#!/usr/bin/perl use warnings; use strict; # by some monk here at perlmonks # wait basically waits until one of the children exits. # It is used to avoid having Zombie processes: if you don't # wait() for them, and you go a long time in your parent # process before exiting, you'll see these processes in # the process list even if they already exited, marked as # "zombie" (this means that resources in the kernel associated # to the process aren't freed). So, wait is the way by which # the parent process acknowledges the kernel of the son's death. # It's interesting that this mechanism actually tries to force # the programmer not to ignore return values from child processes, # much like the good practice of never throwing away return values # from functions. # Moreover, wait returns as soon as the first child exits; in # the OP's post, (s)he created 8 of them, and had to wait until # all of them have exited. According to perldoc -f wait, a call # to the wait function returns -1 if, and only if, there are no # more children alive(1) - thus the cycle. # If you happened to set $SIG{'CHLD'} to 'IGNORE', # this zombie-ridding happens automatically, and you're likely to # have wait always return -1 - but I guess it's not the case with # this thread :) Moreover, in this case I would discourage the # OP to do that, because (s)he's using wait as a synchronisation # mechanism to understand when all the children exited. BTW, this # is documented in perldoc perlipc, but I suggest taking a look in # perldoc perlport as well. for ( my $i = 0 ; $i < 11 ; $i++ ) { defined( my $pid = fork() ) or print STDERR "Unable to fork!"; if ($pid) { print "Child $i is $pid\n"; } else { print "Hi! I'm just a kid.. I'm going to sleep now..\n"; sleep rand(10); print "*Yawn* I'm awake! Huh? It's me! Kid $$! Bye bye! $$ Time to die!\n"; exit 0; } } #this is correct 1 while wait > 0; # wait by itself is different # wait; print "Screw this, I'm done waiting for those kids\n";
In reply to Re: delay loop
by zentara
in thread delay loop
by Vasilis
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