A simple one header solution to supporting utf8 strings in C and C .

Functions provided from the C header string.h but with a utf8* prefix instead of the str* prefix:

API function docs

Functions provided from the C header strings.h but with a utf8* prefix instead of the str* prefix:

Functions provided that are unique to utf8.h:

Just #include "utf8.h" in your code!

The current supported platforms are Linux, macOS and Windows.

The current supported compilers are gcc, clang, MSVC's cl.exe, and clang-cl.exe.

The utf8.h API matches the string.h API as much as possible by design. There are a few major differences though.

utf8.h uses char8_t* in C 20 instead of char*

Anywhere in the string.h or strings.h documentation where it refers to 'bytes' I have changed that to utf8 codepoints. For instance, utf8len will return the number of utf8 codepoints in a utf8 string - which does not necessarily equate to the number of bytes.

int utf8casecmp(const void *src1, const void *src2);

Return less than 0, 0, greater than 0 if src1 < src2, src1 == src2, src1 > src2 respectively, case insensitive.

void *utf8cat(void *dst, const void *src);

Append the utf8 string src onto the utf8 string dst.

void *utf8chr(const void *src, utf8_int32_t chr);

Find the first match of the utf8 codepoint chr in the utf8 string src.

int utf8cmp(const void *src1, const void *src2);

Return less than 0, 0, greater than 0 if src1 < src2,
src1 == src2, src1 > src2 respectively.

void *utf8cpy(void *dst, const void *src);

Copy the utf8 string src onto the memory allocated in dst.

size_t utf8cspn(const void *src, const void *reject);

Number of utf8 codepoints in the utf8 string src that consists entirely
of utf8 codepoints not from the utf8 string reject.

void *utf8dup(const void *src);

Duplicate the utf8 string src by getting its size, mallocing a new buffer
copying over the data, and returning that. Or 0 if malloc failed.

size_t utf8len(const void *str);

Number of utf8 codepoints in the utf8 string str,
excluding the null terminating byte.

size_t utf8nlen(const void *str, size_t n);

Similar to utf8len, except that only at most n bytes of src are looked.

int utf8ncasecmp(const void *src1, const void *src2, size_t n);

Return less than 0, 0, greater than 0 if src1 < src2, src1 == src2,
src1 > src2 respectively, case insensitive. Checking at most n
bytes of each utf8 string.

void *utf8ncat(void *dst, const void *src, size_t n);

Append the utf8 string src onto the utf8 string dst,
writing at most n 1 bytes. Can produce an invalid utf8
string if n falls partway through a utf8 codepoint.

int utf8ncmp(const void *src1, const void *src2, size_t n);

Return less than 0, 0, greater than 0 if src1 < src2,
src1 == src2, src1 > src2 respectively. Checking at most n
bytes of each utf8 string.

void *utf8ncpy(void *dst, const void *src, size_t n);

Copy the utf8 string src onto the memory allocated in dst.
Copies at most n bytes. If n falls partway through a utf8 codepoint, or if dst doesn't have enough room for a null terminator, the final string will be cut short to preserve utf8 validity.

void *utf8pbrk(const void *str, const void *accept);

Locates the first occurrence in the utf8 string str of any byte in the
utf8 string accept, or 0 if no match was found.

void *utf8rchr(const void *src, utf8_int32_t chr);

Find the last match of the utf8 codepoint chr in the utf8 string src.

size_t utf8size(const void *str);

Number of bytes in the utf8 string str,
including the null terminating byte.

size_t utf8size_lazy(const void *str);

Similar to utf8size, except that the null terminating byte is excluded.

size_t utf8nsize_lazy(const void *str, size_t n);

Similar to utf8size, except that only at most n bytes of src are looked and the null terminating byte is excluded.

size_t utf8spn(const void *src, const void *accept);

Number of utf8 codepoints in the utf8 string src that consists entirely
of utf8 codepoints from the utf8 string accept.

void *utf8str(const void *haystack, const void *needle);

The position of the utf8 string needle in the utf8 string haystack.

void *utf8casestr(const void *haystack, const void *needle);

The position of the utf8 string needle in the utf8 string haystack, case insensitive.

void *utf8valid(const void *str);

Return 0 on success, or the position of the invalid utf8 codepoint on failure.

void *utf8nvalid(const void *str, size_t n);

Similar to utf8valid, except that only at most n bytes of src are looked.

int utf8makevalid(void *str, utf8_int32_t replacement);

Return 0 on success. Makes the str valid by replacing invalid sequences with the 1-byte replacement codepoint.

void *utf8codepoint(const void *str, utf8_int32_t *out_codepoint);

Sets out_codepoint to the current utf8 codepoint in str, and returns the address of the next utf8 codepoint after the current one in str.

void *utf8rcodepoint(const void *str, utf8_int32_t *out_codepoint);

Sets out_codepoint to the current utf8 codepoint in str, and returns the address of the previous utf8 codepoint before the current one in str.

size_t utf8codepointsize(utf8_int32_t chr);

Returns the size of the given codepoint in bytes.

void *utf8catcodepoint(void *utf8_restrict str, utf8_int32_t chr, size_t n);

Write a codepoint to the given string, and return the address to the next place after the written codepoint. Pass how many bytes left in the buffer to n. If there is not enough space for the codepoint, this function returns null.

int utf8islower(utf8_int32_t chr);

Returns 1 if the given character is lowercase, or 0 if it is not.

int utf8isupper(utf8_int32_t chr);

Returns 1 if the given character is uppercase, or 0 if it is not.

void utf8lwr(void *utf8_restrict str);

Transform the given string into all lowercase codepoints.

void utf8upr(void *utf8_restrict str);

Transform the given string into all uppercase codepoints.

utf8_int32_t utf8lwrcodepoint(utf8_int32_t cp);

Make a codepoint lower case if possible.

utf8_int32_t utf8uprcodepoint(utf8_int32_t cp);

Make a codepoint upper case if possible.

Various functions provided will do case insensitive compares, or transform utf8 strings from one case to another. Given the vastness of unicode, and the authors lack of understanding beyond latin codepoints on whether case means anything, the following categories are the only ones that will be checked in case insensitive code:

• ASCII
• Latin-1 Supplement
• Latin Extended-A
• Latin Extended-B
• Greek and Coptic
• Cyrillic

• Implement utf8coll (akin to strcoll).
• Implement utf8fry (akin to strfry).
• Investigate adding dst buffer sizes for utf8cpy and utf8cat to catch overwrites (as suggested by @FlohOfWoe in https://twitter.com/FlohOfWoe/status/618669237771608064)

This is free and unencumbered software released into the public domain.

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