views:

114

answers:

4

How can I create global variables that are shared in C? If I put it in a header file, then the linker complains that the variables are already defined. Is the only way to declare the variable in one of my C files and to manually put in externs at the top of all the other C files that want to use it? That sounds not ideal.

+10  A: 

in the header file write it with extern. And in one of the c files declare it without extern.

dig
+6  A: 

In one header file (shared.h):

extern int this_is_global;

In every file that you want to use this global symbol, include header containing the extern declaration:

#include "shared.h"

To avoid multiple linker definitions, just one declaration of your global symbol must be present across your compilation units (e.g: shared.cpp) :

/* shared.cpp */
#include "shared.h"
int this_is_global;
Hernán
+2  A: 

You put the declaration in a header file, e.g.

 extern int my_global;

In one of your .c files you define it at global scope.

int my_global;

Every .c file that wants access to my_global includes the header file with the extern in.

nos
+1  A: 

In the header file

header file

#ifndef SHAREFILE_INCLUDED
#define SHAREFILE_INCLUDED
#ifdef  MAIN_FILE
int global;
#else
extern int global;
#endif
#endif

In the file with the file you want the global to live:

#define MAIN_FILE
#include "share.h"

In the other files that need the extern version:

#include "share.h"
jim mcnamara
Good approach using the preprocessor.
Hernán
ah this is the solution i had a while ago - i forgot about the MAIN_FILE preprocessor variable. i thinik i like the cur accepted answer more tho
Claudiu