $handle, cleaning up the temporary file, but it works.

I have written the Forks::Super module to deal with issues like this (and many others). For this problem you would use it like

use Forks::Super;
my $pid = fork { cmd => "$commandPath $options", child_fh => "out" };
my $job = Forks::Super::Job::get($pid);
while (!$job->is_complete) {
    @someInputToProcess = $job->read_stdout();
    ... process input ...
    ... optional sleep here so you don't consume CPU waiting for input ...
}
waitpid $pid, 0;
@theLastInputToProcess = $job->read_stdout();