Programming Perl, 4th Edition

alarm

alarm EXPR
alarm

This function tells the operating system to send a SIGALRM signal to the current process after EXPR wallclock seconds have elapsed.

Only one timer may be active at once. Each call disables the previous timer, and an EXPR of 0 cancels the previous timer without starting a new one. The return value is the amount of time remaining on the previous timer.

print "Answer me within one minute, or die: ";
alarm(60);             # kill program in one minute
$answer = <STDIN>;
$timeleft = alarm(0);  # clear alarm
say "You had $timeleft seconds remaining";

It is usually a mistake to intermix alarm and sleep calls, because many systems use the alarm(2) syscall mechanism to implement sleep(3). Historically, the elapsed time may be up to one second less than you specified because of how seconds are counted. Additionally, a busy system may not get around to running your process immediately. See Chapter 15 for information on signal handling, such as how to use alarms to time out slow operations.

For alarms of finer granularity than one second, the Time::HiRes module provides functions for this purpose. For a hackier approach, use the four-argument version of select (leaving the first three arguments undefined), or Perl’s syscall function to access setitimer(2) (if your system supports it).