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I am having a linker error in a file of some 3000 lines that I cannot seem to reproduce. I was wondering if anybody had any ideas where the problem could be. Basically I have a function in a header file and whenever I go to link my project it says:

file:               line:   Message:
main.o                      In function 'myFunction@16':
main.h              123     multiple definition of 'myFunction@16'
main.h              123     first defined here

To me this seems iffy at best! What is going on?!

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Last Post by Labdabeta
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Without code to look at, my best guess is that main.h doesn't have an include guard and is somehow getting included more than once.

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What gusano79 said. A header guard is something like this:

#ifndef MAIN_H
#define MAIN_H
// Contents of file below.
.
.
#endif /* MAIN_H */

This will keep the header contents from being processed more than once, even if included in more that one other header file that you are using.

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I figured it out, but now I cannot find a solution. The problem was that they were global in the header file for a DLL, the main.cpp for compiling the DLL uses extern TYPE VARIABLE_NAME; throughout, whereas when any other main.cpp uses the header file it uses TYPE VARIABLE_NAME. The thing is that I absolutely need globals to be used. Is there any way to use globals in a DLL? EG:

myDLL.h:

#ifndef MYDLL_H
#define MYDLL_H
#ifdef BUILD_DLL
#define DLL_EXPORT __declspec(dllexport) __stdcall
#else
#define DLL_EXPORT __declspec(dllimport) __stdcall
#endif
int globalVar;
DLL_EXPORT void init();
DLL_EXPORT void myFunction();
#endif

myDLL.cpp:

#include "myDLL.h"
DLL_EXPORT void init()
{
    globalVar=0;
}
DLL_EXPORT void myFunction()
{
    globalVar++;
}

main.cpp:

#include "myDLL.h"
int main()
{
    init();
    myFunction();
    return globalVar;
}
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Shouldn't this...

#define DLL_EXPORT __declspec(dllexport) __stdcall
#else
#define DLL_EXPORT __declspec(dllimport) __stdcall
#endif

...really be this...

#define DLL_EXPORT __declspec(dllexport) __stdcall
#else
#define DLL_IMPORT __declspec(dllimport) __stdcall
#endif

...?

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No it shouldnt, otherwise i would have to compile with DLL_EXPORT and then change it all to DLL_IMPORT when I want to use it. DLL_EXPORT and DLL_IMPORT are merely macros. Anyways, I am using a pseudo-solution to my problem:

myDLL.h:
#ifndef MYDLL_H#define MYDLL_H#ifdef BUILD_DLL#define DLL_EXPORT __declspec(dllexport) __stdcall#else#define DLL_EXPORT __declspec(dllimport) __stdcall#endifint globalVar;DLL_EXPORT void init();DLL_EXPORT void myFunction();#endif

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Wow... that was not supposed to happen, I just hit ENTER and it saved. Oh well, this is what I meant to post:

myDLL.h:

#ifndef MYDLL_H
#define MYDLL_H

#ifdef BUILD_DLL
    #define DLL_EXPORT __declspec(dllexport) __stdcall
#else
    #define DLL_EXPORT __declspec(dllimport) __stdcall
#endif
struct GLOBAL_STRUCTURE
{
    int globalInt;
};
#ifndef THIS_IS_MAIN_CPP
    GLOBAL_STRUCTURE globals;
#endif
DLL_EXPORT void initG(GLOBAL_STRUCTURE *gs);
void init(){initG(&globals);}
DLL_EXPORT void myFunctionG(GLOBAL_STRUCTURE *gs);
void myFunction(){myFunctionG(&globals);}
DLL_EXPORT int *getIntG(GLOBAL_STRUCTURE *gs);
void getInt(){return getIntG(&globals);}
#endif

myDLL.cpp

#define THIS_IS_MAIN_CPP
#include "myDLL.h"
DLL_EXPORT void initG(GLOBAL_STRUCTURE *gs)
{
    gs->globalInt=0;
}
DLL_EXPORT void myFunctionG(GLOBAL_STRUCTURE *gs)
{
    gs->globalInt++;
}
DLL_EXPORT void getIntG(GLOBAL_STRUCTURE *gs)
{
    return &(gs->globalInt);
}

My question is whether or not this will work, and if it does work is it safe? Also how c compatible is it?

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I was just looking around the web and it looked like this would be a valid solution... is it?

DLL_EXPORT GLOBAL_STRUCTURE globals; //in the above code

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