24.4.3 Nonlocal Control Transfer in Handlers

You can do a nonlocal transfer of control out of a signal handler using the setjmp and longjmp facilities (see Non-Local Exits).

When the handler does a nonlocal control transfer, the part of the program that was running will not continue. If this part of the program was in the middle of updating an important data structure, the data structure will remain inconsistent. Since the program does not terminate, the inconsistency is likely to be noticed later on.

There are two ways to avoid this problem. One is to block the signal for the parts of the program that update important data structures. Blocking the signal delays its delivery until it is unblocked, once the critical updating is finished. See Blocking Signals.

The other way to re-initialize the crucial data structures in the signal handler, or make their values consistent.

Here is a rather schematic example showing the reinitialization of one global variable.

     #include <signal.h>
     #include <setjmp.h>
     
     jmp_buf return_to_top_level;
     
     volatile sig_atomic_t waiting_for_input;
     
     void
     handle_sigint (int signum)
     {
       /* We may have been waiting for input when the signal arrived,
          but we are no longer waiting once we transfer control. */
       waiting_for_input = 0;
       longjmp (return_to_top_level, 1);
     }
     
     int
     main (void)
     {
       ...
       signal (SIGINT, sigint_handler);
       ...
       while (1) {
         prepare_for_command ();
         if (setjmp (return_to_top_level) == 0)
           read_and_execute_command ();
       }
     }
     
     /* Imagine this is a subroutine used by various commands. */
     char *
     read_data ()
     {
       if (input_from_terminal) {
         waiting_for_input = 1;
         ...
         waiting_for_input = 0;
       } else {
         ...
       }
     }