First Windows DLL

I'm using Open Watcom IDE. I have been following all the examples/tutorials I have found for creating a Windows DLL. I have very little experience of C programming, could anyone please give me a pointer to what I am doing wrong here?

I have a Hello.c file in my project as:-

Code:

#include <stdlib.h>
#include <stdio.h>
#include <windows.h>

extern "C"
{
    __declspec(dllexport) double GetNumberFromDLL()
    {
        return 50;
    }
}

When I compile I get lots of errors, the first few being:-

"Invalid Declarator" error on the extern
Expecting ';' but found 'C'
Expecting data or function declaration, but found 'string'

extern"C" is a C++ thing. You're trying to put it in a C file. The usual pattern is this:

Code:

#ifdef __cplusplus
extern "C" {
#endif

// whatever code...

#ifdef __cplusplus
} // extern "C"
#endif

Originally Posted by brewbuck

extern"C" is a C++ thing. You're trying to put it in a C file. The usual pattern is this:

Code:

#ifdef __cplusplus
extern "C" {
#endif

// whatever code...

#ifdef __cplusplus
} // extern "C"
#endif

Thanks for your very prompt reply.

I thought the extern was needed to stop mangling. It complies fine without the extern but then in my calling application I get error "Unable to find an entry point named 'GetNumberFromDLL'".

Originally Posted by g_hickley

Thanks for your very prompt reply.

I thought the extern was needed to stop mangling. It complies fine without the extern but then in my calling application I get error "Unable to find an entry point named 'GetNumberFromDLL'".

It is needed. I guess I wasn't clear enough in my post. The problem is that your file is *.c, not *.cpp, so it is being compile as C instead of C++. Rename it to .cpp

1. DLLs should always have a default entry point (e.g. C has Main(), Windows GUI have WinMain). DLLs have DllMain(). The following compiles in C.

Code:

BOOL APIENTRY DllMain (HINSTANCE hInst, DWORD reason, LPVOID reserved)
{
    return TRUE;
}

2. Try:

Code:

DLLIMPORT void HelloWorld(void)
{
    MessageBox (0, "Hello World from DLL!\n", "Hi", MB_ICONINFORMATION);
}

w/ the header files as:

Code:

#include <windows.h>

#ifndef _DLL_H_
#define _DLL_H_

#if BUILDING_DLL
# define DLLIMPORT __declspec (dllexport)
#else /* Not BUILDING_DLL */
# define DLLIMPORT __declspec (dllimport)
#endif /* Not BUILDING_DLL */

BOOL APIENTRY DllMain (HINSTANCE, DWORD, LPVOID);
DLLIMPORT void HelloWorld(void);

#endif /* _DLL_H_ */