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Re: Re: capturing STDOUT

by DrZaius (Monk)
on Apr 30, 2001 at 08:34 UTC ( #76544=note: print w/ replies, xml ) Need Help??


in reply to Re: capturing STDOUT
in thread capturing STDOUT

I can't seem to get this to work. I have the following code, but on my linux/perl5.6.0 system, strace says it is blocking on the read.

#!/usr/bin/perl -w use strict; my $stuff; { pipe(READ,WRITE); local *STDOUT=*WRITE; print "Into the pipe\n"; local $/; $stuff = <READ>; } print "Pipe has: $stuff";
Any suggestions?


Comment on Re: Re: capturing STDOUT
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Re: Re: Re: capturing STDOUT
by Rhandom (Curate) on Apr 30, 2001 at 18:12 UTC
    Just a guess... But I think that it is still blocking on the READ handle because the WRITE handle is still open.

    Yup, just checked. That is what is happening...
    #!/usr/bin/perl -w pipe(READ,WRITE); print WRITE "Good\n"; close(WRITE); print <READ>;

    Try that code with and without the close(WRITE) line commented out. If you comment it out it hangs.

    Here is some more trouble that you're going to run into. Because you need to close the WRITE handle before the <READ> will stop, you are going to run into a buffer limitation (actually this could happen anyway). On many systems a pipe buffer can only accept 4k to 8k before it blocks. It will block until somebody cleans it out by reading the READ end. This means if the output of your command is too big, just executing it will block as it tries to put all of it's info into the WRITE handle.

    The way to fix all of this is to pipe, fork, have the parent READ and the child WRITE.

    #!/usr/bin/perl -w pipe(READ,WRITE); my $pid = fork; die $! unless defined $pid; ### parent if( $pid ){ local $/ = 1; # never do this global my $out = <READ>; ### child }else{ # local *STDOUT = *WRITE; print WRITE "Good\n"; close(WRITE); exit; } print "I got [$out]\n";
    my @a=qw(random brilliant braindead); print $a[rand(@a)];

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