An example of ualarm and setjmp

The following program is an example of using ualarm and the setjmp/longjmp system calls to interrupt a routine and return control back to the calling routine.

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

static jmp_buf buf;
void signal_handler (int x)
    printf ("Signal with the following number: %d\n", x);
    /* Go back to where setjmp was called. Return the value "1" from
       setjmp. */
    longjmp (buf,1);

/* Run indefinitely */

void run_indefinitely ()
    while (1) {


int main (int argc, char ** argv)
    struct sigaction act;
    act.sa_handler = signal_handler;
    sigaction (SIGALRM, & act, 0);

    /* Set an alarm to go off after 1,000 microseconds (one thousandth
       of a second). */
    ualarm (1000, 0);
    if (! setjmp (buf)) {
        /* The first time setjmp is called, it returns zero. */
        run_indefinitely ();
    else {
        /* This branch is reached from the function "signal_handler"
           above. */
        printf ("Jumped to here.\n");
    return 0;

This should print the following when run:

Signal with the following number: 14
Jumped to here.
and then exit.

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