How can I terminate a system command with alarm in Perl?

Question

I am running the below code snippet on Windows. The server starts listening continuously after reading from client. I want to terminate this command after a time period.

Answer 1

What the hell was I thinking? You don't need a background process for this task. You just need to follow the example in the perldoc -f alarm function and wrap your time-sensitive code in an eval block.

my $command = "adb shell cd /data/app; ./iperf -u -s -p 5001";
my @output;
eval {
    local $SIG{ALRM} = sub { die "Timeout\n" };
    alarm 30;
    @output = `$command`;
    alarm 0;
};
if ($@) {
    warn "$command timed out.\n";
} else {
    print "$command successful. Output was:\n", @output;
}

Inside the eval block, you can capture your output the regular way (with backticks or qx() or readpipe). Though if the call times out, there won't be any output.

If you don't need the output (or don't mind hacking some interprocess communication together), an almost idiot-proof alternative is to set the alarm and run the system call in a child process.

$command = "adb shell cd /data/app; ./iperf -u -s -p 5001";
if (($pid = fork()) == 0) {
    # child process
    $SIG{ALRM} = sub { die "Timeout\n" }; # handling SIGALRM in child is optional
    alarm 30;
    my $c = system($command);
    alarm 0;
    exit $c >> 8;  # if you want to capture the exit status
}
# parent
waitpid $pid, 0;

waitpid will return when either the child's system command is finished, or when the child's alarm goes off and kills the child. $? will hold the exit code of the system call, or something else (142 on my system) for an unhandled SIGALRM or 255 if your SIGALRM handler calls die.