Why strlen function works without #include<string.h>?

Question

Quick question:

strlen[char*] works perfectly regardless whether I #include <string.h> or not

All I get from compiler is a warning about implicit declaration, but functionally it works as intended.

Why is that?

Answer 1

When you invoke undefined behavior, one possible behavior is that the program behaves as you expected it to, one your system, and with the current version of libraries and system software you have installed. This does not mean it's okay to do this. Actually a correct C99 compiler should not allow implicit function declarations; it should give you an error.

Answer 2

The function prototypes in C are not compulsory. They're useful indications to the compiler so that it can do type checking on types which are passed into them. When you don't include string.h, a default signature is assumed for the function which is why you get the warning.