CODEWITHSUNDEEP
cshellunixprocesssignals
M. Chakhtoura
M. Chakhtoura

Reputation: 75

Sigaction handler in C

We have an assignment to explain this text of code. My only problem is understanding the handle_signal function, why did we use 2 new sigaction and then used "old_treatment" with "rien"?

#define DELAY 1
#define NB_ITERATIONS 60
void handle_signal (int num_signal){
    struct sigaction rien, old_ treatment;
    printf ("Signal %d => ", num_signal);
    printf ("I have received a SIGTSTP.\n");
    rien.sa_handler = SIG_DFL;
    rien.sa_flags = 0;
    sigemptyset (&rien.sa_mask);
    sigaction (SIGTSTP, &rien, &old_ treatment);
    printf ("Then I sleep....\n");
    kill (getpid(), SIGSTOP);
    printf ("They wakes me?\n");
    Sigaction (SIGTSTP, &old_ treatment, NULL);
    printf ("Here we go again!\n");
}

int main (void){
    struct sigaction a;
    int i;
    a.sa_handler = handle_signal;
    sigemptyset (&a.sa_mask);
    sigaction (SIGTSTP, &a, NULL);
    for (i = 1; i < NB_ITERATIONS; i++) {
    sleep (DELAY);
    printf ("%d", i % 10);
    fflush (stdout);}
    printf ("End\n");
    return EXIT_SUCCESS;
}

Upvotes: 0

Views: 1804

Answers (1)

Barmar
Barmar

Reputation: 782436

The purpose of this is to temporarily change the action for SIGTSTP, then restore it back.

sigaction(SIGTSTP, &rien, &old_handler);

sets it to the default action, and saves the previous action in old_handler.

Then it sends itself a SIGSTOP signal to actually suspend the process.

When that returns, it means that the process has been continued, so it puts back the old action with:

sigaction(SIGTSTOP, &old_handler, NULL);

It's not clear why this is needed, though. It would make more sense if it suspended the process by sending a SIGTSTP signal rather than SIGSTOP. In that case, it needs to set the default action, otherwise it would just recurse infinitely.

Upvotes: 1

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