Question

In the following program, pause is interrupted once, but then pause never returns. I have set alarm to interrupt pause, so i am confused why pause never returns?

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

static void sig_alrm(int);
static jmp_buf env_alrm;


int main(int arc, char **argv)
{

    int x;
    x = setjmp(env_alrm);
    printf("setjmp was created with return value: %d\n", x);

    if(signal(SIGALRM, sig_alrm) == SIG_ERR)
    {
            printf("Error settting SIGALRM\n");
            exit(1);
    }


    if((x!= 0) && (x!=1))
    {

            printf("Error setting setjmp\n");
            exit(1);
    }

    printf("Line next to setjmp\n");
    x = alarm(2);
    printf("Alarm set for 2 seconds, remaning secs from previous alarm: %d\n");
    pause();
    printf("Line next to pause()\n");
    alarm(0);
    return 0;
}

static void sig_alrm(int signo)
{
    longjmp(env_alrm, 1);
}

Here is the output and the last line shows where the application pauses

setjmp was created with return value: 0
Line next to setjmp
Alarm set for 2 seconds, remaining secs from previous alarm: 0
setjmp was created with return value: 1
Line next to setjmp
Alarm set for 2 seconds, remaining secs from previous alarm: 0

Answer 1

use sigsetjmp() and siglongjmp() instead, to save and restore the signal masks, which are not saved by default in Linux, to clear any pending signals, from man setjmp():

POSIX does not specify whether setjmp() will save the signal mask. In System V it will not.By default, Linux/glibc follows the System V behavior. If you want to portably save and restore signal masks, use sigsetjmp()
and siglongjmp().

Note: I’m not sure what you’re trying to accomplish, but your code looks like it’s supposed to run in an infinite loop, calling longjmp() restores execution as if it had just returned from setjmp() and it goes on forever.