Go to main content

man pages section 2: System Calls

Exit Print View

Updated: July 2017



sigwait - wait until a signal is posted


#include <signal.h>

int sigwait(sigset_t *set);

Standard conforming

cc [ flag ... ] file ... –D_POSIX_PTHREAD_SEMANTICS [ library...]
#include <signal.h>

int sigwait(const sigset_t *set, int *sig);


The sigwait() function selects a signal in set that is pending on the calling thread. If no signal in set is pending, sigwait() blocks until a signal in set becomes pending. The selected signal is cleared from the set of signals pending on the calling thread and the number of the signal is returned, or in the standard–conforming version (see standards(5)) placed in sig. The selection of a signal in set is independent of the signal mask of the calling thread. This means a thread can synchronously wait for signals that are being blocked by the signal mask of the calling thread . To ensure that only the caller receives the signals defined in set, all threads should have signals in set masked including the calling thread.

If more than one thread is using sigwait() to wait for the same signal, no more than one of these threads returns from sigwait() with the signal number. If more than a single thread is blocked in sigwait() for a signal when that signal is generated for the process, it is unspecified which of the waiting threads returns from sigwait(). If the signal is generated for a specific thread, as by pthread_kill(3C), only that thread returns.

Should any of the multiple pending signals in the range SIGRTMIN to SIGRTMAX be selected, it will be the lowest numbered one. The selection order between realtime and non-realtime signals, or between multiple pending non-realtime signals, is unspecified.

Return Values

Upon successful completion, the default version of sigwait() returns a signal number; the standard–conforming version returns 0 and stores the received signal number at the location pointed to by sig. Otherwise, the default version returns -1 and sets errno to indicate an error; the standard-conforming version returns an error number to indicate the error.


The sigwait() function will fail if:


The set argument points to an invalid address.


The wait was interrupted by an unblocked, caught signal.


The set argument contains an unsupported signal number.


Example 1 Creating a thread to handle receipt of a signal

The following sample C code creates a thread to handle the receipt of a signal. More specifically, it catches the asynchronously generated signal, SIGINT.

* compile with -D_POSIX_PTHREAD_SEMANTICS switch;
* required by sigwait()
* sigint thread handles delivery of signal. uses sigwait( ) to wait
* for SIGINT signal.
#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <signal.h>
#include <synch.h>

static void    *threadTwo(void *);
static void    *threadThree(void *);
static void    *sigint(void *);

sigset_t       signalSet;

void *
    pthread_t    t;
    pthread_t    t2;
    pthread_t    t3;
    sigfillset ( &signalSet );
     * Block signals in initial thread. New threads will
     * inherit this signal mask.
    pthread_sigmask ( SIG_BLOCK, &signalSet, NULL );
    printf("Creating threads\n");
    pthread_create(&t, NULL, sigint, NULL);
    pthread_create(&t2, NULL, threadTwo, NULL);
    pthread_create(&t3, NULL, threadThree, NULL);
    printf("press CTRL-C to deliver SIGINT to sigint thread\n");
    pthread_exit((void *)0);
static void *
threadTwo(void *arg)
    printf("hello world, from threadTwo [tid: %d]\n",
    printf("threadTwo [tid: %d] is now complete and exiting\n",
    pthread_exit((void *)0);

static void *
threadThree(void *arg)
    printf("hello world, from threadThree [tid: %d]\n",
    printf("threadThree [tid: %d] is now complete and exiting\n",
    pthread_exit((void *)0);

void *
sigint(void *arg)
    int    sig;
    int    err;
    printf("thread sigint [tid: %d] awaiting SIGINT\n",
    /* use standard-conforming sigwait() -- 2 args: signal set, signum
    err = sigwait ( &signalSet, &sig );
    /* test for SIGINT; could catch other signals */
    if (err || sig != SIGINT)
    printf("\nSIGINT signal %d caught by sigint thread [tid: %d]\n",
                            sig, pthread_self());
    pthread_exit((void *)0);


See attributes(5) for descriptions of the following attributes:

Interface Stability

See Also

sigaction(2), sigpending(2), sigprocmask(2), sigsuspend(2), pthread_create(3C), pthread_kill(3C), pthread_sigmask(3C), signal.h(3HEAD), attributes(5), standards(5)


The sigwait() function cannot be used to wait for signals that cannot be caught (see sigaction(2)). This restriction is silently imposed by the system.

Solaris 2.4 and earlier releases provided a sigwait() facility as specified in POSIX.1c Draft 6. The final POSIX.1c standard changed the interface as described above. Support for the Draft 6 interface is provided for compatibility only and may not be supported in future releases. New applications and libraries should use the standard–conforming interface.