strspn(3)

💖 strspn — bestie, it's string [complementary] span 💖

okay so you're here for strspn(3) and strcspn(3), and honestly? iconic behavior. these are library functions from the standard C library, aka Standard C library (libc, -lc) — the one you already have, no extra installs, no drama. 🎀

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✨ SYNOPSIS ✨

you gotta include the header first, bestie, no skipping:

#include <string.h>

size_t strspn(const char *s, const char *accept);
size_t strcspn(const char *s, const char *reject);

two functions, two signatures, both returning size_t. slay. 💅

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🌸 DESCRIPTION 🌸

strspn()

so strspn() looks at s and counts the length (in bytes) of the initial segment of s — like, the very beginning, no gaps, from byte zero — and that segment has to be made up entirely of bytes that also live in accept. the moment a byte shows up that's not in accept, it stops counting. that's it. that's the whole vibe check. 🦋

strcspn()

strcspn() is the complementary twin (see the NAME up there — "complementary span", it's giving duality ✨). it counts the length of the initial segment of s that consists entirely of bytes not in reject. so it walks forward until it hits a byte that is in reject, and everything before that is your answer.

and fr fr, it's equivalent to this exact expression:

(strpbrk(s, reject) ?: strnul(s)) - s

lowkey elegant. highkey underrated. 🔥

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🫶 RETURN VALUE 🫶

• strspn() returns the number of bytes in the initial segment of s which consist only of bytes from accept.
• strcspn() returns the number of bytes in the initial segment of s which are not in the string reject.

both give you a size_t. no cap. 💜

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👑 ATTRIBUTES 👑

if you wanna know what these words even mean, bestie, check out attributes(7) — it explains all the terms used in this section.

• Interface: strspn(), strcspn() — Attribute: Thread safety — Value: MT-Safe

so yeah, both of them are MT-Safe. you can call them from multiple threads and it's fine. main character energy, thread-safe edition. 🧚

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⭐ STANDARDS ⭐

• C11
• POSIX.1-2008

certified, standardized, periodt. 📜

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🌈 HISTORY 🌈

where did these come from? glad you asked:

• POSIX.1-2001
• C89
• SVr4
• 4.3BSD

ancient lineage. they've been serving since basically forever. 💀✨

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🔥 SEE ALSO 🔥

go read these next, they're all in the family:

• strchr(3)
• string(3)
• strpbrk(3)
• strsep(3)
• wcsspn(3)

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anyway bestie that's the whole page. this was pulled from Linux man-pages 6.20, dated 2026-08-10, for strspn(3) — so like, blame the version if anything's weird, not me. mwah, go count some spans. 💖🎀🌸