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  /* Equivalent to signal( SIGUSR1, handler);
     but make signal persistent */
  struct sigaction act;
  act.sa_handler = &handler;
  act.sa_flags = 0; /* makes signal persistent by default */
  if (sigfillset( &act.sa_mask) != 0) {
    /* handle error */
  }
  if (sigaction(SIGUSR1, &act, NULL) != 0) {
    /* handle error */
  }

  /* Equivalent to signal( SIGUSR1, handler); */
     but make signal non-persistent */
  struct sigaction act;
  act.sa_handler = &handler;
  act.sa_flags = SA_RESETHAND; /* makes signal non-persistent */
  if (sigfillset( &act.sa_mask) != 0) {
    /* handle error */
  }
  if (sigaction(SIGUSR1, &act, NULL) != 0) {
    /* handle error */
  }