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mbrlen - determine number of bytes in next multibyte character
#include <wchar.h> size_t mbrlen (const char *s, size_t n, mbstate_t *ps);
The mbrlen function inspects at most n bytes of the multibyte string starting at s and extracts the next complete multibyte character. It updates the shift state *ps. If the multibyte character is not the null wide character, it returns the number of bytes that were consumed from s. If the multibyte character is the null wide character, it resets the shift state *ps to the initial state and returns 0. If the n bytes starting at s do not contain a complete multibyte character, mbrlen returns (size_t)(-2). This can happen even if n >= MB_CUR_MAX, if the multibyte string contains redundant shift sequences. If the multibyte string starting at s contains an invalid multibyte sequence before the next complete character, mbrlen returns (size_t)(-1) and sets errno to EILSEQ. In this case, the effects on *ps are undefined. If ps is a NULL pointer, a static anonymous state only known to the mbrlen function is used instead.
The mbrlen function returns the number of bytes parsed from the multibyte sequence starting at s, if a non-null wide character was recognized. It returns 0, if a null wide character was recognized. It returns (size_t)(-1) and sets errno to EILSEQ, if an invalid multibyte sequence was encountered. It returns (size_t)(-2) if it couldn't parse a complete multibyte character, meaning that n should be increased.
ISO/ANSI C, UNIX98
The behaviour of mbrlen depends on the LC_CTYPE category of the current locale.