path: root/usr/src/man/man3c/pthread_sigmask.3c

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.TH PTHREAD_SIGMASK 3C "Mar 23, 2005"
.SH NAME
pthread_sigmask \- change or examine calling thread's signal mask
.SH SYNOPSIS
.LP
.nf
cc -mt [ \fIflag\fR... ] \fIfile\fR... -lpthread [ \fIlibrary\fR... ]
#include <pthread.h>
#include <signal.h>

\fBint\fR \fBpthread_sigmask\fR(\fBint\fR \fIhow\fR, \fBconst sigset_t *\fR\fIset\fR, \fBsigset_t *\fR\fIoset\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBpthread_sigmask()\fR 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.
.sp
.LP
If the value of the argument \fIset\fR is not \fINULL\fR, \fIset\fR points to a
set of signals that can modify the currently blocked set. If the value of
\fIset\fR is \fINULL\fR, the value of \fIhow\fR is insignificant and the
thread's signal mask is unmodified; thus, \fBpthread_sigmask()\fR can be used
to inquire about the currently blocked signals.
.sp
.LP
The value of the argument \fIhow\fR specifies the method in which the set is
changed and  takes one of the following values:
.sp
.ne 2
.na
\fB\fBSIG_BLOCK\fR\fR
.ad
.RS 15n
\fIset\fR corresponds to a set of signals to block. They are added to the
current signal mask.
.RE

.sp
.ne 2
.na
\fB\fBSIG_UNBLOCK\fR\fR
.ad
.RS 15n
\fIset\fR corresponds to a set of signals to unblock. These signals are deleted
from the current signal mask.
.RE

.sp
.ne 2
.na
\fB\fBSIG_SETMASK\fR\fR
.ad
.RS 15n
\fIset\fR corresponds to the new signal mask. The current signal mask is
replaced by \fBset\fR.
.RE

.sp
.LP
If the value of \fIoset\fR is not \fINULL\fR, it points to the location where
the previous signal mask is stored.
.SH RETURN VALUES
.sp
.LP
Upon successful completion, the \fBpthread_sigmask()\fR function returns
\fB0\fR. Otherwise, it returns a non-zero value.
.SH ERRORS
.sp
.LP
The \fBpthread_sigmask()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The value of \fIhow\fR is not defined and \fIoset\fR is \fINULL\fR.
.RE

.SH EXAMPLES
.LP
\fBExample 1 \fRCreate a default thread that  can serve as a signal
catcher/handler with its own signal mask.
.sp
.LP
The following example shows how to create a default thread that  can serve as a
signal catcher/handler with its own signal mask. \fBnew\fR will have a
different value from the creator's signal mask.

.sp
.LP
As POSIX threads and Solaris threads are fully compatible even within the same
process, this example uses \fBpthread_create\fR(3C) if you execute \fBa.out
0\fR, or \fBthr_create\fR(3C) if you execute \fBa.out 1\fR.

.sp
.LP
In this example:

.RS +4
.TP
.ie t \(bu
.el o
The \fBsigemptyset\fR(3C) function initializes a null signal set, \fBnew\fR.
The \fBsigaddset\fR(3C) function packs the signal, \fBSIGINT\fR, into that new
set.
.RE
.RS +4
.TP
.ie t \(bu
.el o
Either \fBpthread_sigmask()\fR or \fBthr_sigsetmask()\fR is used to mask the
signal, \fBSIGINT\fR (CTRL-C), from the calling thread, which is \fBmain()\fR.
The signal is masked to guarantee that only the new thread will  receive this
signal.
.RE
.RS +4
.TP
.ie t \(bu
.el o
\fBpthread_create()\fR or \fBthr_create()\fR creates the signal-handling
thread.
.RE
.RS +4
.TP
.ie t \(bu
.el o
Using \fBpthread_join\fR(3C) or \fBthr_join\fR(3C), \fBmain()\fR then waits for
the termination of that signal-handling thread, whose \fBID\fR number is
\fBuser_threadID\fR; \fBmain()\fR will then \fBsleep\fR(3C) for 2 seconds,
after which the program terminates.
.RE
.RS +4
.TP
.ie t \(bu
.el o
The signal-handling thread, \fBhandler\fR:
.RS +4
.TP
.ie t \(bu
.el o
Assigns the handler \fBinterrupt()\fR to handle the signal \fBSIGINT\fR, by the
call to  \fBsigaction\fR(2).
.RE
.RS +4
.TP
.ie t \(bu
.el o
Resets its own signal set to \fInot block\fR the signal, \fBSIGINT\fR.
.RE
.RS +4
.TP
.ie t \(bu
.el o
Sleeps for 8 seconds to allow time for the user to deliver the signal,
\fBSIGINT\fR, by pressing the  \fBCTRL-C\fR.
.RE
.RE
.sp
.in +2
.nf
/* 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\en",user_threadID);
	sleep(2);
	printf("main thread, # %d is done\en", 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("\en Press CTRL-C to deliver SIGINT signal to the
     process\en");
	sleep(8);  /* give user time to hit CTRL-C */
 return (NULL)
}

