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;
 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)