My very basic knowledge of C and compilation process has gone rusty lately. I was trying to figure out answer to the following question but I could not connect compilation, link and pre-processing phase basics. A quick search on the Google did not help much either. So, I decided to come to the ultimate source of knowledge :)
I know: Variables should not be defined in the .h files. Its ok to declare them there.
Why: Because a header file might get included from multiple places, thus redefining the variable more than one time (Linker gives the error).
Possible work-around: Use header-guards in header files and define variable in that.
Is it really a solution: No. Because header-guards are for preprocessing phase. That is to tell compiler that this part has been already included and do not include it once again. But our multiple definition error comes in the linker part - much after the compilation.
This whole thing has got me confused about how preprocessing & linking work. I thought that preprocessing will just not include the code, if the header guard symbol has been defined. In that case, shouldn't multiple definition of a variable problem also get solved?
What happens that these preprocessing directives save the compilation process from redefining symbols under header guards, but the linker still gets multiple definitions of the symbol?
Header guard protects you from multiple inclusions in a single source file, not from multiple source files. I guess your problem stems from not understanding this concept.
It is not that pre-processor guards are saving during the compile time from this problem. Actually during compile time, one only source file gets compiled into an obj, symbol definitions are not resolved. But, in case of linking when the linker tries to resolve the symbol definitons, it gets confused seeing more than one definition casuing it to flag the error.
One thing that I've used in the past (when global variables were in vogue):
var.h file:
...
#ifdef DEFINE_GLOBALS
#define EXTERN
#else
#define EXTERN extern
#endif
EXTERN int global1;
EXTERN int global2;
...
Then in one .c file (usually the one containing main()):
#define DEFINE_GLOBALS
#include "var.h"
The rest of the source files just include "var.h" normally.
Notice that DEFINE_GLOBALS is not a header guard, but rather allows declaring/defining the variables depending on whether it is defined. This technique allows one copy of the declarations/definitions.
You have two .c files. They get compiled separately. Each one includes your header file. Once. Each one gets a definition. They conflict at link time.
The conventional solution is:
#ifdef DEFINE_SOMETHING
int something = 0;
#endif
Then you #define DEFINE_SOMETHING in only one .c file.
Header guards stop a header file being included multiple times in the same translation unit (i.e. in the same .c source file). They have no effect if you include the file in two or more translation units.
来源：https://stackoverflow.com/questions/2216765/variable-definition-in-header-files