strlen not checking for NULL

Question

Why is strlen() not checking for NULL?

if I do strlen(NULL), the program segmentation faults.

Trying to understand the rationale be

Answer 1

size_t strlen ( const char * str );

http://www.cplusplus.com/reference/clibrary/cstring/strlen/

Strlen takes a pointer to a character array as a parameter, null is not a valid argument to this function.