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Re^2: IPC::Open3 woes

by LNEDAD (Novice)
on Mar 24, 2010 at 13:15 UTC ( #830552=note: print w/ replies, xml ) Need Help??


in reply to Re: IPC::Open3 woes
in thread IPC::Open3 woes

I am hopeing that somebody can add some clarity to this old thread. Can somebody tell me why the following coded hangs if the line which defines the @outlines array is commented out?

I have used open3 in other places w/o reading the the child's output and the process didn't hang. I expect it is because in that case the command didn't have output.

I understand how some of the other example programs in this thread prevent locking while read from open3's handles but it seems strange that this would lock when I am not reading from them and just calling waitpid as soon as possible.

Also, should I explicitly close $wtr, $rtr, and $err?

... my($wtr,$rdr); my $err = gensym; ### $logcmd is a command that returns approx. 460 lines. my $pid = open3($wtr, $rdr, $err, "$logcmd"); my @outlines = <$rdr>; ### if this line is commented out we hang on w +aitpid waitpid( $pid, 0 ); ...


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Re^3: IPC::Open3 woes
by Anonymous Monk on Apr 21, 2010 at 23:04 UTC
    Please somebody do comment, this is great information. I fear it is now out of date. I can't get this to work under Perl 5.8 any more. The documentation does not help. The code hangs on the open3 call.
Re^3: IPC::Open3 woes
by ikegami (Pope) on Apr 21, 2010 at 23:36 UTC

    I am hopeing that somebody can add some clarity to this old thread. Can somebody tell me why the following coded hangs if the line which defines the @outlines array is commented out?

    You have a deadlock.

    • waitpid never returns because the child never exits.
    • The child never exits because it's blocked trying to write a full pipe.
    • The pipe is full because noone is emptying it.

    If it doesn't happen for all your children, it's because they don't output enough data to fill the pipe.

    use strict; use warnings; use IPC::Open3 qw( open3 ); for (my $size = 1024; ; $size += 1024) { print("$size\n"); my $pid = open3( local our $to_chld, local our $fr_chld, local our $fr_chld_err, perl => ( -e => 'print "x" x $ARGV[0]', $size ) ); waitpid($pid, 0); }
    ... 62464 63488 64512 65536 66560 [hangs]^C

    The solution is to redirect the output to /dev/null

    use strict; use warnings; use IPC::Open3 qw( open3 ); open(local *DEVNULL, '>', '/dev/null') or die; for (my $size = 1024; ; $size += 1024) { print("$size\n"); my $pid = open3( local our $to_chld, '>&DEVNULL', '>&DEVNULL', perl => ( -e => 'print "x" x $ARGV[0]', $size ) ); waitpid($pid, 0); }
    ... 365568 366592 367616 368640 369664 370688 371712 372736 373760 374784 ...

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