strlen

strlen, strnlen_s

Defined in header <string.h>
size_t strlen( const char *str );
(1)
size_t strnlen_s( const char *str, size_t strsz );
(2) (since C11)
1) Returns the length of the given null-terminated byte string, that is, the number of characters in a character array whose first element is pointed to by str up to and not including the first null character.
The behavior is undefined if str is not a pointer to a null-terminated byte string.