pthread_sigmask(3)

✨ NAME ✨

pthread_sigmask — the glow-up version of blocking signals: examine and change your mask of blocked signals 💅

Feels like a libpthread (POSIX threads library) moment, ngl — link it with -lpthread. 🧵

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✨ SYNOPSIS ✨

#include <signal.h>

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

🎀 Feature Test Macro Requirements for glibc

(go check feature_test_macros(7) bestie, it's all there)

pthread_sigmask():

_POSIX_C_SOURCE >= 199506L || _XOPEN_SOURCE >= 500

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💖 DESCRIPTION 💖

okay so pthread_sigmask() is literally just like sigprocmask(2), except — and this is the part that eats — its use in multithreaded programs is explicitly specified by POSIX.1. That's the whole plot twist. The other differences are noted further down this page, so don't skip them. 🙈

For the full tea on the arguments and how this function actually operates, go read sigprocmask(2). No cap, everything's there.

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🌸 RETURN VALUE 🌸

Success? She returns 0. Error? She returns an error number. That's it, that's the vibe. ✨

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🦋 ERRORS 🦋

See sigprocmask(2). Yeah bestie, we're pointing you there again — it's giving "read the other page." 👑

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💜 ATTRIBUTES 💜

For an explanation of the terms used in this section, see attributes(7).

• Interface: pthread_sigmask()
• Attribute: Thread safety
• Value: MT-Safe 🔥

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🌈 STANDARDS 🌈

POSIX.1-2008. Periodt.

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⭐ HISTORY ⭐

POSIX.1-2001. She's been iconic for a minute. 👑

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🍓 NOTES 🍓

A new thread inherits a copy of its creator's signal mask. Genetics, but make it signals. 🧬

The glibc pthread_sigmask() function silently ignores attempts to block the two real-time signals that are used internally by the NPTL threading implementation. Silently!! She just pretends she didn't hear you. 🙈 See nptl(7) for the details.

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🧚 EXAMPLES 🧚

The program below blocks some signals in the main thread, and then creates a dedicated thread to fetch those signals via sigwait(3). Here's a shell session showing it in action, live and in 4K:

$ ./a.out &
[1] 5423
$ kill -QUIT %1
Signal handling thread got signal 3
$ kill -USR1 %1
Signal handling thread got signal 10
$ kill -TERM %1
[1]+  Terminated              ./a.out

🌸 Program source 🌸

#include <err.h>
#include <errno.h>
#include <pthread.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

/* Simple error handling functions */

static void *
sig_thread(void *arg)
{
    sigset_t *set = arg;
    int s, sig;

    for (;;) {
        s = sigwait(set, &sig);
        if (s != 0)
            errc(EXIT_FAILURE, s, "sigwait");
        printf("Signal handling thread got signal %d\n", sig);
    }
}

int
main(void)
{
    pthread_t thread;
    sigset_t set;
    int s;

    /* Block SIGQUIT and SIGUSR1; other threads created by main()
       will inherit a copy of the signal mask.  */

    sigemptyset(&set);
    sigaddset(&set, SIGQUIT);
    sigaddset(&set, SIGUSR1);
    s = pthread_sigmask(SIG_BLOCK, &set, NULL);
    if (s != 0)
        errc(EXIT_FAILURE, s, "pthread_sigmask");

    s = pthread_create(&thread, NULL, &sig_thread, &set);
    if (s != 0)
        errc(EXIT_FAILURE, s, "pthread_create");

    /* Main thread carries on to create other threads and/or do
       other work.  */

    pause();            /* Dummy pause so we can test program */
}

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👑 SEE ALSO 👑

sigaction(2), sigpending(2), sigprocmask(2), pthread_attr_setsigmask_np(3), pthread_create(3), pthread_kill(3), sigsetops(3), pthreads(7), signal(7)

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okay that's the whole page, you ate that 💅 this was Linux man-pages 6.20, dated 2025-09-21 — yes, pthread_sigmask(3) said it, bestie. go forth and mask responsibly 🫶🎀