Tribblix: manual page: pthread

PTHREAD_SIGMASK(3C) Standard C Library Functions PTHREAD_SIGMASK(3C)

NAME

pthread_sigmask - change or examine calling thread's signal mask

SYNOPSIS

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

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

DESCRIPTION

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:

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

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

SIG_SETMASK
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.

ERRORS

The pthread_sigmask() function will fail if:

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

EXAMPLES

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:

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

o 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.

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

o 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.

o The signal-handling thread, handler:

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

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

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

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

int
main( int argc, char *argv[] ) {
test_argv(argv[1]);

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

case '0': /* POSIX */
pthread_sigmask(SIG_BLOCK, &new, NULL);
pthread_create(&user_threadID, NULL, handler,
argv[1]);
pthread_join(user_threadID, NULL);
break;

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

printf("thread handler, # %d, has exited\n",user_threadID);
sleep(2);
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);
break;
case '1': /* Solaris */
thr_sigsetmask(SIG_UNBLOCK, &new, NULL);
break;
}
printf("\n Press CTRL-C to deliver SIGINT signal to the
process\n");
sleep(8); /* give user time to hit CTRL-C */
return (NULL)
}

void
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");
exit(NULL);
}
}

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(),
signal);
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).

ATTRIBUTES

NOTES

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(7)).
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.

March 23, 2005 PTHREAD_SIGMASK(3C)