Mixing C & C++ in codeblocks linker error

Question

I have a C header as follows:

Utilities.h:

extern SHM shm; //a struct.

extern void GetDesktopResolution(int* width, int* height);

extern bool CreateSharedMemory(uintptr_t id);

extern bool OpenSharedMemory(uintptr_t id);

extern bool UnMapSharedMemory();

and the implementation is a .c file that just implements the above functions:

SHM shm = {0};

bool CreateSharedMemory(uintptr_t id)
{
    //...
}

bool OpenSharedMemory(uintptr_t id)
{
    //...
}

bool UnMapSharedMemory()
{
    //...
}

This compiles perfectly fine.

Then I have a main.cpp file as follows:

#include "Utilities.h"


void swapBuffers(void* dsp, unsigned long wnd)
{
    if (!shm.mapped)
    {
        #if defined _WIN32 || defined _WIN64
        OpenSharedMemory((uintptr_t)dsp) || CreateSharedMemory((uintptr_t)dsp);
        #else
        OpenSharedMemory((uintptr_t)wnd) || CreateSharedMemory((uintptr_t)wnd);
        ftruncate(shm.hFile, shm.size);
        #endif // defined
    }
}
#endif // defined

When I compile it, I get:

obj\Release\src\main.o:main.cpp:(.text+0x249): undefined reference to `OpenSharedMemory(unsigned int)'

obj\Release\src\main.o:main.cpp:(.text+0x26c): undefined reference to `CreateSharedMemory(unsigned int)'

c:/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.8.1/../../../../x86_64-w64-mingw32/bin/ld.exe: obj\Release\src\main.o: bad reloc address 0x1 in section `.text.startup'

collect2.exe: error: ld returned 1 exit status

However, if I change "main.cpp" to "main.c" it compiles fine.. I checked and the .cpp file is compiled with g++ and the .c files are compiled with gcc but for some odd reason, the two object files cannot be linked together.

Any ideas what I'm doing wrong?

did you try: `extern "C" void GetDesktopResolution(int* width, int* height);` ? — Piotr Skotnicki, Sep 12 '14 at 19:27
I did. When I do that, it prints: `Utilities.h error: expected identifier or '(' before string constant` because it's a `C` header file. — Brandon, Sep 12 '14 at 19:29
so in main.cpp try: `extern "C" {\n #include "Utilities.h"\n}` — Piotr Skotnicki, Sep 12 '14 at 19:30
:o That worked! If you put that as an answer, I'll accept it.. — Brandon, Sep 12 '14 at 19:32
possible duplicate of [In C++ source, what is the effect of extern "C"?](http://stackoverflow.com/questions/1041866/in-c-source-what-is-the-effect-of-extern-c) — Niall, Sep 12 '14 at 19:42

Piotr Skotnicki · Accepted Answer · 2015-06-28T15:49:41.963

The Utilities.c file was/is probably compiled with C linkage, so the compiler does not perform name-mangling. As such, those functions are invisible for a C++ compiler.

In order to explicitly tell the compiler about a specific linkage of functions from given headers, wrap the include directive in *.cpp files with extern "C":

extern "C"
{
    #include "Utilities.h"
}

or create a separate header file (e.g. Utilities.hpp) with the following content:

extern "C" void GetDesktopResolution(int* width, int* height);

extern "C" bool CreateSharedMemory(uintptr_t id);

extern "C" bool OpenSharedMemory(uintptr_t id);

extern "C" bool UnMapSharedMemory();

Mixing C & C++ in codeblocks linker error

1 Answers1