24.3.4 sigaction Function Example

In Basic Signal Handling, we gave an example of establishing a simple handler for termination signals using signal. Here is an equivalent example using sigaction:

     #include <signal.h>
     
     void
     termination_handler (int signum)
     {
       struct temp_file *p;
     
       for (p = temp_file_list; p; p = p->next)
         unlink (p->name);
     }
     
     int
     main (void)
     {
       ...
       struct sigaction new_action, old_action;
     
       /* Set up the structure to specify the new action. */
       new_action.sa_handler = termination_handler;
       sigemptyset (&new_action.sa_mask);
       new_action.sa_flags = 0;
     
       sigaction (SIGINT, NULL, &old_action);
       if (old_action.sa_handler != SIG_IGN)
         sigaction (SIGINT, &new_action, NULL);
       sigaction (SIGHUP, NULL, &old_action);
       if (old_action.sa_handler != SIG_IGN)
         sigaction (SIGHUP, &new_action, NULL);
       sigaction (SIGTERM, NULL, &old_action);
       if (old_action.sa_handler != SIG_IGN)
         sigaction (SIGTERM, &new_action, NULL);
       ...
     }

The program just loads the new_action structure with the desired parameters and passes it in the sigaction call. The usage of sigemptyset is described later; see Blocking Signals.

As in the example using signal, we avoid handling signals previously set to be ignored. Here we can avoid altering the signal handler even momentarily, by using the feature of sigaction that lets us examine the current action without specifying a new one.

Here is another example. It retrieves information about the current action for SIGINT without changing that action.

     struct sigaction query_action;
     
     if (sigaction (SIGINT, NULL, &query_action) < 0)
       /* sigaction returns -1 in case of error. */
     else if (query_action.sa_handler == SIG_DFL)
       /* SIGINT is handled in the default, fatal manner. */
     else if (query_action.sa_handler == SIG_IGN)
       /* SIGINT is ignored. */
     else
       /* A programmer-defined signal handler is in effect. */