Sigaction handler in C

Question

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; }

You can only safely make async-signal-safe function calls from within a signal handler. printf() is not async-signal-safe. See pubs.opengroup.org/onlinepubs/9699919799/functions/… — Andrew Henle
– Andrew Henle, Commented Nov 7, 2018 at 20:06
@TimRandall He's edited that out of the question, but now I think he was asking why it switched from lowercase sigaction to mixed case Sigaction, not about case and switch statements. — Barmar
– Barmar, Commented Nov 7, 2018 at 20:23
Yes, but the point about the Minimal, Complete, and Verifiable example is that we don't have to fumble around asking whether a typo is a real typo or an accidental typo, or whether the code is the actual code or only resembles it. — Weather Vane
– Weather Vane, Commented Nov 7, 2018 at 20:27

Barmar · Accepted Answer · 2018-11-07 20:30:58Z

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.

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