SIGWAIT(2) System Calls SIGWAIT(2)


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(7))
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

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.


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


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(7) for descriptions of the following attributes:

|Interface Stability | Committed |
|MT-Level | Async-Signal-Safe |
|Standard | See standards(7). |


sigaction(2), sigpending(2), sigprocmask(2), sigsuspend(2),
pthread_create(3C), pthread_kill(3C), pthread_sigmask(3C),
signal.h(3HEAD), attributes(7), standards(7)


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

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.

illumos April 16, 2009 SIGWAIT(2)