topic Re: Perl Question in Operating System - Linux

Perl Question

Steve Start — Mon, 07 Aug 2006 11:33:55 GMT

Hello Perl Experts:

How do I temporarily replace a signal handler inside a perl function and then restore the previous signal handler?

TIA,
Steve

Re: Perl Question

A. Clay Stephenson — Mon, 07 Aug 2006 11:39:20 GMT

This is rather easy if I correctly understand your question.

my $Flag = 0;

sub my_local_handler
{
$SIG{INT} = \&my_local_handler;
++$Flag;
return(0);
} # my_local_handler

sub my_function
{
local $SIG{INT} = \&my_local_handler;
...
...
return(0);
} # my_function

The key is the "local" scope. When my_function falls out of scope then the signal handler for SIGINT reverts to its prior value.

Re: Perl Question

James R. Ferguson — Mon, 07 Aug 2006 11:41:49 GMT

Hi Steve:

Use a 'local' declaration for your handler. For instance:

sub mything {
local $SIG{INT} = \&myhandler;
...
}

Regards!

...JRF...

Re: Perl Question

Steve Start — Mon, 07 Aug 2006 12:02:56 GMT

Thanks guys. That was very quick.

A. Clay, why do you do this inside the signal handler?

sub my_local_handler
{
$SIG{INT} = \&my_local_handler;
++$Flag;
return(0);
} # my_local_handler

It looks like you are setting the signal handler to itself. Was this a typo?

Thanks,
Steve

Re: Perl Question

A. Clay Stephenson — Mon, 07 Aug 2006 12:11:31 GMT

No Steve, t'weren't no typo nor was it no mistake neither. It was (probably) paranoia on my part leftover from signal handling in C in the dim mists of time. Consider what would happen if the process received another SIGINT while already processing a SIGINT. Resetting the signal handler to itself does no harm but is probably not necessary in modern versions of UNIX and Perl but the paranoid programmer will include the signal redirection in order to make truly portable code.

Re: Perl Question

A. Clay Stephenson — Mon, 07 Aug 2006 12:14:53 GMT

Oh, and I should add that the truly paranoid programmer also knows that signal handlers should be short and sweet. Do no more than absolutely necessary and then return. It depends upon how re-entrant the code is at the lowest levels -- and this applies to C and Perl.