Re: Perl Quiz of the Week #23

On Sep 3, 2004, at 15:43, Mark Jason Dominus wrote:

>> 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.

I see.

> 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.

This is zsh, I remember that "Broken pipe" message from other shells.

> 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'.

Exactly:

% (perl balanced_gen.pl 12; echo $? >/dev/tty) | head
(((((((((((())))))))))))
((((((((((()()))))))))))
((((((((((())())))))))))
((((((((((()))()))))))))
((((((((((())))())))))))
((((((((((()))))()))))))
((((((((((())))))())))))
((((((((((()))))))()))))
((((((((((())))))))())))
((((((((((()))))))))()))
141

Now everything makes sense, thank you very much Mark!

-- fxn