Thread: [Dev-C++] Macro problem

Hi,
Please help me with the following code. We have three files: a header fil=
e =20
and two
implementation files.

// Source of "header.h"
#ifndef HEADER_H
#define HEADER_H

int p;

#endif
--------------------------------
// Source of "main.cpp"
#include <cstdio>
#include "header.h"

int main()
{
     printf("%i\n",p);
     return 0;
}
--------------------------------
// Soruce of "func.cpp"
#include "header.h"
int func()
{
     return p;
}

The compiler generates the error " multiple definition of 'p' ".
What is wrong with the code?
When the header file is included for the first time, the macro "HEADER_H"=
 =20
is defined
but when the header file is included in the second implementation file,
"HEADER_H" can not be recognized in the second file and "p" is defined =20
once more!!
It seems macros have file scope.

Thanks,


--=20
Using Opera's revolutionary e-mail client: http://www.opera.com/mail/

The #ifndef HEADER_H... construct only protects from inclusion of a
specific header file once from the SAME C-filer. Not from inclusion by
multiple C files.

In the header file, you should normally write

extern int p;

and then in one of the C files write

int p;

Or you can modify the files to look like:

--- header.h ---
#ifndef HEADER_H
#define HEADER_H

#if defined IMPLEMENTATION
#define EXTERN
#else
#define EXTERN extern
#endif

EXTERN int p;

#endif
---------

--- main.c ---
#define IMPLEMENTATION
#include "header.h"

int main() {
   ...
}
----------

--- other.c ---
#include "header.h"
...
------------

/Per W


On Mon, 30 May 2005, Farzan Hajian wrote:

> Hi,
> Please help me with the following code. We have three files: a header file
> and two
> implementation files.
>
> // Source of "header.h"
> #ifndef HEADER_H
> #define HEADER_H
>
> int p;
>
> #endif
> --------------------------------
> // Source of "main.cpp"
> #include <cstdio>
> #include "header.h"
>
> int main()
> {
>      printf("%i\n",p);
>      return 0;
> }
> --------------------------------
> // Soruce of "func.cpp"
> #include "header.h"
> int func()
> {
>      return p;
> }
>
> The compiler generates the error " multiple definition of 'p' ".
> What is wrong with the code?
> When the header file is included for the first time, the macro "HEADER_H"
> is defined
> but when the header file is included in the second implementation file,
> "HEADER_H" can not be recognized in the second file and "p" is define