identifying function prototype from dll

Question

I have an exe file and a dll file. This exe file uses the dll to decode input file. This exe file is gui based and does not support command line execution.

So my requirement is to create a program which loads this dll and use the funciton to decode an input file so that I can use this program in some scripts.

The exe expects two inputs. Source file name and destination file name.

When I disassembled the exe, only 1 parameter is being passed to the dll function call, which is the source file name. I couldn't find how the destination file name is passed to the function.

Disassembly of dll function shows 2 exported functions.

assembly code for dll function from IDA is as follows

While calling the dll function from exe, the stack is as follows

From this I understood that the a pointer to the source file name is passed as the argument. The pointer points to the following memory location.

From this much information, is it possible to identify the function prototype of the dll function.

Answer 1

It’s hard to say for sure without full binary or all called functions but going by the debugger screens it looks like the argument is pointer to a structure with layout similar to the following:

struct PARAMS {
  char *inputfname;
  int flag1;
  char *outfname;
  int flag2;
};

There maybe other fields not obvious from the posted info, but you could try to start with this.