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)
    }

The conventional way is to put declarations in a header file and definitions in a source file:

foo.h:

Code:

#ifndef FOO_H
#define FOO_H

// declarations

extern int myvariable;
int foo(int x);

#endif

foo.c:

Code:

#include "foo.h"

// definitions

int myvariable;

int foo(int x)
{
    return x+5;
}

main.c:

Code:

#include <stdio.h>
#include "foo.h"

int main(void)
{
    int y  =  foo(3);
    printf("%d\n",y)
}

"extern" means "this object is declared here but defined elsewhere". It's usually used to declare variables in a header file. It's optional (that is, it's not needed) for function declarations.