Unix Command Internals

Signal Handling

Related to signal handling is the alarm clock
Every process has an alarm clock
When the alarm clock goes off, the kernel sends the process an ALRM signal
The alarm clock is preserved across exec, but not copied on fork
We can use it to make a useful utility, stopafter:

        #!/usr/bin/perl
        my ($limit, $command, @args) = @ARGV;
        $SIG{ALARM} = 'IGNORE';
        alarm $limit;
        exec $command, @args;
        die "Couldn't run '$command': $!";

To use:

        stopafter 30 command arg arg arg...

stopafter just sets up an alarm clock and then execs the program you asked for
30 seconds later, the alarm clock goes off
- The ALRM signal is delivered
- The process dies
(Unless the command cancelled the alarm clock or caught or ignored the ALRM signal)