sigwait.c

/*
 * sigwait.c
 *
 * Demonstrate use of sigwait() to synchronously handle
 * asynchrnous signals within a threaded program.
 */
#include <sys/types.h>
#include <unistd.h>
#include <pthread.h>
#include <signal.h>
#include "errors.h"

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
int interrupted = 0;
sigset_t signal_set;

/*
 * Wait for the SIGINT signal. When it has occurred 5 times, set
 * the "interrupted" flag (the main thread's wait predicate) and
 * signal a condition variable. The main thread will exit.
 */
void *signal_waiter (void *arg)
{
    int sig_number;
    int signal_count = 0;
    int status;

    while (1) {
        sigwait (&signal_set, &sig_number);
        if (sig_number == SIGINT) {
            printf ("Got SIGINT (%d of 5)\n", signal_count+1);
            if (++signal_count >= 5) {
                status = pthread_mutex_lock (&mutex);
                if (status != 0)
                    err_abort (status, "Lock mutex");
                interrupted = 1;
                status = pthread_cond_signal (&cond);
                if (status != 0)
                    err_abort (status, "Signal condition");
                status = pthread_mutex_unlock (&mutex);
                if (status != 0)
                    err_abort (status, "Unlock mutex");
                break;
            }
        }
    }    
    return NULL;
}

int main (int argc, char *argv[])
{
    pthread_t signal_thread_id;
    int status;

    /*
     * Start by masking the "interesting" signal, SIGINT in the
     * initial thread. Because all threads inherit the signal mask
     * from their creator, all threads in the process will have
     * SIGINT masked unless one explicitly unmasks it. The
     * semantics of sigwait require that all threads (including
     * the thread calling sigwait) have the signal masked, for
     * reliable operation. Otherwise, a signal that arrives
     * while the sigwaiter is not blocked in sigwait might be
     * delivered to another thread.
     */
    sigemptyset (&signal_set);
    sigaddset (&signal_set, SIGINT);
    status = pthread_sigmask (SIG_BLOCK, &signal_set, NULL);
    if (status != 0)
        err_abort (status, "Set signal mask");

    /*
     * Create the sigwait thread.
     */
    status = pthread_create (&signal_thread_id, NULL,
        signal_waiter, NULL);
    if (status != 0)
        err_abort (status, "Create sigwaiter");

    /*
     * Wait for the sigwait thread to receive SIGINT and signal
     * the condition variable.
     */
    status = pthread_mutex_lock (&mutex);
    if (status != 0)
        err_abort (status, "Lock mutex");
    while (!interrupted) {
        status = pthread_cond_wait (&cond, &mutex);
        if (status != 0)
            err_abort (status, "Wait for interrupt");
    }
    status = pthread_mutex_unlock (&mutex);
    if (status != 0)
        err_abort (status, "Unlock mutex");
    printf ("Main terminating with SIGINT\n");
    return 0;
}