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Man Pages
MBRLEN(3) FreeBSD Library Functions Manual MBRLEN(3)

mbrlen
get number of bytes in a character (restartable)

Standard C Library (libc, -lc)

#include <wchar.h>

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

The mbrlen() function inspects at most n bytes pointed to by s to determine the number of bytes needed to complete the next multibyte character.

The mbstate_t argument, ps, is used to keep track of the shift state. If it is NULL, mbrlen() uses an internal, static mbstate_t object, which is initialized to the initial conversion state at program startup.

It is equivalent to:

mbrtowc(NULL, s, n, ps);

Except that when ps is a NULL pointer, mbrlen() uses its own static, internal mbstate_t object to keep track of the shift state.

The mbrlen() functions returns:
0
The next n or fewer bytes represent the null wide character (L'\0').
>0
The next n or fewer bytes represent a valid character, mbrlen() returns the number of bytes used to complete the multibyte character.
(size_t)-2
The next n contribute to, but do not complete, a valid multibyte character sequence, and all n bytes have been processed.
(size_t)-1
An encoding error has occurred. The next n or fewer bytes do not contribute to a valid multibyte character.

A function that calculates the number of characters in a multibyte character string:
size_t
nchars(const char *s)
{
	size_t charlen, chars;
	mbstate_t mbs;

	chars = 0;
	memset(&mbs, 0, sizeof(mbs));
	while ((charlen = mbrlen(s, MB_CUR_MAX, &mbs)) != 0 &&
	    charlen != (size_t)-1 && charlen != (size_t)-2) {
		s += charlen;
		chars++;
	}

	return (chars);
}

The mbrlen() function will fail if:
[]
An invalid multibyte sequence was detected.
[]
The conversion state is invalid.

mblen(3), mbrtowc(3), multibyte(3)

The mbrlen() function conforms to ISO/IEC 9899:1999 (“ISO C99”).
April 7, 2004 FreeBSD 13.1-RELEASE

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