On Wed, Jul 9, 2008 at 9:56 PM, Bradley Hanna <brahan.ml@xxxxxxxxx> wrote:
> I am trying to define a variable that needs to be manipulated by one c
> file in the kernel and read by another c file in a different directory.
> I have defined the variable in a header file that is #included in both c
> files.
A variable can be DECLARED multiple times but only DEFINED once.
Define means give it real memory space, and declare means like saying
the compiler "someone else allocated this variable, and I just want to
use it". Get the point?
So you should define it on a single module (int var), and put its
declaration on a header file, like
extern int var;
Whenever you link a module with the one that defines the variable, you
should #include the header file that declares it. For example:
var.c:
---8<------
int var; /* variable defined: compiler actually allocates space */
---8-<-----
var.h:
---8<------
#ifndef VAR_H /* Rembember, symbols beginning by "_" are reserved by
the compiler */
#define VAR_H
extern int var; /* variable declared. Just state its type and make it
"available" */
#endif
---8<-----
use.c
---8<-----
#include "var.h"
int main(void) { /* you are compiling -ansi -pedantic, aren't you? */
var++;
return 0;
}
---8<-----
Makefile:
---8<-----
use: use.c var.c var.h
gcc -ansi -pedantic -Wall -o use use.c var.c
---8<-----
Cheers
--
Alberto Giménez
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