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mbrtowc - convert a multibyte sequence to a wide character
#include <wchar.h> size_t mbrtowc (wchar_t *pwc, const char *s, size_t n, mbstate_t *ps);
The main case for this function is when s is not NULL and pwc is not NULL. In this case, the mbrtowc function inspects at most n bytes of the multibyte string starting at s, extracts the next complete multibyte character, converts it to a wide character and stores it at *pwc. It updates the shift state *ps. If the converted wide character is not L'\0', it returns the number of bytes that were consumed from s. If the converted wide character is L'\0', 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, mbrtowc 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, mbrtowc returns (size_t)(-1) and sets errno to EILSEQ. In this case, the effects on *ps are undefined. A different case is when s is not NULL but pwc is NULL. In this case the mbrtowc function behaves as above, excepts that it does not store the converted wide character in memory. A third case is when s is NULL. In this case, pwc and n are ignored. The mbrtowc function puts *ps in the initial state and returns 0. In all of the above cases, if ps is a NULL pointer, a static anonymous state only known to the mbrtowc function is used instead.
The mbrtowc function returns the number of bytes parsed from the multibyte sequence starting at s, if a non-L'\0' wide character was recognized. It returns 0, if a L'\0' 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 mbrtowc depends on the LC_CTYPE category of the current locale.