C Preprocessor Guards?

Question

I am working on a C project, and am trying to use pre-processor guards as can be used in C++:

#ifndef CONFIG_H
#define CONFIG_H

... exciting stuff in C ....

#endif

Including this in my source appears to have no effect in Visual Studio, as when I include a given file, such as Config.h, in multiple files, the compiler gives me the following errors:

1>main.obj : error LNK2005: _OPCodes already defined in lib.obj
1>main.obj : error LNK2005: _OPTotal already defined in lib.obj
1>main.obj : error LNK2005: _RegCodes already defined in lib.obj
1>main.obj : error LNK2005: _RegTotal already defined in lib.obj
1>main.obj : error LNK2005: _UDSTotal already defined in lib.obj

Could anyone give me any pointers (no pun intended) on this, please?

@maditya Sorry, it's 3 in the morning, it was a typo in my post. Fixed now and still not working. — Oliver Spryn, Apr 10 '13 at 07:09
When you say "including this in my source," you mean you put the include guard in the header, not in the `.cpp` files, right? In any case, try putting `#pragma once` in there as well, just to see what happens. — Kyle Strand, Apr 10 '13 at 07:23

score 9 · Accepted Answer · edited May 23 '17 at 12:23

9

The guards will prevent defining things twice in a compilation unit. They will not prevent defining the same thing in different compilation units. And the linker messages indicates that it is what occurs _OPCodes for instance is defined in lib and in main.

Usually, a header should have only declarations for functions and global variables, corresponding definitions would be provided in one of the source files.

(See for instance What is the difference between a definition and a declaration? for more information)

edited May 23 '17 at 12:23

Community

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answered Apr 10 '13 at 07:14

AProgrammer

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Thank you that was the answer to my problem. I was (stupidly) defining stuff in my `.h` file. Move them to a `.c`, and my problems disappeared. – Oliver Spryn Apr 10 '13 at 07:32

Kyle Strand · Answer 2 · 2013-04-10T07:15:42.090

4

EDIT: this is based on the original post, which had a typo. It's NOT the OP's real problem, apparently.

You've given your guards two different names. They must match.

#ifndef CONFIG_H
#define CONFIG_H  // not CONFIG_G!

edited Apr 10 '13 at 07:15

answered Apr 10 '13 at 06:55

Kyle Strand

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Sorry, it's 3 in the morning, it was a typo in my post. Fixed now and still not working. – Oliver Spryn Apr 10 '13 at 07:08
Try adding `#pragma once` just for kicks to see if that fixes it. If not, then your problem is not in this header file. If so, you might have your `#endif` in the wrong place or some such thing. In any case, adding `#pragma once` can't hurt anything, and it miiiiiight speed up compilation just a tiiiiiiiiny bit. (Probably not, though.) – Kyle Strand Apr 10 '13 at 07:13

score 2 · Answer 3 · answered Apr 10 '13 at 06:59

2

#ifndef CONFIG_H
#define CONFIG_G

... exciting stuff in C ....

#endif

its a typo because of that you are getting 'already defined error'

in your header file you are defining CONFIG_G instead of CONFIG_H , so from the next source file the #ifndef CONFIG_H is true so it is again including the same contents

answered Apr 10 '13 at 06:59

user2247801

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Sorry, it's 3 in the morning, it was a typo in my post. Fixed now and still not working. – Oliver Spryn Apr 10 '13 at 07:09

score 0 · Answer 4 · answered Apr 10 '13 at 07:24

If you have renamed any files you should remove them from the solution and re-add them. Sometimes Visual Studio is a bit weird with this and they will cause linker errors when left in.

Make sure to do a rebuild.

You may also have accidentally included a .cpp file instead of a .h file. Double check all the includes just in case.

C Preprocessor Guards?

4 Answers4