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Re: close() on opened pipe fails in forked child

by kennethk (Monsignor)
on May 09, 2013 at 17:04 UTC ( #1032800=note: print w/ replies, xml ) Need Help??


in reply to close() on opened pipe fails in forked child

Is there a particular reason you have to die on a close failure? I assume the close is necessary for a buffer flush. Is there a particular reason you need a global filehandle? This issue would seem to go away if you use Indirect Filehandles and let the Perl garbage collection resolve the closing.

open my $pipe, "| cat" or die $!; my $pid = fork(); if( $pid == 0 ) { # child print $pipe "Child\n"; exit 0; } sleep 1; print $pipe "Parent\n"; my $reaped = waitpid(-1, 0 ); print "pid $reaped\n";

#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.


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Re^2: close() on opened pipe fails in forked child
by saintmike (Vicar) on May 09, 2013 at 20:04 UTC
    Is there a particular reason you have to die on a close failure?

    I just used die() for illustration purposes, the real module handles it differently, but it must somehow handle errors on close() because they could be real errors with the command run by the pipe, and the user needs to know what's going on.

    Is there a particular reason you need a global filehandle? This issue would seem to go away if you use Indirect Filehandles and let the Perl garbage collection resolve the closing.

    The file handle is stored within the module and therefore persists until the object of the class that the module implements goes out of scope. In this case, this happens when the child terminates.

    I guess my issue is that this is not a "real" error, neither from the user's perspective (who just forks), or from the module owner's perspective (who just opens/closes a pipe), and therefore it shouldn't even be surfaced.

      Well, you could come up with a hackish work around by caching the value of $$ when you open your pipe, and then check to see if the value is equivalent when you close it to decide if the error should be propagated or not, e.g.
      my $status = $pipe->close; die $! if $$ != $mainpid;
      If you are confident that only the parent thread will access the output methods, you could write the above more like:
      if ($$ == $mainpid) { $pipe->close or die $!; }
      Both of these solutions require you to put the effort into handling the issue, which is less aesthetically pleasing to me, but gets the job done. I cannot come up with an effective way of propagating real errors and blocking apparent errors from the fork that seems elegant.

      The only other possibility that occurs to me is to check if the pipe process is a child of the current process before error propagation (as opposed to caching pid), but this does not strike me as fundamentally cleaner.


      #11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

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