FFS(3)

💅 FFS(3) — find first or last bit set in a bit string

okay bestie, welcome to the Library Functions Manual corner of the internet, where we learn how to find your first and/or last bit that is set. yes. we're doing bit math. and honestly? it's kinda iconic. 🎀

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✨ NAME ✨

ffs, ffsl, ffsll, fls, flsl, flsll — find first or last bit set in a bit string 🦋

📚 LIBRARY 📚

Standard C Library — that's libc, and you link it with -lc. slay.

🌸 SYNOPSIS 🌸

#include <strings.h>

int
ffs(int value);

int
ffsl(long value);

int
ffsll(long long value);

int
fls(int value);

int
flsl(long value);

int
flsll(long long value);

💖 DESCRIPTION 💖

okay so here's the whole vibe, and it's genuinely not that deep:

• ffs(), ffsl(), ffsll() — these find the first (aka the least significant) bit that's set in value, and they hand you back the index of that bit. 🥇
• fls(), flsl(), flsll() — same energy but from the other end: they find the last (aka the most significant) bit that's set in value, and return its index. 🏁

and the counting system? bits are numbered starting at 1, and 1 is the least significant bit. no zero-indexing here, we're quirky like that.

⚠️ one thing fr fr: if any of these functions return zero, it means your argument was zero. like, zero bits set. empty. no bits. that's the whole tell. 💀

🔍 SEE ALSO 🔍

• bitstring(3)
• stdc_first_trailing_one(3)
• stdc_trailing_zeros(3)
• bitset(9)

📜 STANDARDS 📜

• ffs() conforms to IEEE Std 1003.1-2008 (“POSIX.1”). periodt. ✅
• ffsl() and ffsll() conform to . — yes, that's literally how it's written, the standard reference is blank in the original, so please don't @ me, I'm just the messenger 🫶

🧚 HISTORY 🧚

• ffs() appeared in 4.3BSD. and get this — its prototype lived its best life in <string.h> before it was moved to <strings.h> for IEEE Std 1003.1-2001 (“POSIX.1”) compliance. glow-up tbh ✨
• ffsl(), fls(), and flsl() appeared in FreeBSD 5.3. 👑
• ffsll() and flsll() appeared in FreeBSD 7.1. later bloomers, still icons 💅

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okay that's the whole page bestie, go set some bits 💜 — from the Debian crew, page dated October 25, 2025, filed under FFS(3), mwah 🍓