Thread: Header files question

In response to one of my Cboard queries I was told by a responding OP that my header file treatment was wrong. The way I did it, was conform an Internet tutorial. This is how (re below). Please tell me what is wrong and also how it should be done. I understand that the "externs" in the example signify that no memory is to be allotted. I was also told that no function definitions should be in a header file. Where then should they be defined?

Code:

//in foo.h

    #ifndef temp
    #define temp
         extern int foo(int x)
    #end if
    extern int foo(int x)
    {
               return x+5;
    }

//in main.c
    #include "foo.h"
    int main(void)
    {
          int y  =  foo(3);
          printf("%d\n",y)
    }