> -----Original Message-----
> From: Paul D. Kraus [mailto:[EMAIL PROTECTED]
> Sent: Monday, May 09, 2005 9:19 AM
> To: Perl Beginners List
> Subject: Fork - Process ID
>
>
> Sceniro.
> I have many cron jobs that run perl scripts. These perl
> scripts launch TbredBasic Applications. The problem I keep
> finding these processes hung but I have noway of finding out
> which program is actually causing the problem. When the
> tbredbasic application launches the processes name is always BASIC.
>
> I wanted to write a program that would write out the process
> id and the perl program that spawned it to a file then
> destroy that file when the program fails. This way I can just
> look at the "log" and see that program X is what keeps hanging.
>
> So I have been experminting with the fork command but I am
> confused as to the out put I get. Below is the code and its
> output follows. First please explain why the pid prints twice
> and second let me know if there is a better way to achive
> what I am trying to do.
>
> TIA
> Paul
>
Hi Paul. Fork is fun to play with!
I've put some comments below:
> #!/bin/perl
>
> use strict;
> use warnings;
>
> defined( my $pid = fork ) or die "Cannot fork: $!";
# right here, there are two processes running. The first one, and the
one that got forked.
# both processes have the $pid variable. The original process should
receive the new processes
# PID as the return value from fork, so it's $pid = 1111 (or, whatever
the system returns).
# The new process will receive a 0 (zero) from fork. Now you can use
that $pid value to split
# your code down two different paths.
> print "pid:$pid\n"
# When I ran your code, the following was printed:
# pid: 1275
# pid: 0
# Your output was not included in the email. Was your output
different than this?
> unless( $pid ) {
> exec "date";
> die "cannot exec date: $!";
> }
# This unless block should run date command if the process is the NEW
process. The old process
# with a 0 (zero) value for $pid, will skip the 'exec' and 'die' lines
of code. Is that what you intended?
Here is a subroutine template I use in my daemon scripts, if it helps.
The 'setsid' function requires that you import the POSIX module
somewhere in your code:
use POSIX 'setsid';
sub daemonize {
# A forked process usually doesn't want to be in some random
directory, so we change to root (/)
chdir '/' or die "Can't chdir to /: $!";
# The forked process doesn't want to read from standard-in, nor do we
want it to spam the
# standard-out with random messages.
open STDIN, '/dev/null' or die "Can't read /dev/null: $!";
open STDOUT, '/dev/null' or die "Can't write to /dev/null: $!";
# Here is where the actual fork takes place
defined( my $pid = fork ) or die "Can't fork the daemon: $!";
# Here the script that starts the forked process will end
exit if $pid; # original (terminal) process dies here
# This will give the forked process it's own session and make the
process it's own session leader
setsid or die "Can't start a new session: $!";
# finally, we'll duplicate standard-error to wherever standard-out is,
again to prevent spamming standard-out
open STDERR, '>&STDOUT' or die "Can't dup STDOUT: $!";
}
Hope this helps you some.
--Errin
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