How to pass string from autoit to my own c++ dll and back

[emcodem]

Dears, 

i am trying to come up with my own c/++ dll to connect autoit with mongodb or similar (without ADO). The goal is to exchange "unmodified" strings between autoit and my dll, we want to migrate a very large autoit application from using the filesystem as a database to a real database (potentially hundreds of thousands files are written and listed/read...), not yet sure if it really pays off to go the dll approach is worth it. One very simple alternative would be to use some http rest api bridge to talk with the database but this adds a little overhead because of the http protocol. 

The result of my tests below is as you see in the .au3 comments, the "Add" function call works including parameters and return int but the "Echo" functions parameter wchar_t* is always 0.

 

Any clues be greatly appreciated!

Besides some old forum posts i did not really find any examples how to pass string or binary data unmodified between dlls and autoit...

 

test.au3:

#AutoIt3Wrapper_UseX64=Y
Global Const $g_sFileDll      = 'C:\dev\FileBridge++\FileBridge++\x64\Debug\FileBridge.dll'
$hdll = DllOpen($g_sFileDll)

#this works, $test is 5
$test = DllCall($hdll, "int:cdecl", "Add", "int", 2, "int", 3)

#this works, we get a console print in scite and the messagebox initiated by dll pops up
$test = DllCall($hdll, "int:cdecl", "Function", "int", 2, "int", 3)

#this does not work, we only get 0 pointer int the dll
$utf16String = "Hello, World"
$ptr = DllStructCreate("wchar_t [" & StringLen($utf16String) + 1 & "]") ; +1 for null terminator
DllStructSetData($ptr, 1, $utf16String)
DllCall($hdll, "int", "Echo", "ptr", DllStructGetPtr($ptr))

DllClose($hdll)

 

C++ Part:

FileBridge++.h:

#include <wchar.h>

#define DLL_EXPORT
#if defined DLL_EXPORT
#define DECLDIR __declspec(dllexport)
#else
#define DECLDIR __declspec(dllimport)
#endif

int CppAdd(int a, int b);

extern "C"
{
    // Declare all functions here
    DECLDIR void Function(void);
    DECLDIR int Add(int a, int b);
    DECLDIR void Echo( wchar_t* utf16String);  // Function takes a pointer to MyStructure
}

FileBridge++.cpp:

#include "pch.h"
#include <iostream>
#include <Windows.h> 
#include "FileBridge++.h"

using namespace std;

extern "C"
{

    DECLDIR int Add(int a, int b)
    {
        return CppAdd(a, b);
    }

    DECLDIR void Function(void)
    {
        std::cout << "DLL Called! Hello AutoIt!" << std::endl;
        MessageBox(0, TEXT("This MsgBox is created by the dll"), TEXT("Simple Dll..."), 0);
    }

    DECLDIR void Echo(wchar_t* utf16String)
    {
        if (utf16String == nullptr) {
            std::wcout << L"Received NULL pointer!" << std::endl;
            return;
        }
        std::wcout << L"You wrote: [" << utf16String << L"]" << std::endl;
        
    }
}

int CppAdd(int a, int b)
{
    return(a + b);
}

 

Edited December 27, 2024 by emcodem

[jchd]

1 hour ago, emcodem said:

$ptr = DllStructCreate("wchar_t [" & StringLen($utf16String) + 1 & "]") ; +1 for null terminator

wchar_t is not a valid type. Use wchar instead and test result of function calls.

[emcodem]

@jchd thank you so much for the fast response!

It works, in my examples above, only the au3 part needs to be changed as you describe:

$ptr = DllStructCreate("wchar [" & StringLen($utf16String) + 1 & "]") ; +1 for null terminator

 

For the way back from the DLL to autoit, we can just return wchar_t* instead of void from the C function and retrieve it like

Local $aDLL = DllCall($hdll, "WSTR", "Echo", "ptr", DllStructGetPtr($ptr))

 

Edited December 27, 2024 by emcodem