#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);
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:
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.
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.
The pthread_sigmask() function shall not return an error code of [EINTR].
The following sections are informative.
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; } }
See kill() for a discussion of the requirement on delivery of signals.
The Base Definitions volume of POSIX.1-2017, <signal.h>
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