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This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
this interface may differ (consult the corresponding Linux manual page for details of
Linux behavior), or the interface may not be implemented on Linux.
NAME
pthread_sigmask, sigprocmask — examine and change blocked signals
SYNOPSIS
#include <signal.h>
int pthread_sigmask(int how, const sigset_t *restrict set,
sigset_t *restrict oset);
int sigprocmask(int how, const sigset_t *restrict set,
sigset_t *restrict oset);
DESCRIPTION
The pthread_sigmask() function shall examine or change (or both) the calling thread's
signal mask, regardless of the number of threads in the process. The function shall be
equivalent to sigprocmask(), without the restriction that the call be made in a single-
threaded process.
In a single-threaded process, the sigprocmask() function shall examine or change (or both)
the signal mask of the calling thread.
If the argument set is not a null pointer, it points to a set of signals to be used to
change the currently blocked set.
The argument how indicates the way in which the set is changed, and the application shall
ensure it consists of one of the following values:
SIG_BLOCK The resulting set shall be the union of the current set and the signal set
pointed to by set.
SIG_SETMASK The resulting set shall be the signal set pointed to by set.
SIG_UNBLOCK The resulting set shall be the intersection of the current set and the
complement of the signal set pointed to by set.
If the argument oset is not a null pointer, the previous mask shall be stored in the
location pointed to by oset. If set is a null pointer, the value of the argument how is
not significant and the thread's signal mask shall be unchanged; thus the call can be used
to enquire about currently blocked signals.
If there are any pending unblocked signals after the call to sigprocmask(), at least one
of those signals shall be delivered before the call to sigprocmask() returns.
It is not possible to block those signals which cannot be ignored. This shall be enforced
by the system without causing an error to be indicated.
If any of the SIGFPE, SIGILL, SIGSEGV, or SIGBUS signals are generated while they are
blocked, the result is undefined, unless the signal was generated by the action of another
process, or by one of the functions kill(), pthread_kill(), raise(), or sigqueue().
If sigprocmask() fails, the thread's signal mask shall not be changed.
The use of the sigprocmask() function is unspecified in a multi-threaded process.
RETURN VALUE
Upon successful completion pthread_sigmask() shall return 0; otherwise, it shall return
the corresponding error number.
Upon successful completion, sigprocmask() shall return 0; otherwise, −1 shall be returned,
errno shall be set to indicate the error, and the signal mask of the process shall be
unchanged.
ERRORS
The pthread_sigmask() and sigprocmask() functions shall fail if:
EINVAL The value of the how argument is not equal to one of the defined values.
The pthread_sigmask() function shall not return an error code of [EINTR].
The following sections are informative.
EXAMPLES
Signaling in a Multi-Threaded Process
This example shows the use of pthread_sigmask() in order to deal with signals in a multi-
threaded process. It provides a fairly general framework that could be easily
adapted/extended.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <signal.h>
#include <string.h>
#include <errno.h>
...
static sigset_t signal_mask; /* signals to block */
int main (int argc, char *argv[])
{
pthread_t sig_thr_id; /* signal handler thread ID */
int rc; /* return code */
sigemptyset (&signal_mask);
sigaddset (&signal_mask, SIGINT);
sigaddset (&signal_mask, SIGTERM);
rc = pthread_sigmask (SIG_BLOCK, &signal_mask, NULL);
if (rc != 0) {
/* handle error */
...
}
/* any newly created threads inherit the signal mask */
rc = pthread_create (&sig_thr_id, NULL, signal_thread, NULL);
if (rc != 0) {
/* handle error */
...
}
/* APPLICATION CODE */
...
}
void *signal_thread (void *arg)
{
int sig_caught; /* signal caught */
int rc; /* returned code */
rc = sigwait (&signal_mask, &sig_caught);
if (rc != 0) {
/* handle error */
}
switch (sig_caught)
{
case SIGINT: /* process SIGINT */
...
break;
case SIGTERM: /* process SIGTERM */
...
break;
default: /* should normally not happen */
fprintf (stderr, "\nUnexpected signal %d\n", sig_caught);
break;
}
}
APPLICATION USAGE
None.
RATIONALE
When a thread's signal mask is changed in a signal-catching function that is installed by
sigaction(), the restoration of the signal mask on return from the signal-catching
function overrides that change (see sigaction()). If the signal-catching function was
installed with signal(), it is unspecified whether this occurs.
See kill() for a discussion of the requirement on delivery of signals.
FUTURE DIRECTIONS
None.
SEE ALSO
exec, kill(), sigaction(), sigaddset(), sigdelset(), sigemptyset(), sigfillset(),
sigismember(), sigpending(), sigqueue(), sigsuspend()
The Base Definitions volume of POSIX.1‐2008, <signal.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any
discrepancy between this version and the original IEEE and The Open Group Standard, the
original IEEE and The Open Group Standard is the referee document. The original Standard
can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .