mbrlen(3)

💖 mbrlen — the girl who counts bytes for you 💖

okay bestie, so this page is all about mbrlen, a lil function from the Linux man-pages, and she is literally the byte-counter of the multibyte world. she tells you how many bytes the next complete multibyte character is eating. no cap. 🦋

🌈 LIBRARY 🌈

Standard C library (libc, -lc) — she lives in there, rent free 💅

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✨ SYNOPSIS ✨

#include <wchar.h>

size_t mbrlen(size_t n;
              const char s[restrict n], size_t n,
              mbstate_t *restrict ps);

yes bestie, that's the exact signature, don't come for me, I copied it character for character 😭

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👑 DESCRIPTION 👑

so here's the tea. mbrlen() peeks at at most n bytes of the multibyte string starting at s, and it extracts the next complete multibyte character. she also updates the shift state *ps while she's at it. main character energy fr.

• if that multibyte character is not the null wide character → she returns the number of bytes that were consumed from s 💫
• if that multibyte character is the null wide character → she resets the shift state *ps to the initial state and returns 0 🎀

now, plot twist. if the n bytes starting at s don't contain a complete multibyte character, mbrlen() returns (size_t) -2. and yes, this can happen even if n >= MB_CUR_MAX, specifically if the multibyte string contains redundant shift sequences. the way that can just... happen 🙈

and if the multibyte string starting at s contains an invalid multibyte sequence before the next complete character? she returns (size_t) -1 and sets errno to EILSEQ. in that case, the effects on *ps are undefined — so like, don't touch that, she's not answering questions 💀

one more thing: if ps is NULL, a static anonymous state known only to the mbrlen() function is used instead. she's got a secret state, iykyk 🤫

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🫶 RETURN VALUE 🫶

here's the recap so you never have to ask twice:

• she returns the number of bytes parsed from the multibyte sequence starting at s, if a non-null wide character was recognized ✅
• she returns 0, if a null wide character was recognized 🌸
• she returns (size_t) -1 and sets errno to EILSEQ, if an invalid multibyte sequence was encountered 😭
• she returns (size_t) -2 if she couldn't parse a complete multibyte character — which literally just means n should be increased, bestie, go make it bigger 📈

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⭐ ATTRIBUTES ⭐

want the whole explanation of the terms used up in this section? see attributes(7). iconic.

• mbrlen() — Thread safety — MT-Unsafe race:mbrlen/!ps 💀

fr fr pay attention to that one. MT-Unsafe race:mbrlen/!ps is not a vibe you want in your threaded code.

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🎀 STANDARDS 🎀

C11, POSIX.1-2008. certified, periodt.

🦋 HISTORY 🦋

POSIX.1-2001, C99. she's been around, she's a legacy icon.

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💅 NOTES 💅

the behavior of mbrlen() depends on the LC_CTYPE category of the current locale. so like, your locale is running the show, not her 🙈

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👀 SEE ALSO 👀

• mbrtowc(3) — her sibling, go say hi 🫶

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okay that's the whole page bestie, go forth and count your bytes 💖✨

this was mbrlen(3) from Linux man-pages 6.20, dated 2026-02-08 — mwah, slay, bye 🎀💜