SIGSETOPS(3)

✨ SIGSETOPS(3) ✨ — the signal-set girlies of libc

okay bestie, pull up a chair 🫶 this is the <signal.h> signal set operation squad, living in the Standard C library (libc, -lc). Five absolute icons: sigemptyset, sigfillset, sigaddset, sigdelset, sigismember. POSIX coded, no cap. 💅

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🌸 SYNOPSIS 🌸

#include <signal.h>

int sigemptyset(sigset_t *set);
int sigfillset(sigset_t *set);

int sigaddset(sigset_t *set, int signum);
int sigdelset(sigset_t *set, int signum);

int sigismember(const sigset_t *set, int signum);

🎀 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

• sigemptyset(), sigfillset(), sigaddset(), sigdelset(), sigismember():
  • _POSIX_C_SOURCE

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🦋 DESCRIPTION 🦋

So these functions are literally just here to let you mess with POSIX signal sets. That's it, that's the job. ✨

• sigemptyset() — initializes the signal set given by set to empty, with all signals excluded from the set. she's giving minimalism 💖
• sigfillset() — initializes set to full, including all signals. maximalist era 👑
• sigaddset() and sigdelset() — add and delete respectively signal signum from set.
• sigismember() — tests whether signum is a member of set.

Now the important part, bestie, do not skip this one 🚨:

Objects of type sigset_t must be initialized by a call to either sigemptyset() or sigfillset() before being passed to the functions sigaddset(), sigdelset(), and sigismember() or the additional glibc functions described below (sigisemptyset(), sigandset(), and sigorset()). The results are undefined if this is not done.

undefined behaviour?? not the vibe. init your sigset_t first, periodt. 😭

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💫 RETURN VALUE 💫

• sigemptyset(), sigfillset(), sigaddset(), and sigdelset() return 0 on success and -1 on error.
• sigismember() returns 1 if signum is a member of set, 0 if signum is not a member, and -1 on error. three whole outcomes, she's versatile 💅

On error, these functions set errno to indicate the error. always check ya girl errno 🫶

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🔥 ERRORS 🔥

• EINVAL — signum is not a valid signal.

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🌈 ATTRIBUTES 🌈

for the explanation of the terms used in this section, go see attributes(7) (iykyk). here's the tea ☕:

• Interface: sigemptyset(), sigfillset(), sigaddset(), sigdelset(), sigismember(), sigisemptyset(), sigorset(), sigandset()
• Attribute: Thread safety
• Value: MT-Safe

all of them! every single one! safe to hit from multiple threads, no drama. major slay 🧚

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💜 VERSIONS 💜

🍓 GNU

if you define the _GNU_SOURCE feature test macro, then <signal.h> exposes three other functions for manipulating signal sets. glibc said "and I took that personally" ✨

int sigisemptyset(const sigset_t *set);
int sigorset(sigset_t *dest, const sigset_t *left,
              const sigset_t *right);
int sigandset(sigset_t *dest, const sigset_t *left,
              const sigset_t *right);

• sigisemptyset() returns 1 if set contains no signals, and 0 otherwise.
• sigorset() places the union of the sets left and right in dest.
• sigandset() places the intersection of the sets left and right in dest.
• Both functions return 0 on success, and -1 on failure.

these functions are nonstandard (a few other systems provide similar functions) and their use should be avoided in portable applications.

portable girls, stay away from these unless you know what you're doing 🙈

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⭐ STANDARDS ⭐

POSIX.1-2008.

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🌸 HISTORY 🌸

POSIX.1-2001. she's been iconic since the early 2000s, we love a veteran 💖

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🎀 NOTES 🎀

when creating a filled signal set, the glibc sigfillset() function does not include the two real-time signals used internally by the NPTL threading implementation. see nptl(7) for the details. so "full" is doing some light gatekeeping, ngl 💀

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🫶 SEE ALSO 🫶

• sigaction(2)
• sigpending(2)
• sigprocmask(2)
• sigsuspend(2)

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and that's the whole page, bestie! 💖 this was brought to you by Linux man-pages 6.20, dated 2026-02-08. go forth and initialize your sigset_ts before you do literally anything else with them. mwah 💋✨