Multithreaded Programming Guide

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;

    sigaddset(&set, SIGINT);
    pthread_sigsetmask(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() {

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().