G'day ristov,

Welcome to the Monastery.

"I am trying to write the code which would wait for the child process to exit -- but if the code gets the TERM signal while waiting, the child process should be terminated with the same signal. ... I am interested whether the same task can be accomplished with blocking waitpid() ..."

This code demonstrates how you could do this:

#!/usr/bin/env perl -l

use strict;
use warnings;
use constant SLEEP_TIME => 20;

my $pid = fork;
die "Can't fork()" unless defined $pid;

if ($pid) {
    print "Parent: $$; Child: $pid";

    local $SIG{TERM} = sub {
        print "Parent received signal: @_";
        kill TERM => $pid if kill 0 => $pid;
    };

    print "Waiting on child ...";
    waitpid($pid, 0);
    print "Child terminated.";
}
else {
    print "Child: $$";

    local $SIG{TERM} = sub {
        print "Child received signal: @_";
        die "Child died via signal handler.\n";
    };

    sleep SLEEP_TIME;
    exit;
}
[download]

Running without any intervention:

Parent: 28186; Child: 28187
Waiting on child ...
Child: 28187
... SLEEP_TIME seconds pass
Child terminated.
[download]

Running and sending TERM to the parent:

Parent: 28192; Child: 28193
Waiting on child ...
Child: 28193
... on another command line: kill -TERM 28192
Parent received signal: TERM
Child received signal: TERM
Child died via signal handler.
Child terminated.
[download]

Running and sending TERM to the child:

Parent: 28201; Child: 28202
Waiting on child ...
Child: 28202
... on another command line: kill -TERM 28202
Child received signal: TERM
Child died via signal handler.
Child terminated.
[download]

The signal handler in the child was purely for demonstration purposes. Here's the output, from similar runs, with it removed.

Parent: 28405; Child: 28406
Waiting on child ...
Child: 28406
... SLEEP_TIME seconds pass
Child terminated.
[download]

Parent: 28419; Child: 28420
Waiting on child ...
Child: 28420
... kill -TERM 28419
Parent received signal: TERM
Child terminated.
[download]

Parent: 28430; Child: 28431
Waiting on child ...
Child: 28431
... kill -TERM 28431
Child terminated.
[download]

"... checking if the child process exists, in order to avoid sending TERM to a non-existing process."

That's handled by "... if kill 0 => $pid;". See kill.

See also: perlipc: Signals.

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