C预处理程序防护？

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?

Answer 1

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)

Answer 2

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!

Answer 3

#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

Answer 4

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.