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ALARM(3) Library Functions Manual ALARM(3)

alarm
set signal timer alarm

#include <unistd.h>
unsigned int
alarm(unsigned int seconds);

This is a simplified interface to setitimer(2).
The alarm() function waits a count of seconds before asserting the terminating signal SIGALRM. When the signal has successfully been caught, alarm() returns the amount of time left on the clock. The maximum number of seconds allowed is 100000000.
If an alarm has been set with alarm(), another call to alarm() will supersede the prior call. The request alarm(0) voids the current alarm.

If the call succeeds, any time left remaining from a previous call is returned. If an error occurs, the value -1 is returned, and a more precise error code is placed in the global variable errno.

setitimer(2), sigaction(2), sigsuspend(2), signal(3), sleep(3), ualarm(3)

The alarm() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

An alarm() system call appeared in the Programmer's Workbench (PWB/UNIX) and was ported to Version 7 AT&T UNIX. For 4.1cBSD, it was reimplemented as a wrapper around the setitimer(2) system call.
January 28, 2016 OpenBSD-current