Open, ye! And read from your child!

gryng has asked for the wisdom of the Perl Monks concerning the following question:

I have a much larger program that this goes with, but even the smaller example does not seem to work. I tried initially to use the pipe command, and then switched to open(READ, "-|") command -- but both exhibit the same behavior: I do not get any data from the child until the child closes his end. Here is my example code:

#!/usr/bin/perl -w
use strict;

if (my $pid = open(READ, "-|")) {
  print "child said: '", <READ>, "'; ",time,"\n";
  print "child said: '", <READ>, "'; ",time,"\n";
} else {
  print "hello; ",time,"\n";
  sleep 2;
  print "how are you; ",time,"\n";
}
[download]

I receive this as output:

gryn@echor:~$ ./testcase.pl
child said: 'hello; 1005587480
how are you; 1005587482
'; 1005587482
child said: ''; 1005587482
[download]

I'm quite perplexed, and I've read over and over the various examples in the documentations (such as perlipc's numerous ones). I just can figure it out.

I am running with perl 5.6, on a intel debian unstable box; in case that matters.

Ciao,
Gryn

Comment on Open, ye! And read from your child! Select or Download Code

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Re: Open, ye! And read from your child! by runrig (Abbot) on Nov 12, 2001 at 23:08 UTC
Along with having buffering on, your `<READ>` is in list context, sucking up all input, which doesn't appear to be what you want. Try this: `$\|++; if (my $pid = open(READ, "-\|")) { my $line = <READ>; print "child said: [", $line, "]; ",time,"\n"; $line = <READ>; print "child said: [", $line, "]; ",time,"\n"; } else { print "hello; ",time,"\n"; sleep 2; print "how are you; ",time,"\n"; }` [download]	[reply] [d/l] [select]
Re: Re: Open, ye! And read from your child! by gryng (Hermit) on Nov 13, 2001 at 01:42 UTC
Doh! You would think a (relatively) seasoned Perl Junkie would have noticed that list context blurp. Ok the new code that works is: `#!/usr/bin/perl -w use strict; if (my $pid = open(READ, "-\|")) { print "child said: '", scalar <READ>, "'; ",time,"\n"; print "child said: '", scalar <READ>, "'; ",time,"\n"; } else { $\|=1; print "hello; ",time,"\n"; sleep 2; print "how are you; ",time,"\n"; }` [download] And output: `gryn@echor:~$ ./testcase.pl child said: 'hello; 1005597768 '; 1005597768 child said: 'how are you; 1005597770 '; 1005597770` [download] The buffering is necessary, as you all pointed out. However, with the list context problem in the way, I couldn't figure out where to properly put the (de)buffering until it was fixed. Thanks and kudos to you three, Gryn	[reply] [d/l] [select]
Re: Open, ye! And read from your child! by belg4mit (Prior) on Nov 12, 2001 at 23:01 UTC
Looks like buffering is getting you, try disabling it (well setting it to auto-flush rather): `select(READ); $\|=1;` [download]	[reply] [d/l]
Re: Open, ye! And read from your child! by Zaxo (Archbishop) on Nov 12, 2001 at 23:17 UTC
The parent reads READ in list contest, so the first `print` prints everything child has to say. The parent's second `print` statement immediately gets eof. Update: Oops, kept this up too long before answering, didn't mean to dup runrig's answer After Compline, Zaxo	[reply]