Thread: DLL Interface Oddity

For a few days now i've been investigating how to best send data structures (specifically STL vectors) from a c++ DLL to some indie gamedev software based on Delphi (which i dont have internal access to). I've narrowed it down to two options, either i simply call the DLL multiple times to recieve one number at a time and incur some nasty overhead, or I send an appropiately formatted string (convert 16bit ints to 2x 8bit chars) containing the vector.

The problem is that when it tries to send a number below 256, the string length is apparently only 1 instead of being 2 with a zero character. At first i thought it was the signed 16bit number messing with the bitwise operators.... but beyond that i've really got no idea. Is it possible that im inserting a null character as opposed to a zero? How would i stop this?

As i test i wrote the DLL below:

Code:

#include <windows.h>
#include <string>
#define export extern "C" __declspec(dllexport) __stdcall

using namespace std;

string str = "";

export const char* send_ds(){
	str = "";
	unsigned __int16 j0 = 1;
	__int8 j1 = ((j0<<8)>>8);
	__int8 j2 = (j0>>8);
	str += char(j1);
	str += char(j2);
	return (str.c_str());
	}
		
BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved){
    switch (fdwReason){
        case DLL_PROCESS_ATTACH: {break;}
        case DLL_PROCESS_DETACH: {break;}
        case DLL_THREAD_ATTACH: {break;}
        case DLL_THREAD_DETACH: {break;}
    	}
    return TRUE;
    }

PS: Sorry for the sloppy coding format, i've never had any formal training, so i kinda just made it up as i went along. Also, Im using Dev-C++ and Mingw if thats of any importance....

>Is it possible that im inserting a null character as opposed to a zero?
That's what it looks like is happening. One idea is to convert the number to a string first:

Code:

#include <sstream>
.
.
	int j0 = 1;
	std::ostringstream converter;

	converter << j0;

	return converter.str().c_str();

Are you expecting a string at the other end? It sounds like that's the case, becasue otherwise the null character wouldn't even be a problem.

So based on the assumption you are looking for a string at the other end, converting the number to a string makes sense. But I'm likely missing something. But you can convert any number the fits in an int (or bigger if you use a larger integer type). You could even send a group of numbers separated by spaces or commas:

Code:

int a = 111111;
int b = 222222;
converter << a << " " << b;

If you're just expecting an array of numbers at the other end (not a string), then the null character won't be a problem. As you said it's the same as sending the number zero. So at the other end, it's interpreted as the number 0. The issue comes into play with strings.

Here's another suggestion: send a second parameter which contains the length of the data:

Code:

export const char* send_ds(int *length){
	str = "";
	unsigned __int16 j0 = 1;
	__int8 j1 = ((j0<<8)>>8);
	__int8 j2 = (j0>>8);
	str += char(j1);
	str += char(j2);
	*length = str.length();
	return (str.c_str());
	}

when i pass the string it regards it as a string instead of a char array, and wont let me access the numbers after the null.

That being the case, I guess passing the length as another parameter isn't going to help. But it looks like you've found something which will work. Perhaps you can find a method to pass an int array to Delphi. Something like:

Code:

export int* send_ds(){

Or:

Code:

export send_ds(int array[]){