void
interrupt(int sig)
{
 printf("thread %d caught signal %d\en", thr_self(\|), sig);
}

void test_argv(char argv1[\|])    {
	if(argv1 == NULL)  {
		printf("use 0 as arg1 to use thr_create(\|);\en \e
		or use 1 as arg1 to use pthread_create(\|)\en");
		exit(NULL);
	}
}
.fi
.in -2

.sp
.LP
In the last example, the \fBhandler\fR 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 \fBsigwait\fR(2). The two subroutines in the previous example,
\fBhandler()\fR and \fBinterrupt()\fR, could have been replaced with the
following routine:

.sp
.in +2
.nf
void *
handler(void *unused)
{
    int signal;
    printf("thread %d is waiting for you to press the CTRL-C keys\en",
            thr_self(\|));
    sigwait(&new, &signal);
    printf("thread %d has received the signal %d \en", thr_self(\|),
        signal);
    return (NULL);
}
/* pthread_create(\|) and thr_create(\|) would use NULL instead
   of argv[1] for the arg passed to handler(\|) */
.fi
.in -2

.sp
.LP
In this routine, one thread is dedicated to catching and handling the  signal
specified by the set \fBnew\fR, which allows  \fBmain()\fR and all of its other
sub-threads, created \fIafter\fR \fBpthread_sigmask()\fR or
\fBthr_sigsetmask()\fR masked that signal, to continue uninterrupted. Any use
of  \fBsigwait\fR(2) should be such that all threads block the signals passed
to \fBsigwait\fR(2) at all times. Only the thread that calls \fBsigwait()\fR
will get the signals. The call to \fBsigwait\fR(2) takes two arguments.

.sp
.LP
For this type of background dedicated signal-handling routine, a Solaris daemon
thread can be used by passing the argument \fBTHR_DAEMON\fR to
\fBthr_create\fR(3C).

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	MT-Safe and Async-Signal-Safe
.TE

.SH SEE ALSO
.sp
.LP
\fBsigaction\fR(2), \fBsigprocmask\fR(2), \fBsigwait\fR(2),
\fBcond_wait\fR(3C), \fBpthread_cancel\fR(3C), \fBpthread_create\fR(3C),
\fBpthread_join\fR(3C), \fBpthread_self\fR(3C), \fBsigaddset\fR(3C),
\fBsigemptyset\fR(3C), \fBsigsetops\fR(3C), \fBsleep\fR(3C),
\fBattributes\fR(5), \fBcancellation\fR(5), \fBstandards\fR(5)
.SH NOTES
.sp
.LP
It is not possible to block signals that cannot be caught or ignored (see
\fBsigaction\fR(2)). It is also not possible to block or unblock
\fBSIGCANCEL\fR, as \fBSIGCANCEL\fR is reserved for the implementation of POSIX
thread cancellation (see \fBpthread_cancel\fR(3C) and \fBcancellation\fR(5)).
This restriction is quietly enforced by the standard C library.
.sp
.LP
Using \fBsigwait\fR(2) in a dedicated thread allows asynchronously generated
signals to be managed synchronously; however, \fBsigwait\fR(2) should never be
used to manage synchronously generated signals.
.sp
.LP
Synchronously generated signals are exceptions that are generated by a thread
and are directed at the thread causing the exception. Since \fBsigwait()\fR
blocks waiting for signals, the blocking thread cannot receive a synchronously
generated signal.
.sp
.LP
The \fBsigprocmask\fR(2) function behaves the same as if
\fBpthread_sigmask()\fR has been called. POSIX leaves the semantics of the call
to \fBsigprocmask\fR(2) unspecified in a multi-threaded process, so programs
that care about POSIX portability should not depend on this semantic.
.sp
.LP
If a signal is delivered while a thread is waiting on a condition variable, the
\fBcond_wait\fR(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.
.sp
.LP
Although \fBpthread_sigmask()\fR is Async-Signal-Safe with respect to the
Solaris environment, this safeness is not guaranteed to be portable to other
POSIX domains.
.sp
.LP
Signals that are generated synchronously should not be masked. If such a signal
is blocked and delivered, the receiving process is killed.