Completion Semantics

Another way to deal with signals is with completion semantics.

Use completion semantics when a signal indicates that something so catastrophic has happened that no reason exists to continue executing the current code block. The signal handler runs instead of the remainder of the block that had the problem. In other words, the signal handler completes the block.

In Example 5–3, the block in question is the body of the then part of the if statement. The call to setjmp(3C) saves the current register state of the program in jbuf and returns 0, thereby executing the block.

Example 5–3 Completion Semantics

sigjmp_buf jbuf;
void mult_divide(void) {
    int a, b, c, d;
    void problem();

    sigset(SIGFPE, problem);
    while (1) {
        if (sigsetjmp(&jbuf) == 0) {
            printf("Three numbers, please:\n");
            scanf("%d %d %d", &a, &b, &c);
            d = a*b/c;
            printf("%d*%d/%d = %d\n", a, b, c, d);
        }
    }
}

void problem(int sig) {
    printf("Couldn't deal with them, try again\n");
    siglongjmp(&jbuf, 1);
}

If a SIGFPE floating-point exception occurs, the signal handler is invoked.

The signal handler calls siglongjmp(3C), which restores the register state saved in jbuf, causing the program to return from sigsetjmp() again. The registers that are saved include the program counter and the stack pointer.

This time, however, sigsetjmp(3C) returns the second argument of siglongjmp(), which is 1. Notice that the block is skipped over, only to be executed during the next iteration of the while loop.

You can use sigsetjmp(3C) and siglongjmp(3C) in multithreaded programs. Be careful that a thread never does a siglongjmp() that uses the results of another thread's sigsetjmp().

Also, sigsetjmp() and siglongjmp() restore as well as save the signal mask, but setjmp(3C) and longjmp(3C) do not.

Use sigsetjmp() and siglongjmp() when you work with signal handlers.

Completion semantics are often used to deal with exceptions. In particular, the Sun Ada^TM programming language uses this model.

Remember, sigwait(2) should never be used with synchronous signals.