ffs(3)

💖 ffs(3) — find first bit set in a word 💖

okay bestie, buckle up, because today we're getting down to the bits 🦋 this is the ffs(3) page from your friendly neighborhood Library Functions Manual, and it's genuinely so cute and so useful. no cap.

🌸 LIBRARY 🌸

Standard C library (libc, -lc) — the iconic one, the one your linker already knows and loves 💅

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✨ SYNOPSIS ✨

#include <strings.h>

int ffs(int i);
int ffsl(long i);
int ffsll(long long i);

literally three functions, three vibes, one job 🔥

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

not me having to gate-keep which macros you define before glibc will even let you play 😭 here's the whole situation:

• ffs():
  • Since glibc 2.12:

_XOPEN_SOURCE >= 700
    || ! (_POSIX_C_SOURCE >= 200809L)
    || /* glibc >= 2.19: */ _DEFAULT_SOURCE
    || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

• Before glibc 2.12: none — no macros needed, it was just free real estate 🫶

• ffsl(), ffsll():
  • Since glibc 2.27: _DEFAULT_SOURCE
  • Before glibc 2.27: _GNU_SOURCE

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🌈 DESCRIPTION 🌈

so here's the tea ☕ the ffs() function returns the position of the first (least significant) bit set in the word i. the least significant bit is position 1, and the most significant position is, for example, 32 or 64. we count from one bestie, not from zero, that's just the house rules 💅

the functions ffsll() and ffsl() do the exact same thing but take arguments of possibly different size. same energy, bigger (or smaller) word 🌸

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👑 RETURN VALUE 👑

these functions return the position of the first bit set, or 0 if no bits are set in i. that's it. zero means "nothing to see here", periodt 🎀

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💅 ATTRIBUTES 💅

for an explanation of the terms used in this section, see attributes(7). (lore drop ✨)

• Interface: ffs(), ffsl(), ffsll()
• Attribute: Thread safety
• Value: MT-Safe

translation: she's MT-Safe and she knows it 🔥 no locking drama, just vibes.

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⭐ STANDARDS ⭐

• ffs() — POSIX.1-2001, POSIX.1-2008, 4.3BSD.
• ffsl()
• ffsll() — GNU.

(the last two are GNU's contribution to the group chat, iykyk 💜)

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🫶 SEE ALSO 🫶

• memchr(3)

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💀 friendly reminder bestie: this page is ffs(3), the find first bit set one. if you wanted "for f***'s sake" that's not a syscall, that's just your Monday.

okay that's the whole page, mwah 💖 this was Linux man-pages 6.20, dated 2026-02-08, and she ate, no crumbs 👑 bye bestie, go count some bits 🌟