Question

Can you decompile a c dll to use pinvoke on or use reflector?

How do I get the method names and signatures?

Answer 1

Simply put there is no trivial way to do what you want. You can use a disassembler library such as distorm to disassemble the code around the exported entry points, though. There are some heuristics one can use, but many of those will only work with 32bit calling conventions (__stdcall and __cdecl) in particular. Personally I find the Python bindings for it useful, but libdasm can do the same.

Any other tool with disassembler capabilities will be of great value, such as OllyDbg or Immunity Debugger.

Note: if you have a program that already calls the DLL in question, it is most of the time very worthwhile to run that under a debugger (of course only if the code can be trusted, but your question basically implies that) and set breakpoints at the exported functions. From that point on you can infer a lot more from the runtime behavior and the stack contents of the running target. However, this will still be tricky – particularly with __cdecl where a function may take an arbitrary amount of parameters. In such a case you’d have to sift through the calling program for xrefs to the respective function and infer from the stack cleanup following the call how many parameters/bytes it discards. Of course looking at the push instructions before the call will also have some value, though it requires a little experience especially when calls are nested and you have to discern which push belongs to which call.

Basically you will have to develop a minimal set of heuristics matching your case, unless you have already licensed one of the expensive tools (and know how to wield them) that come with their own heuristics that have usually been fine-tuned for a long time.

If you happen to own an IDA Pro (or Hex-Rays plugin) license already you should use that, of course. Also, the freeware versions of IDA, although lagging behind, can handle 32bit x86 PE files (which includes DLLs, of course), but the license may be an obstacle here depending on the project you’re working on (“no commercial use allowed”).