sigwait(2)

For multithreaded programs, sigwait(2) is the preferred interface to use, because it deals well with asynchronously generated signals.

sigwait() causes the calling thread to wait until any signal identified by its set argument is delivered to the thread. While the thread is waiting, signals identified by the set argument are unmasked, but the original mask is restored when the call returns.

All signals identified by the set argument must be blocked on all threads, including the calling thread; otherwise, sigwait() might not work correctly.

Use sigwait() to separate threads from asynchronous signals. You can create one thread that is listening for asynchronous signals while your other threads are created to block any asynchronous signals that might be set to this process.

New sigwait() Implementations

Two versions of sigwait() are available beginning with the Solaris Operating Environment 2.5 release: the new Solaris Operating Environment 2.5 version, and the POSIX.1c version. New applications and libraries should use the POSIX standard interface, as the Solaris Operating Environment version might not be available in future releases.

The syntax for the two versions of sigwait() is shown below.

#include <signal.h>

/* the Solaris 2.5 version*/
int sigwait(sigset_t *set);

/* the POSIX.1c version */
int sigwait(const sigset_t *set, int *sig);

When the signal is delivered, the POSIX.1c sigwait() clears the pending signal and places the signal number in sig. Many threads can call sigwait() at the same time, but only one thread returns for each signal that is received.

With sigwait() you can treat asynchronous signals synchronously--a thread that deals with such signals simply calls sigwait() and returns as soon as a signal arrives. By ensuring that all threads (including the caller of sigwait()) have such signals masked, you can be sure that signals are handled only by the intended handler and that they are handled safely.

By always masking all signals in all threads, and just calling sigwait() as necessary, your application will be much safer for threads that depend on signals.

Usually, you create one or more threads that call sigwait() to wait for signals. Because sigwait() can retrieve even masked signals, be sure to block the signals of interest in all other threads so they are not accidentally delivered.

When a signal arrives, a thread returns from sigwait(), handles the signal, and calls sigwait() again to wait for more signals. The signal-handling thread is not restricted to using Async-Signal-Safe functions and can synchronize with other threads in the usual way. (The Async-Signal-Safe category is defined in "MT Interface Safety Levels".)

Note - sigwait() cannot receive synchronously-generated signals.

sigtimedwait(3RT)

sigtimedwait(3RT) is similar to sigwait(2) except that it fails and returns an error when a signal is not received in the indicated amount of time.

Thread-Directed Signals

The UNIX signal mechanism is extended with the idea of thread-directed signals. These are just like ordinary asynchronous signals, except that they are sent to a particular thread instead of to a process.

Waiting for asynchronous signals in a separate thread can be safer and easier than installing a signal handler and processing the signals there.

A better way to deal with asynchronous signals is to treat them synchronously. By calling sigwait(2), discussed in "sigwait(2)", a thread can wait until a signal occurs.

Example 5-2 Asynchronous Signals and sigwait(2)

main() {
    sigset_t set;
    void runA(void);
    int sig;

    sigemptyset(&set);
    sigaddset(&set, SIGINT);
    pthread_sigmask(SIG_BLOCK, &set, NULL);
    pthread_create(NULL, 0, runA, NULL, PTHREAD_DETACHED, NULL);

    while (1) {
        sigwait(&set, &sig);
        printf("nestcount = %d\n", nestcount);
        printf("received signal %d\n", sig);
    }
}

void runA() {
    A(4,4);
    exit(0);
}

This example modifies the code of Example 5-1: the main routine masks the SIGINT signal, creates a child thread that calls the function A of the previous example, and then issues sigwait() to handle the SIGINT signal.

Note that the signal is masked in the compute thread because the compute thread inherits its signal mask from the main thread. The main thread is protected from SIGINT while, and only while, it is not blocked inside of sigwait().

Also, note that there is never any danger of having system calls interrupted when you use sigwait().

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 there is no reason 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 (a 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 (among the registers saved are 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.

Note that you can use sigsetjmp(3C) and siglongjmp(3C) in multithreaded programs, but be careful that a thread never does a siglongjmp() using the results of another thread's sigsetjmp().

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

It is best to use sigsetjmp() and siglongjmp() when you work with signal handlers.

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

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