c written language, the two documents, how to call main function subroutine

Write a header file named MAX.H
#ifndef max_h
#define max_h
void max (void);
#endif

Then the main document file header function contains MAX.H
#include <stdio.h>
#include "MAX.H"
int main (void) {
max ();
return 0;
}

Functions file also contains MAX.H
#include <stdio.h>
#include "MAX.H"
void max (void) {
the printf ( "Hello world \ n-!");
}