#include <stdlib.h> int mblen(const char *s, size_t n);
If s is not a null pointer, mblen() determines the number of bytes constituting the character pointed to by s. It is equivalent to:
mbtowc((wchar_t *)0, s, n);
A call with s as a null pointer causes this function to return 0. The behavior of this function is affected by the LC_CTYPE category of the current locale.
If s is a null pointer, mblen() returns 0. It s is not a null pointer, mblen() returns 0 (if s points to the null byte), the number of bytes that constitute the character (if the next n or fewer bytes form a valid character), or −1 (if they do not form a valid character) and may set errno to indicate the error. In no case will the value returned be greater than n or the value of the MB_CUR_MAX macro.
The mblen() function may fail if:
Invalid character sequence is detected.
The mblen() function can be used safely in multithreaded applications, as long as setlocale(3C) is not being called to change the locale.
See attributes(5) for descriptions of the following attributes: