mbrtowc(3)

💖 mbrtowc(3) — convert a multibyte sequence to a wide character 💖

okay bestie, welcome to the Library Functions Manual, section 3, page mbrtowc(3). this one's a lil crunchy but i got you, fr fr 🫶

📚 LIBRARY

Standard C library (libc, -lc) — yeah she lives in libc, standard issue, no cap.

✨ SYNOPSIS ✨

#include <wchar.h>

size_t mbrtowc(size_t n;
               wchar_t *restrict pwc, const char s[restrict n],
               size_t n, mbstate_t *restrict ps);

like. look at that prototype. it's giving chaos but we move 💅

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🧠 DESCRIPTION

okay so the main case is when s is not NULL and pwc is not NULL. in that case, mbrtowc() peeks at at most n bytes of the multibyte string starting at s, yoinks the next complete multibyte character, converts it to a wide character, and drops it at *pwc. it also updates the shift state *ps. then:

• if the converted wide character is not L'\0' (the null wide character), she returns the number of bytes that were consumed from s. 🎀
• if the converted wide character is L'\0', she resets the shift state *ps to the initial state and returns 0.

now. if the n bytes starting at s do not contain a complete multibyte character, mbrtowc() returns (size_t) -2. and bestie, listen — this can happen even if n >= MB_CUR_MAX, if the multibyte string contains redundant shift sequences. sneaky.

if the multibyte string starting at s contains an invalid multibyte sequence before the next complete character, mbrtowc() returns (size_t) -1 and sets errno to EILSEQ. and in this case? the effects on *ps are undefined. not a vibe. 💀

a different case is when s is not NULL but pwc is NULL. then mbrtowc() behaves exactly as above, except she does not store the converted wide character in memory. she's just doing the conversion and keeping it to herself, main character energy.

a third case is when s is NULL. in this case, pwc and n are ignored. so:

• if the conversion state represented by *ps denotes an incomplete multibyte character conversion, mbrtowc() returns (size_t) -1, sets errno to EILSEQ, and leaves *ps in an undefined state. 🫠
• otherwise, mbrtowc() puts *ps in the initial state and returns 0. slay.

in all of the above cases, if ps is NULL, a static anonymous state known only to the mbrtowc() function is used instead. otherwise, *ps must be a valid mbstate_t object. and bestie, an mbstate_t object a can be initialized to the initial state by zeroing it, for example using:

memset(&a, 0, sizeof(a));

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🔁 RETURN VALUE

so what does she give back? depends:

• the number of bytes parsed from the multibyte sequence starting at s, if a non-L'\0' wide character was recognized 🌸
• 0, if a L'\0' wide character was recognized
• (size_t) -1 and sets errno to EILSEQ, if an invalid multibyte sequence was encountered 😭
• (size_t) -2 if the consumed n bytes form a valid partial character sequence — meaning more subsequent data needs to be fed in. she's waiting, she's patient, she's hydrated 🧚

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🏷️ ATTRIBUTES

for an explanation of the terms used in this section, see attributes(7). (no tables on my watch, so here it is all cute:)

• Interface: mbrtowc()
• Attribute: Thread safety
• Value: MT-Unsafe race:mbrtowc/!ps

ok so thread safety said MT-Unsafe race:mbrtowc/!ps. read it. respect it. she is not out here being safe across threads unless you handle ps yourself, iykyk ✨

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📏 STANDARDS

C11, POSIX.1-2008. certified, standardized, iconic.

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🕰️ HISTORY

POSIX.1-2001, C99. she's been around. a legend from the jump 💜

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📝 NOTES

the behavior of mbrtowc() depends on the LC_CTYPE category of the current locale. so yeah, set your locale or she'll be confused. 🦋

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👀 SEE ALSO

• mbsinit(3)
• mbsrtowcs(3)

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and that's the whole page bestie!! this was Linux man-pages 6.20, dated 2026-02-08 — go forth and convert those multibyte sequences responsibly, mwah 💅💖🌈