C Pass arguments as void-pointer-list to imported function from LoadLibrary()

Question

The problem I have is that I want to create a generic command line application that can be used to load a library DLL and then call a function in the library DLL. The function name is specified on the command line with the arguments also provided on the utility command line.

I can access the external function from a DLL dynamically loaded using the LoadLibrary() function. Once the library is loaded I can obtain a pointer to the function using GetProcAddress() I want to call the function with the arguments specified on the command line.

Can I pass a void-pointer-list to the function-pointer which I got returned by the LoadLibrary() function similar to the example below?

To keep the example code simple, I deleted the error-checking. Is there a way to get something like this working:

    //Somewhere in another dll
    int DoStuff(int a, int b)
    {
        return a + b;
    }

    int main(int argc, char **argv)
    {
        void *retval;
        void *list = argv[3];
        HMODULE dll;
        void* (*generic_function)(void*);

        dll = LoadLibraryA(argv[1]);

        //argv[2] = "DoStuff"
        generic_function = GetProcAddress(dll, argv[2]);

        //argv[3] = 4, argv[4] = 7, argv[5] = NULL
        retval = generic_function(list);
    }

If I forgot to mention necessary information, please let me know. Thanks in advance

Calling convention could eventually give you grief. The actual function expectations how you're calling it are an obvious assumption. And "... with passed arguments.", noting the plurality of that statement, is interesting. You're passing one. If you need more than that (i.e. you're expecting this to magically send argv[3]...argv[argc-1] as function arguments), this won't do it, and doing it right gets complicated quickly. — WhozCraig
It is not clear what you mean by if I could pass a void-pointer-list to the function-pointer. If the function you are calling is defined as ... MyFunction(void *), then yes, you can call it like this, otherwise you would not be able to. Also, make sure it is labeled as declspec(stdcall). — seva titov
I just looked for a way to get a command-line tool to call e.g. kernel32.dll and any of its function and pass arguments to it. I was hoping - as WhozCraig said - for a magic way to find the argument list from an imported function — h4x0r
@WhozCraig thanks for your comment. "you're expecting this to magically send argv[3]...argv[argc-1] as function arguments), this won't do it" is exactly what I was looking for (or at least a workaround for this). — h4x0r

Chris Dodd Chris Dodd · Accepted Answer · 2015-03-12T00:35:57

You need to cast the function pointer returned by LoadLibrary to one with the right argument types before calling it. One way to manage it is to have a number call-adaptor functions that do the right thing for every possible function type you might want to call:

void Call_II(void (*fn_)(), char **args) {
    void (*fn)(int, int) = (void (*)(int, int))fn_;
    fn(atoi(args[0]), atoi(args[1]));
}
void Call_IS(void (*fn_)(), char **args) {
    void (*fn)(int, char *) = (void (*)(int, char *))fn_;
    fn(atoi(args[0]), args[1]);
}
...various more functions

Then you take the pointer you got from GetProcAddress and the additional arguments and pass them to the correct Call_X function:

void* (*generic_function)();

dll = LoadLibraryA(argv[1]);

//argv[2] = "DoStuff"
generic_function = GetProcAddress(dll, argv[2]);

//argv[3] = 4, argv[4] = 7, argv[5] = NULL

Call_II(generic_function, &argv[3]);

The problem is that you need to know what the type of the function you're getting the pointer for is and call the appropriate adaptor function. Which generally means making a table of function name/adaptors and doing a lookup in it.

The related problem is that there's no function analogous to GetProcAddress that will tell you the argument types for a function in the library -- that information simply isn't stored anywhere accessable in the dll.

C Pass arguments as void-pointer-list to imported function from LoadLibrary()

2 Answers