alarm() — schedule a SIGALRM in N seconds

Lesson

alarm(n) asks the kernel to deliver SIGALRM to your process n seconds from now. The classic use is making a blocking syscall — read(), accept(), connect() — give up after a timeout: install a SIGALRM handler that does nothing (or sets a flag), call alarm(n), then issue the syscall. When SIGALRM fires the syscall returns EINTR.

alarm(0) cancels a pending alarm. For sub-second or repeating timers, use setitimer() or — modern Linux — timerfd_create().

Code examples

#include <signal.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>

static void on_alarm(int sig) { (void)sig; /* nothing; just interrupt */ }

int main(void) {
    struct sigaction sa = {0};
    sa.sa_handler = on_alarm;
    sigemptyset(&sa.sa_mask);
    sigaction(SIGALRM, &sa, NULL);

    char buf[32];
    alarm(3);                              /* 3-second budget */
    ssize_t n = read(STDIN_FILENO, buf, sizeof buf);
    alarm(0);                              /* cancel */
    if (n < 0 && errno == EINTR) {
        printf("read timed out\n");
        return 1;
    }
    printf("got %zd bytes\n", n);
    return 0;
}

Common mistakes

• Forgetting to install a SIGALRM handler. Default action for SIGALRM is terminate — the timeout will kill your program instead of interrupting the read.
• Forgetting alarm(0) on the success path. A stale alarm fires later and surprises you.

Summary

alarm(n) schedules SIGALRM for n seconds later. Default action is terminate, so install a handler first. Cancel with alarm(0).

Practice with these exercises

• Time-out a blocking read with alarm()