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