a demonstration of signal(3)

output

0x0
0x1f2e
dummy handler received 99

signals.c

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

void
dummy_handler(int signal)
{
  printf("dummy handler received %d\n", signal);
}

void
another_dummy_handler(int signal)
{

}

int
main(int argc, char** argv)
{
  sig_t initial_handler;
  sig_t old_handler;

  initial_handler = signal(SIGUSR1, dummy_handler);

  /* this will be NULL, because no handler was previously registered */
  printf("%p\n", initial_handler);

  old_handler = signal(SIGUSR1, another_dummy_handler);

  /* this won't be, and in fact old_handler will equal the address of
   * dummy_handler */
  printf("%p\n", old_handler);

  /* just to prove what I said above */
  (*old_handler)(99);

  return 0;
}