Perl sleep terminated by SIGCHLD - perl

Question

#!/usr/bin/perl
use POSIX ":sys_wait_h";
$SIG{CHLD} = \&REAPER;
sub REAPER {
my $pid;
while (($pid = waitpid(-1, WNOHANG)) > 0) {
print "where is here,$pid\n";
}
}
sub child {
print "I'm child, pid=$$.\n";
sleep 2;
}
$lid = fork();
if ($lid == 0) {
&child;
exit;
} else {
sleep 1000;
print "I am parent, child pid : $lid\n";
}
Output:
I'm child, pid=11839.
where is here,11839
I am parent, child pid : 11839
The above is my Perl script. The output is right, but one strange thing is that it prints I am parent, child pid : 11839 immediately after the last output. Why didn't the sleep 1000 have any effect?

Answer 1

This is documented: "May be interrupted if the process receives a signal". The whole point is to allow signal handlers to run. Just go back to sleep if it's not time to wake up.
use Time::HiRes qw( time sleep ); # Optional.
sub unint_sleep($) {
my $sleep_til = time + $_[0];
for (;;) {
my $sleep_dur = time - $sleep_til;
last if $sleep_dur <= 0;
sleep($sleep_dur);
}
}

Answer 2

Please always use strict and use warnings, and declare your symbols using my at their point of definition. This applies especially when you are asking for help, as these measures can reveal simple bugs that are otherwise easily overlooked
sleep is implemented by using setitimer to request a SIGALRM after a specified interval, and then pause to suspend the process until it gets the signal.
But if a SIGCHLD comes first this will also wake the process.
The signal mask cannot be set to prevent this as otherwise the SIGCHLD wouldn't get serviced