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Linkage problems using visual c++ tool

Posted on 2004-04-05
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Last Modified: 2012-05-04
I was trying to connect 2 projects - one in C and the other in C++ to one C++ project.
i converted all the extensions of the C files into cpp ones.
Then , when i tryied to linkage all the project together i got more then 900 errors in the form of

A.obj : error LNK2005 : "unsigned char VAR" (?VAR@3PAPAEA) already defined in B.obj

all the A files that gives the linkage errors are the c files converted into cpp ones.
all the VAR variables that gives the linkage errors are in the file global.h which was belong to the former C project.
i suspected that this is an "IFNDEF" problem in the global.h problems but i don't think this is the case.

any advice of how to solve this errors will be highly appreciated since this question concerned to my final project which i need to finish very soon so thanks in advance
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Question by:haboles
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6 Comments
 
LVL 48

Accepted Solution

by:
AlexFM earned 500 total points
ID: 10756192
Global variables should be defined by the following way:

// global.h
#ifnfef GLOBAL_H
#define GLOBAL_H

extern int VAR;

// other global variables, all with extern keyword

#endif

// cpp file (define each global variable only in one cpp file, use it in all cpp files where necessary)
#include "Global.h"

int VAR;


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LVL 48

Expert Comment

by:AlexFM
ID: 10756207
Global.h file contains all global variables with extern keyword. This doesn't create variable, only defines it's type. To create global variable, it should be written without extern keyword:

extern int VAR;    // like forward declaration

int VAR;    // variable is created

Create variable in only one cpp file. Use it in any cpp file - include "Global.h" to it and compiler is happy.


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LVL 17

Expert Comment

by:rstaveley
ID: 10756642
For the benefit of the C file (assuming you are compiling it as C rather than C++), you should use the following to ensure that the symbol doesn't get C++ name mangling:

header.h
--------8<--------
extern
#ifdef __cplusplus
          "C"
#endif
                int VAR;
--------8<--------

source.c/.cpp
--------8<--------
#ifdef __cplusplus
extern "C"
#endif
                int VAR = 0;
--------8<--------
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LVL 39

Expert Comment

by:itsmeandnobodyelse
ID: 10758019
>> i converted all the extensions of the C files into cpp ones.

Because of this all sources are C++ sources now.

Name mangling is an issue of functions only.

Regards, Alex

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LVL 17

Expert Comment

by:rstaveley
ID: 10758180
> Name mangling is an issue of functions only.

Stone me you're right... and structs, classes and static data members in classes I now see. Thanks for the correction, Alex.
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Author Comment

by:haboles
ID: 10758453
thanks all for the quick answers.
good to know that i have place to go now when i have c/c++ question.
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