shared global variables in C

Question

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.

Solution 1

In the header file write it with extern. And at the global scope of one of the c files declare it without extern.

Solution 2

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;

Solution 3

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"

Solution 4

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.

Solution 5

If you're sharing code between C and C++, remember to add the following to the shared.hfile:

#ifdef __cplusplus
extern "C" {
#endif

extern int my_global;
/* other extern declarations ... */

#ifdef __cplusplus
}
#endif

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Comments

[Claudiu]

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

[NickO]

do you have any preferred references to learn more about IPC mechanisms?

[AntonioCS]

Please put more emphasis on "just one declaration of your global symbol..." Kind of tripped on that one. I had the declaration on all the c files I wanted to use the global variables on :(

[Geremia]

Can we declare it in that *.c file's header file instead?

[GDavid]

You can also add #ifndef common #define common extern #endif to the header if you want to avoid defining common every time globals.h is included. The only caveat is that common is not undefined automatically.