Macro/globals question

**newlearner** · 07-18-2009

Hi,

I have a globals header that contains all my global variables and constants. The basic layout looks like this:

Code:

#ifndef _GLOBALS_H_
#define _GLOBALS_H_

int foo;

const int bar = 2;

#endif

Obviously I include this header in a lot of different files. The loader is complaining that there are multiple definitions of bar but does not complain about foo. The problem is obviously that I'm including globals.h in two different files, but if I add static in front of const int bar, doesn't that create separate global variables for the two different files? Also, why doesn't the loader complain about foo being defined multiple times? Thanks in advance.

EDIT: Fixed typo.

**Spidey** · 07-18-2009

#ifndef _GLOBALS_H_
#define _GLOBAL_H_

What this translates too is

If _GLOBALS_H is not defined
Define _GLOBAL_H_

So, any time the compiler reaches this statement, _GLOBALS_H will never be defined so it will go ahead and define _GLOBAL_H_

Inclusion guards always need to be of the same name.

**newlearner** · 07-18-2009

Sorry my mistake....typo in writing out code. Thanks for catching that.

**Spidey** · 07-18-2009

Sorry my mistake....typo in writing out code. Thanks for catching that.

Are you still getting an error ?

**Elysia** · 07-18-2009

Yes, the OP is.
Obviously, the same variable is created in each source file that includes the header, so it's natural for multiple definition errors.
What you need to do is place the globals in ONE .c file, then put "extern" in front of those variables in the headers.

**newlearner** · 07-18-2009

@Spidey, yes I meant I made the typo while typing the post. Sorry for not being clear.
@Elysia, excellent trick. I was looking through some source code of other projects and found a way to make your trick even better. I found this in the source code for mutt.

Code:

#ifdef MAIN
#define EXTERN
#else
#define EXTERN extern
#endif

Now define all your globals in globals.h as:

Code:

EXTERN int bar;

Then in main, define MAIN. This makes it slightly better because now all your global variables are defined in main, and everywhere else they are defined as extern. Now you don't need an extra source file to contain all your globals.

I still would like to know why it doesn't complain about foo when it complains about bar...

EDIT: added extra code for clarification

**Elysia** · 07-18-2009

Globals can be anywhere you want. In main, in a separate source file or in an existing source file, so long as it is defined only once.

**newlearner** · 07-18-2009

Yes, but why did the loader only complain about "bar" being defined multiple times and not "foo"?

**grumpy** · 07-18-2009

Originally Posted by newlearner

Yes, but why did the loader only complain about "bar" being defined multiple times and not "foo"?

Because your declaration of foo had no modifiers (extern, etc). It's definition is therefore local to each translation unit. In other words, each source file that #include's your header receives a variable named foo that is only visible to functions in that source file, and distinct from the variables named foo visible to other source files.

**Spidey** · 07-18-2009

Yes, but why did the loader only complain about "bar" being defined multiple times and not "foo"?

I would think that this would have something to do with the symbol-table the compiler generates for variables.

Anytime the compiler comes across a declaration which already exists in the table, it throws an error. However, there are a few cases where a symbol table entry is not added -
These would be typedefs, structs, classes templates ,function prototypes ,extern variables and a few more I cant remember.

You declared foo as an int however the default linkage for global variables in C is extern. So a symbol table entry was not created. However since you declared bar as a const, it was added to the symbol table until you explicitly declared it as extern.

Thread: Macro/globals question

Thread Tools

Search Thread

Display

Macro/globals question

Similar Threads

Alice....

Debugging question

Question about pointers #2

Question...

Question, question!