CODEWITHSUNDEEP
c++cvisual-c++functionpointers
ash
ash

Reputation: 1200

Making a C function pointer work with C-style stack based calling mechanics in C++

I want to call a pure C style function from a dll in my C++ program. I tried casting my function pointer using reinterpret_cast to __cdecl and still the calling convention of _stdcall seems to be preserved. I am new to Windows C++ programming.

Edit Code from comment 

reinterpret_cast< Error ( __cdecl*)(int,int)> (GetProcAddress(Mydll::GetInstance()->ReturnDLLInstance(), "add"))(1,10)

is my call. The actual function syntax seems to have been declared as 

Error __cdecl add(int,int);

Debugger throws me the error run time check failure #0. I am working in Windows-C++

Upvotes: 3

Views: 612

Answers (4)

ash
ash

Reputation: 1200

This helped me out!

http://www.codeguru.com/forum/archive/index.php/t-70673.html

Upvotes: 0

doron
doron

Reputation: 28902

There are two things at work over here.

The first is the calling convention. The calling convention is a part of the Application Binary Interface (ABI) that decides whether the caller or the callee is responsible for cleaning up the stack. If you want your functions to behave correctly both you harness and your dll will need to use the same calling convention. In WIN32 APIs this is typically __stdcall although C typically uses __cdecl.

The other issue is name mangling. Since the arguments of a function form part of the function signature in C++ (to allow for function overloading) this information is incorporated into the symbol table of you object code. This will typically be a whole bunch of extra strange characters. C does not need to do name mangling since it does not allow function overloading.

Sometimes in C++ you want to call C functions (ie C function symbols compiled by a C and not a C++ compiler). In such a case you need to define the function in an extern "C" {} block.

Hopefully this will help you out

Upvotes: 3

Sergio Martinez
Sergio Martinez

Reputation: 239

Usually you need to use extern "C" for this...

--- c_code.h ---

void func(int arg);
void (*func_ptr)(int arg);

--- cpp_code.cpp ---

extern "C" void func(int arg);
extern "C" void (*func_ptr)(int arg);

int main()
{
    func(20);
    *func_ptr(20);
}

Upvotes: 6

mattst88
mattst88

Reputation: 1502

I believe the solution to your question is 'extern "C" { ...'

See http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html#faq-32.3

Upvotes: 8

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