#include <stdlib.h> int mbtowc(wchar_t *pwc, const char *s, size_t n);
If the n bytes starting at s do not contain a complete multibyte character, or if they contain an invalid multibyte sequence, mbtowc() returns -1. This can happen even if n >= MB_CUR_MAX, if the multibyte string contains redundant shift sequences.
A different case is when s is not NULL but pwc is NULL. In this case, the mbtowc() function behaves as above, except 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 mbtowc() function resets the shift state, only known to this function, to the initial state, and returns nonzero if the encoding has nontrivial shift state, or zero if the encoding is stateless.
|mbtowc()||Thread safety||MT-Unsafe race|
This function is not multithread safe. The function mbrtowc(3) provides a better interface to the same functionality.