Access value of a variable after exit 0 in fork

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

Hello All,

I am not sure if I am describing it correctly but will paste a chunk of my code to explain properly, where I am getting stuck.

$trigger='OFF';
my $pid = fork();
if ($pid == 0) {
#some process invoked on xterm
if (process invoked on xterm gives a signal){
        $trigger='ON';
}
exit 0;
}

my $pid = fork();
if ($pid == 0) {
#some other process on xterm
if( $trigger eq 'ON') {      #------->This has value OFF as declared g
+lobally. I want this to have value from previous process. What do I d
+o?
           #do something
exit 0;
}
[download]

Comment on Access value of a variable after exit 0 in fork Download Code

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Re: Access value of a variable after exit 0 in fork by NetWallah (Canon) on Dec 27, 2012 at 07:33 UTC
The simplest would be to return a small numeric value via the exit code of the sub-process, read that in the parent process, and translate that in you main code to set $trigger to "ON". The problem you currently have is that $trigger in the child process is in a different memory space, and modifying that in the child has no effect on the parent's instance of $trigger. "By three methods we may learn wisdom: First, by reflection, which is noblest; Second, by imitation, which is easiest; and third by experience, which is the bitterest." -Confucius	[reply]
Re^2: Access value of a variable after exit 0 in fork by Anonymous Monk on Dec 27, 2012 at 08:53 UTC
Can you please elaborate via some code? Sorry to give you trouble,but just want to understand thoroughly.	[reply]
Re^3: Access value of a variable after exit 0 in fork by Anonymous Monk on Dec 27, 2012 at 10:52 UTC
See perlipc and waitpid `if( my $pid = fork() ){ print "parent ($$) is waiting on child $pid\n"; my $kid ; do { $kid = waitpid(-1, WNOHANG); } while $kid > 0; my $exit = $? >> 8; die qq{Kid ($kid) exited with $? / $exit \n}; } else { print "I am the kid $$\n"; sleep 1; #~ exit 666; ## too big exit 12; } __END__ parent (1724) is waiting on child -880 I am the kid -880 Kid (-880) exited with 3072 / 12` [download] There are abstractions to help you manage this like Parallel::ForkManager, Proc::Fork, forks, threads ....	[reply] [d/l]