24.4.1 Signal Handlers that Return

Handlers which return normally are usually used for signals such as SIGALRM and the I/O and interprocess communication signals. But a handler for SIGINT might also return normally after setting a flag that tells the program to exit at a convenient time.

It is not safe to return normally from the handler for a program error signal, because the behavior of the program when the handler function returns is not defined after a program error. See Program Error Signals.

Handlers that return normally must modify some global variable in order to have any effect. Typically, the variable is one that is examined periodically by the program during normal operation. Its data type should be sig_atomic_t for reasons described in Atomic Data Access.

Here is a simple example of such a program. It executes the body of the loop until it has noticed that a SIGALRM signal has arrived. This technique is useful because it allows the iteration in progress when the signal arrives to complete before the loop exits.

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

/* This flag controls termination of the main loop. */
volatile sig_atomic_t keep_going = 1;

/* The signal handler just clears the flag and re-enables itself. */
void
catch_alarm (int sig)
{
  keep_going = 0;
  signal (sig, catch_alarm);
}

void
do_stuff (void)
{
  puts ("Doing stuff while waiting for alarm....");
}

int
main (void)
{
  /* Establish a handler for SIGALRM signals. */
  signal (SIGALRM, catch_alarm);

  /* Set an alarm to go off in a little while. */
  alarm (2);

  /* Check the flag once in a while to see when to quit. */
  while (keep_going)
    do_stuff ();

  return EXIT_SUCCESS;
}