in reply to Forks, Pipes and Exec
The problem presumably is the following: the way you're connecting STDOUT to the pipe is changing the file descriptor number so that it's no longer 1. However, file descriptor 1 is what your exec'ed child process is assuming to be stdout (i.e. the default).
printf STDERR "before: fileno(STDOUT)=%d\n", fileno(STDOUT); open $old_stdout, ">&STDOUT" or die "open: $!"; close( STDOUT ); pipe( $smash_stdout, STDOUT ); printf STDERR "after: fileno(STDOUT)=%d\n", fileno(STDOUT);
prints
before: fileno(STDOUT)=1 after: fileno(STDOUT)=6
Note that fileno changed from 1 to 6. In other words, you now have fileno 6 connected to the pipe (something an exec'ed child would know nothing about...)
Something like this might work better
printf STDERR "before: fileno(STDOUT)=%d\n", fileno(STDOUT); open $old_stdout, ">&STDOUT" or die "open: $!"; # _don't_ explicitly close STDOUT here local(*RH, *WH); pipe RH, WH; open STDOUT, ">&WH" or die "open: $!"; printf STDERR "after: fileno(STDOUT)=%d\n", fileno(STDOUT);
because this way, STDOUT keeps being associated with file descriptor 1:
before: fileno(STDOUT)=1 after: fileno(STDOUT)=1
Update: a simple demo:
open $old_stdout, ">&STDOUT" or die "open: $!"; local(*RH, *WH); pipe RH, WH; open STDOUT, ">&WH" or die "open: $!"; # write something to the pipe print "foo"; # and have some fork/exec'ed process write to the pipe (via stdout) system('echo bar'); close WH; my $out = <RH>; print STDERR "got from pipe: $out"; # $out is "foobar\n"
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Re^2: Forks, Pipes and Exec (file descriptors)
by diabelek (Beadle) on Oct 30, 2008 at 19:05 UTC | |
by diabelek (Beadle) on Nov 06, 2008 at 18:01 UTC | |
by BrowserUk (Patriarch) on Nov 06, 2008 at 19:17 UTC | |
by diabelek (Beadle) on Nov 06, 2008 at 19:44 UTC | |
by BrowserUk (Patriarch) on Nov 06, 2008 at 20:48 UTC | |
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