#include <string.h>
size_t strlen(const char *s);

#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
size_t strnlen_s(const char *s, size_t n);

#include <wchar.h>
size_t wcslen(const wchar_t *s);

#define __STDC_WANT_LIB_EXT1__ 1
#include <wchar.h>
size_t wcsnlen_s(const wchar_t *s, size_t n);

These functions compute the length of a string of bytes or wide characters starting at s by finding the position of the first null terminator, and returning it. strnlen_s and wcsnlen_s only search the first n elements, and return n if no null character is found. Also, strnlen_s(NULL, anything) and wcsnlen_s(NULL, anything) return zero.

Note that strlen and strnlen_s count bytes, not multibyte characters.