Re: Perl Quiz of the Week #23

Xavier Noria <[email protected]>:
> On Sep 3, 2004, at 14:24, colin.rafferty-/PgpppG8B+R7qynMiXIxWgC/[email protected] wrote:
> 
> >> % time perl balanced_gen.pl 12
> >> perl balanced_gen.pl 12  4.38s user 0.03s system 83% cpu 5.255 total
> >
> > You are looking at the wrong number.  In the one you're showing, I'm
> > doing `head', so it's cut off after 10 results.  You really should be
> > looking at the results of the full run to see that we're in the same
> > ballpark.
> 
> I knew head(1) just showed the first 10 lines by default, but I assumed 
> the rest were ignored but printed no matter what by the script, since 
> there's nothing about the pipeline in the code.

What's happening here is that 'head' exits after printing the first
ten lines, and this closes the pipe.  balanced_gen continues running,
writing data into the pipe, until the pipe becomes full.  When it
tries to write to a full pipe, Unix sends it a PIPE signal, which
kills the balanced_gen process.

So it probably writes about ten lines plus 8192 bytes, more or less,
and then gets prematurely terminated.

Some Unix shells will display the message "Broken pipe" when a process
is killed by a PIPE signal, analogous to the "Segmentation fault" or
"Killed" messages that are displayed when processes are killed by SEGV
or KILL signals, but PIPE happens so often and it's such a normal
occurrence that many shells suppress the message.  You can see the
abnormal termination with

        (perl balanced_gen 12; echo $? >/dev/tty) | head

which should print '141'.  This is 128 (indicating that the process
was killed via a signal) plus 13 (the signal number arbitrarily
assigned to PIPE).  The 13 may vary on your system, but whatever
number you get should match the output of 'kill -l'.