man pages section 3: Basic Library Functions

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Updated: July 2014

pthread_sigmask (3C)


pthread_sigmask - change or examine calling thread's signal mask


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

int pthread_sigmask(int 
how, const sigset_t *set, 
sigset_t *oset);


The pthread_sigmask() function changes or examines a calling thread's signal mask. Each thread has its own signal mask. A new thread inherits the calling thread's signal mask and priority; however, pending signals are not inherited. Signals pending for a new thread will be empty.

If the value of the argument set is not NULL, set points to a set of signals that can modify the currently blocked set. If the value of set is NULL, the value of how is insignificant and the thread's signal mask is unmodified; thus, pthread_sigmask() can be used to inquire about the currently blocked signals.

The value of the argument how specifies the method in which the set is changed and takes one of the following values:


set corresponds to a set of signals to block. They are added to the current signal mask.


set corresponds to a set of signals to unblock. These signals are deleted from the current signal mask.


set corresponds to the new signal mask. The current signal mask is replaced by set.

If the value of oset is not NULL , it points to the location where the previous signal mask is stored.

Return Values

Upon successful completion, the pthread_sigmask() function returns 0. Otherwise, it returns a non-zero value.


The pthread_sigmask() function will fail if:


The value of how is not defined and oset is NULL.


Example 1 Create a default thread that can serve as a signal catcher/handler with its own signal mask.

The following example shows how to create a default thread that can serve as a signal catcher/handler with its own signal mask. new will have a different value from the creator's signal mask.

As POSIX threads and Solaris threads are fully compatible even within the same process, this example uses pthread_create(3C) if you execute a.out 0, or thr_create(3C) if you execute a.out 1.

In this example:

  • The sigemptyset(3C) function initializes a null signal set, new. The sigaddset(3C) function packs the signal, SIGINT, into that new set.

  • Either pthread_sigmask() or thr_sigsetmask () is used to mask the signal, SIGINT (CTRL-C), from the calling thread, which is main(). The signal is masked to guarantee that only the new thread will receive this signal.

  • pthread_create() or thr_create() creates the signal-handling thread.

  • Using pthread_join(3C) or thr_join(3C), main() then waits for the termination of that signal-handling thread, whose ID number is user_threadID; main() will then sleep(3C) for 2 seconds, after which the program terminates.

  • The signal-handling thread, handler:

    • Assigns the handler interrupt() to handle the signal SIGINT, by the call to sigaction (2).

    • Resets its own signal set to not block the signal, SIGINT.

    • Sleeps for 8 seconds to allow time for the user to deliver the signal, SIGINT, by pressing the CTRL-C.

/* cc thisfile.c */
#define _REENTRANT    /* basic first 3-lines for threads */
#include <pthread.h>
#include <thread.h>
thread_t user_threadID;
sigset_t new;
void *handler( ), interrupt( );

main( int argc, char *argv[ ] )  {

	sigaddset(&new, SIGINT);
	switch(*argv[1])  {

		case '0':   /* POSIX */
			pthread_sigmask(SIG_BLOCK, &new, NULL);
			pthread_create(&user_threadID, NULL, handler,
			pthread_join(user_threadID, NULL);

		case '1':   /* Solaris */
			thr_sigsetmask(SIG_BLOCK, &new, NULL);
			thr_create(NULL, 0, handler, argv[1], 0,
			thr_join(user_threadID, NULL, NULL);
		}  /* switch */

	printf("thread handler, # %d, has exited\n",user_threadID);
	printf("main thread, # %d is done\n", thr_self( ));
 return (0)
} /* end main */

struct sigaction act;

void *
handler(char *argv1)
	act.sa_handler = interrupt;
	sigaction(SIGINT, &act, NULL);
	switch(*argv1)	{
		case '0':     /* POSIX */
			pthread_sigmask(SIG_UNBLOCK, &new, NULL);
		case '1':   /* Solaris */
			thr_sigsetmask(SIG_UNBLOCK, &new, NULL);
	printf("\n Press CTRL-C to deliver SIGINT signal to the 
	sleep(8);  /* give user time to hit CTRL-C */
 return (NULL)

interrupt(int sig)
 printf("thread %d caught signal %d\n", thr_self( ), sig);

void test_argv(char argv1[ ])    {
	if(argv1 == NULL)  {
		printf("use 0 as arg1 to use thr_create( );\n \ 
		or use 1 as arg1 to use pthread_create( )\n");

In the last example, the handler thread served as a signal-handler while also taking care of activity of its own (in this case, sleeping, although it could have been some other activity). A thread could be completely dedicated to signal-handling simply by waiting for the delivery of a selected signal by blocking with sigwait (2). The two subroutines in the previous example, handler() and interrupt(), could have been replaced with the following routine:

void *
handler(void *unused)
    int signal;
    printf("thread %d is waiting for you to press the CTRL-C keys\n",
            thr_self( ));
    sigwait(&new, &signal);
    printf("thread %d has received the signal %d \n", thr_self( ),
    return (NULL);
/* pthread_create( ) and thr_create( ) would use NULL instead
   of argv[1] for the arg passed to handler( ) */

In this routine, one thread is dedicated to catching and handling the signal specified by the set new, which allows main () and all of its other sub-threads, created after pthread_sigmask() or thr_sigsetmask() masked that signal, to continue uninterrupted. Any use of sigwait (2) should be such that all threads block the signals passed to sigwait(2) at all times. Only the thread that calls sigwait() will get the signals. The call to sigwait(2) takes two arguments.

For this type of background dedicated signal-handling routine, a Solaris daemon thread can be used by passing the argument THR_DAEMON to thr_create(3C).


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

Interface Stability
MT-Safe and Async-Signal-Safe

See Also

sigaction(2), sigprocmask(2), sigwait (2), cond_wait(3C), pthread_cancel(3C), pthread_create(3C), pthread_join(3C), pthread_self(3C), sigaddset(3C), sigemptyset(3C), sigsetops(3C), sleep(3C), attributes (5), cancellation (5), standards (5)


It is not possible to block signals that cannot be caught or ignored (see sigaction(2)). It is also not possible to block or unblock SIGCANCEL, as SIGCANCEL is reserved for the implementation of POSIX thread cancellation (see pthread_cancel(3C) and cancellation(5) ). This restriction is quietly enforced by the standard C library.

Using sigwait(2) in a dedicated thread allows asynchronously generated signals to be managed synchronously; however, sigwait(2) should never be used to manage synchronously generated signals.

Synchronously generated signals are exceptions that are generated by a thread and are directed at the thread causing the exception. Since sigwait () blocks waiting for signals, the blocking thread cannot receive a synchronously generated signal.

The sigprocmask(2) function behaves the same as if pthread_sigmask () has been called. POSIX leaves the semantics of the call to sigprocmask(2) unspecified in a multi-threaded process, so programs that care about POSIX portability should not depend on this semantic.

If a signal is delivered while a thread is waiting on a condition variable, the cond_wait(3C) function will be interrupted and the handler will be executed. The state of the lock protecting the condition variable is undefined while the thread is executing the signal handler.

Although pthread_sigmask() is Async-Signal-Safe with respect to the Solaris environment, this safeness is not guaranteed to be portable to other POSIX domains.

Signals that are generated synchronously should not be masked. If such a signal is blocked and delivered, the receiving process is killed.