[C] Helper function to call arbitrary x86 Delphi functions

[zeffy]

A project I was working on a while back required me to hook several functions in an application written in Delphi. I was writing my code in C, so I needed to figure out some way to inter-operate with the main application's code. I ended up deciding to write a normal __cdecl function that translated its parameters to Borland Register (aka Delphi "fastcall", different from Microsoft's fastcall, read more here).

This is what I came up with. The helper function takes a pointer to a delphi function, the number of arguments, and then the arguments you want to pass to the delphi function. The result of the invoked function is returned as a void *, but can be easily cast to whatever the return type actually is.

void *Delphi_InvokeMethod(void *pfn, size_t param_count, ...)
{
        va_list ap;
        void *tmp;
        void *registers[3];
        void *result;

        va_start(ap, param_count);
        for ( size_t i = 0; i < param_count; i++ ) {
                tmp = va_arg(ap, void *);

                if ( i < _countof(registers) )
                        registers[i] = tmp;
                else
                        __asm push tmp
        }
        va_end(ap);
        __asm {
                mov eax, registers[0]
                mov edx, registers[type registers * 1]
                mov ecx, registers[type registers * 2]
                call pfn
                mov result, eax
        }
        return result;
}

Keep in mind this is only for x86. As far as I know x64 delphi applications use the universal Microsoft x64 calling convention (someone feel free to correct me here, I haven't had the opportunity to reverse a x64 delphi application yet, so I could be wrong).

It might not be perfect, but it worked for what I needed it for. Feel free to suggest ways to improve it or any other kinds of criticism.

I hope somebody finds this useful, cheers!