The right way to use IPC::Open3 in Perl

Question

Here is the function which does exactly what is specified by its name:

sub runCmd_getStdoutStderrAndExitCode(@)
{
 my @cmd = @_;
 # fork a child
 my ($child_pid, $child_stdout, $child_stderr);
 eval
 {
 $child_pid = open3(\*STDIN, # is /dev/null when collectl is a daemon, UPDATE: can pass 'undef' here
 $child_stdout,
 $child_stderr,
 @cmd);
 };
 if ($@ =~ /^open3:/)
 {
 syslog(LOG_ERR, 'Error in "%s" : %s', "@cmd", $@);
 return (undef, undef, -1);
 }
 # result buffers
 my $stdout = '';
 my $stderr = '';
 # read into result buffers avoid blocking
 my $selector = IO::Select->new();
 $selector->add($child_stdout, $child_stderr);
 while (my @ready = $selector->can_read(SOME_TIMEOUT_CONSTANT))
 {
 foreach my $fh (@ready) {
 my $target_buf = ($fh == $child_stdout)
 ? \$stdout
 : \$stderr;
 if (!sysread($fh, $$target_buf, 4096, length($$target_buf)))
 {
 $selector->remove($fh);
 close $fh;
 }
 }
 }
 close $_ for ($selector->handles());
 my $finished = waitpid($child_pid, WNOHANG);
 return ($stdout, $stderr, $?) if $finished;
 if (kill 'TERM' => $child_pid) {
 # hang until SIGTERM is processed then bury child process
 waitpid($child_pid, 0);
 } else {
 syslog(LOG_WARNING, 'Error sending TERM to child process "%s"', "@cmd");
 }
 return ($stdout, $stderr, -1);
}

After the script successfully completes its job and before Perl interpreter exits, I get this message in the terminal:

stty: standard input: Bad file descriptor

It would be nice to have your opinion about the code correctness, and get a piece of advice about how to avoid that message, as a bonus.

UPDATE: The message mentioned gone away when I replaced '\*STDIN' arg with 'undef'.

Major update: It appears that this code is not functioning properly. It mixes stderr and stdout from child process, and returns everything back as stdout. I have no clue, why it is so, and I started looking towards replacing my own bicycle with IPC::Run.

UPDATE 2: Finally, here is valid and working replacement of aforementioned bicycle masterpiece. The only disadvantage, in my opition, is that IPC::Run is not part of standard Perl distribution:

sub _runCmd_getStdoutStderrAndExitCode(@)
{
 use IPC::Run qw(run timeout);
 my @cmd = @_;
 my ($out, $err, $ok);
 my @args = (\@cmd, \*STDIN, \$out, \$err);
 push @args, timeout(SOME_TIMEOUT_CONSTANT) if SOME_TIMEOUT_CONSTANT;
 eval{ $ok = run @args };
 if ($@) 
 {
 return ($out, $err, -1);
 }
 else
 {
 return ($out, $err, ($ok? 0 : $?));
 }
}

Thanks for your comments!

• Phrancis: Could that message be related to this bug perhaps? As long as the code works as intended (which is sounds like it does) this should still be on-topic regardless of that message, I think.
• 200_success: By before Perl interpreter exists, do you mean before the Perl interpreter exits?
• 200_success: That message makes some sense if your STDIN is /dev/null, as stated in your comment.

Answer 1

as it say in the IPC::Open3 documentation, the filehandle for STDERR cannot be autovivified. You need to

use Symbol qw/ gensym /; my $stderr = gensym;

to create a file handle before opening your command.