Question

To allow access to the Win32 API from a scripting language (written in C), I would like to write a function such as the following:

void Call(LPCSTR DllName, LPCSTR FunctionName,    LPSTR ReturnValue, USHORT ArgumentCount, LPSTR Arguments[]) 

which will call, generically, any Win32 API function.

(the LPSTR parameters are essentially being used as byte arrays – assume that they have been correctly sized to take the correct data type external to the function. Also I believe that some additional complexity is required to distinguish between pointer and non-pointer arguments but I’m ignoring that for the purposes of this question).

The problem I have is passing the arguments into the Win32 API functions. Because these are stdcall I can’t use varargs so the implementation of ‘Call’ must know about the number of arguments in advance and hence it cannot be generic…

I think I can do this with assembly code (by looping over the arguments, pushing each to the stack) but is this possible in pure C?

Update: I’ve marked the ‘No it is not possible’ answer as accepted for now. I will of course change this if a C-based solution comes to light.

Update: ruby/dl looks like it may be implemented using a suitable mechanism. Any details on this would be appreciated.

Answer 1

No, I don’t think its possible to do with without writing some assembly. The reason is you need precise control over what is on the stack before you call the target function, and there’s no real way to do that in pure C. It is, of course, simple to do in Assembly though.

Also, you’re using PCSTR for all of these arguments, which is really just const char *. But since all of these args aren’t strings, what you actually want to use for return value and for Arguments[] is void * or LPVOID. This is the type you should use when you don’t know the true type of the arguments, rather than casting them to char *.