Problem using select() to wait on pipes

[Atlant-Schmidt]

(It's my first time here; please be gentle!)

I've got a program that I wrote for the version of Perl distributed with Cygwin. It's creating a child process and managing that child using the select() call (so that the parent can keep processing while the child process is busy).

It works a treat in the Cygwin Perl environment but when I run it in the Strawberry Perl environment, select() returns -1 and the error is "An operation was attempted on something that is not a socket.".

Interestingly enough, the child process WAS created and it goes on its way correctly; I just can't collect any output from it because of the failure of the select() call. And because of the way the rest of my code works, I fail to reap the child process when it completes.

Here (in simplified form) is my code:

use strict;
use warnings FATAL => 'all';            # See the cautions at https://perldoc.perl.org/warnings regarding FATAL => all

use IPC::Open3;

use POSIX 'WNOHANG';

my $command = 'ls -l run_a_subprocess.pl ; echo "sleeping" ; sleep 3 ; echo "Done sleeping" ; ls -l run_a_subprocess.pl';

my $ParentPid = $$;                                                             # Just for debugging, get the Parent (our) PID
print STDERR "ParentPid = $ParentPid\n";                                        #   :

my $ChildPid = open3( my $child_in, my $child_out, undef, $command );           # The child reads from the child_in file handle, send STDOUT and STDERR to the child_out file handle
print STDERR "ChildPid = $ChildPid\n";                                          #   :

my $ChildFileno_in  = fileno( $child_in  );                                     # Get the fd of the child's STDIN (We don't actually need or use that)
print STDERR "Child stream in fileno = $ChildFileno_in\n";                      #   :

my $ChildFileno_out = fileno( $child_out );                                     # Get the fd of the child's STDOUT/STDERR (That's where we'll find the output from the child)
print STDERR "Child stream out fileno = $ChildFileno_out\n";                    #   :

my $rin = my $win = my $ein = '';                                               # Set the read, write, and exception select() bit masks to nil
vec( $rin, $ChildFileno_out, 1 ) = 1;                                           # Set the read bit mask to refer to the child's STDOUT/STDERR

# Now run the subprocess (the child process) and process its output, line-by-line

while ( 1 )                                                                             # Forever...
{
   my $FdCount = select( my $rout = $rin, my $wout = $win, my $eout = $ein, 0.250 );    # Wait 250 ms for our one-and-only waited-on FDs to become ready
   print STDERR "\n\$FdCount = $FdCount\n";

   if ( $FdCount == -1 )
   {
      print "\$\! = $!\n";
      exit( 1 );
   }

   # (more child output-processing stuff)
}

Here's the output I receive:

$command = "ls -l run_a_subprocess.pl ; echo "sleeping" ; sleep 3 ; echo "Done sleeping" ; ls -l run_a_subprocess.pl"
ParentPid = 32724
ChildPid = 20236
Child stream in fileno = 4
Child stream out fileno = 5

$FdCount = -1
$! = An operation was attempted on something that is not a socket.

[shawnlaffan]

Are you calling the code from Cmd, Powershell or Cygwin?

The code in $command does not run when called at the Cmd prompt as there is no ls tool.

Substituting dir for ls still does not work as Cmd uses a different syntax for multiple commands on one line (I'm still looking for that - hopefully others can chime in).

[Atlant-Schmidt]

I apologize for creating confusion with my rather-complex command-to-be-executed.

You may safely take it as written that:

my $command = 'c:/Strawberry/perl/bin/perl.exe --version';

And that single command should work in any command shell.

[sisyphus]

I get the same output as reported by @Atlant-Schmidt when I run the given program in the cmd.exe shell with Strawberry-Perl-5.42.0.

I notice that the perlport documentation has this to say wrt select :

select  (Win32, VMS) Only implemented on sockets.

I don't know whether that's related to the problem, or whether the actual issue is simply the one that @shawnlaffan pointed out.

The only way I know of running multiple commands on one cmd.exe command line is to separate them with && instead of ;.
But then there's still the issue of making those commands compatible with cmd.exe. (I don't even know what to replace sleep with.)

Maybe there's better mileage to be obtained using PowerShell. (I don't know about that, either.)

As an afterthought, I changed $command to something that both cmd and Cygwin's bash shell will understand:

$command = 'echo "HELLO"';

When run in cygwin, the altered script still runs fine .... and in Windows it still dies in exactly the same way. (So maybe it is simply the portability issue with select after all ?)

@Atlant-Schmidt, do you really need to run the program outside of Cygwin ?

[Atlant-Schmidt]

I've run the problem program in Cygwin windows, in ordinary Windows cmd.exe windows, and in PowerShell windows with the same results: Using the Cygwin Perl it succeeds and using Strawberry Perl it fails as described.

(Please note the simplified command that I've now offered in a reply above.)

Background:

The reason for all of this is that this is a fragment of a larger script to be used by all of the engineers on my team. By fiat, they all have Strawberry Perl installed but they don't all have Cygwin (or Cygwin's version of Perl) so we wanted to be able to describe to them that they should run the script using Strawberry Perl (in whatever command shell they like).

[sisyphus]

I think there will be a workaround for the non-portability of that select() call, but I don't know what that workaround is.
If no-one here can provide decent help, I suggest seeking assistance on perlmonks or stackoverflow.

Anyone ??

Unfortunately connecting to perlmonks (https://www.perlmonks.org/?node=Seekers%20of%20Perl%20Wisdom) can be a bit slow of late, but I think stackoverflow should be able to help out if you have trouble getting through to perlmonks.

[shawnlaffan]

If this can be replicated on Windows with a non-Strawberry perl (probably non-gcc also) then it's likely a more general perl-on-windows issue. Checking on perlmonks or stackoverflow would be the right path.

[sisyphus]

If this can be replicated on Windows with a non-Strawberry perl (probably non-gcc also) then it's likely a more general perl-on-windows issue.

Yes - I see the same behaviour on my own builds of perl (both gcc and MSVC builds).
It's definitely not specific to Strawberry Perl.

[sisyphus]

I wonder if switching to IO::Select (as a replacement to the select core function) would provide the portable solution that's required here.
The IO::Select module is part of perl core (and therefore ships as part of Strawberry Perl) and its documentation makes no mention of any Windows limitations.

From the documentation:

DESCRIPTION
    The "IO::Select" package implements an object approach to the system
    "select" function call. It allows the user to see what IO handles, see
    IO::Handle, are ready for reading, writing or have an exception pending.

@Atlant-Schmidt, it might be worth your while to investigate that.