How to tell if a pipe opened for reading has data to be read

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

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Re: How to tell if a pipe opened for reading has data to be read by superfrink (Curate) on Mar 30, 2007 at 03:11 UTC
Have a look at the IO::Select module on CPAN or the select system call.	[reply]
Re^2: How to tell if a pipe opened for reading has data to be read by Anonymous Monk on Mar 30, 2007 at 03:57 UTC
Doesn't seem to be working. This code prints `select()'ed`, then blocks for 5 seconds and prints `[Piped output]` instead of printing `nothing yet` immediately and exiting. `#!/usr/bin/perl -w use strict; use Symbol; my $sym = gensym; open($sym, "perl -e 'sleep(5); print qq!Piped output!;' \|") or die $!; my ($rin, $rout); $rin = ''; vec($rin, fileno $sym, 1) = 1; my $changed = select($rout = $rin, undef, undef, 0); print "select()'ed\n"; if ($changed && vec($rout, fileno $sym, 1) == 1) { my $buf; sysread($sym, $buf, 4096); print "[$buf]"; } else { print "nothing yet"; }` [download]	[reply] [d/l] [select]
Re^3: How to tell if a pipe opened for reading has data to be read by Joost (Canon) on Mar 30, 2007 at 12:11 UTC
You're suffering from buffering - that program acutally print()s "nothing yet" immediately but you won't see it till the program ends because your output is buffered. try this: `#!/usr/bin/perl -w use strict; use Symbol; $\|=1; # turn off buffering my $sym = gensym; open($sym, "perl -e 'sleep(5); print qq!Piped output!;' \|") or die $!; my ($rin, $rout); $rin = ''; vec($rin, fileno $sym, 1) = 1; my $changed = select($rout = $rin, undef, undef, 0); print "select()'ed: changed=$changed\n"; if ($changed && vec($rout, fileno $sym, 1) == 1) { my $buf; sysread($sym, $buf, 4096); print "[$buf]\n"; } else { print "nothing yet\n"; }` [download] "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l]
Re: How to tell if a pipe opened for reading has data to be read by saintly (Scribe) on Mar 30, 2007 at 05:51 UTC
Perhaps you might benefit from having a look at the answers from the last time this question was asked: - Non blocking read on a filehandle One example that was reported to work was the one about halfway down using Fcntl. Only worked on UNIX though, which doesn't seem like it will be a hindrance for you. Although setting select's timeout to 0 doesn't work, you could set a minimal timeout like 0.01 (1 ms) which might work in your current implementation scheme, but is still inelegant.	[reply]
Re^2: How to tell if a pipe opened for reading has data to be read by dk (Chaplain) on Mar 30, 2007 at 13:10 UTC
Hm. Why does setting select's timeout to 0 not work? It should work, at least in POSIX-compatible implementations.	[reply]
Re^3: How to tell if a pipe opened for reading has data to be read by saintly (Scribe) on Mar 30, 2007 at 22:32 UTC
I was hoping that Perl would turn an 'undef' for $timeout into a NULL pointer and '0' for $timeout into a struct timeval with a 0 value, but I hadn't tested it. One of the comments in the earlier thread Re: Re: Non blocking read on a filehandle indicated it wasn't working then (back in 2001).	[reply]
Re: How to tell if a pipe opened for reading has data to be read by zentara (Cardinal) on Mar 30, 2007 at 13:17 UTC
Look at "perldoc -q filehandle" and search for the word ioctl. There is a low-level method to tell how many characters are waiting in a pipe. I tried to use it on "bc" in IPC3 buffer limit problem I'm not really a human, but I play one on earth. Cogito ergo sum a bum	[reply]