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); }  

Thank you for your time and Brains.
The problem was that i was just poking around but not really understand what exactly is going on even if you told me already that i need to return the struct.
I don't want to use statics and globals because this will be used in a UDF that allows to access the exportet methods from my C dll.
This whole Echo code was just created to explain and debug this very issue.

From this perspective it might be good to just dump the idea of the MakeWstrPtr funciton completely and write out the for DllStructCreate,SetData and GetPtr everywhere. This would make the final code very ugly to read, there would be hundred lines like this, it would be hard to track the variables for me.
    $struct = DllStructCreate("wchar [" & StringLen($s_str) + 1 & "]") ; +1 for null terminator     DllStructSetData($struct, 1, $s_str)
But i think i understand now every Aspect of this and i may go with the following solution:
Func __MakeWstrPtr($s_str, ByRef $struct)     $struct = DllStructCreate("wchar [" & StringLen($s_str) + 1 & "]") ; +1 for null terminator     DllStructSetData($struct, 1, $s_str)     return DllStructGetPtr($struct) EndFunc Func CallEcho($sToEcho)     Local $struct_1     Local $_ptr1 = __MakeWstrPtr($sToEcho,$struct_1)     DllCall($hdll, "WSTR", "Echo", "ptr", $_ptr1) ;the C side prints You wrote: to scite console EndFunc We still need 2 Lines for using MakeWstrPtr function where i would prefer one line but i think at least this way we will end up with much better readable code. This way all the releases of structs and ptrs is done when CallEcho returns which is highly desired.
If i understood correctly, to avoid the Byref Struct parameter, we could potentially return an Array holding struct and ptr but performance is a big concern so we want to avoid spending time on allocating arrays or similar just to make the code more readable.
 
The final result using above solution can look like:
Func UpdateOne($ptr_mongocollection, $search, $update, $options) ;~https://www.mongodb.com/docs/manual/reference/method/db.collection.updateOne/ ;~commonly used options: {"upsert":true} ;~returns json str like { "modifiedCount" : 1, "matchedCount" : 1, "upsertedCount" : 0 } Local $_s1,$_s2,$_s3; Local $_searchptr = __MakeWstrPtr($search,$_s1) Local $_updateptr = __MakeWstrPtr($update,$_s2) Local $_optptr = __MakeWstrPtr($options,$_s3) Local $a_result = DllCall($hdll, "WSTR", "UpdateOne", "ptr",$ptr_mongocollection, "ptr", $_searchptr, "ptr", $_updateptr, "ptr", $_optptr); ConsoleWrite("UpdateOne: " & $a_result[0] & " Error: " & _WinAPI_GetLastError() & @CRLF) return $a_result[0] EndFunc  

Hello,
Instead of creating the Global variable $out in the main body of the script and passing 2 parameters to MakeWstrPtr() , why not simply return the structure variable name when the function ends, something like this :
Global $g_tStruct = WstrPtr("test") For $i = 5 To 1 Step -1 CallEcho() Next Func WstrPtr($str) Local $tStruct = DllStructCreate("wchar [" & StringLen($str) + 1 & "]") ; +1 for null terminator DllStructSetData($tStruct, 1, $str) Return $tStruct EndFunc Func CallEcho() Local Static $pStruct = DllStructGetPtr($g_tStruct) DllCall($hDll, "WSTR", "Echo", "ptr", $pStruct) EndFunc Concerning the pointer, juste use a Static variable inside CallEcho() so its value won't be destroyed when the function ends. I tested the memory content and it was ok (e.g.  "t e s t" was constantly there, during the 5 calls to CallEcho)

I have found a solution. I will describe the relevant parts of the project -- for the benefit of others
I use Pegasus Mail and back up the settings and data daily: just the files that have changed, i.e. have the archive bit set. Pegasus organizes saved emails into what it calls "folders" (but which are actually files). The "folders' are organized into a tree structure.
Very occasionally a Pegasus "folder" disappears, and when it does, it needs to be retrieved from back ups. But to retrieve a "folder" one needs to know its filename, and the name of the file is FOL followed by seemingly random hex-looking digits.
Pegasus maintains a file, hierarch.pm. It is in CSV format and contains the tree structure including the name of the "folder" and the filename. It is, of course, flat,
My project is to show hierarch.pm in a treeview so the user can select a "folder" and have the script tell him the name of the corresponding file.
So the treeview needs a string to be associated with each "folder" item.
To do this, I avoided using GuiTreeviewCreateItem() so I, not calling this native AutoIt funcion, get to store the file name in the $iParam parameter of the treeview struct. So I created the treeview by calling _GuiCtrlTreeview_Add() and _GuiCtrlTreeview_AddChild().
I thought of calling _GuiCtrlTreeview_SetItemParam() to store the file name with each treeview item. But $iParam cannot be used directly to store a string.
In Structureconstants.au3, $iParam is a 4-byte integer. In 32-bit AutoIt, a pointer is also 4 bytes. I decided to try storing a pointer to a string there.
To allocate memory and get a pointer back, I call _WinAPI_CreateString(). To get the string back, I call PointerToStringW():
Func _PointerToStringW($ptr) Return DllStructGetData(DllStructCreate("wchar[" & _WinAPI_StringLenW($ptr) & "]", $ptr), 1) EndFunc To store the string with the treeview item, I coded;
$pStr = _WinAPI_CreateString(StringInStr($a20[$i][$kFull],':FOL') ? $a20[$i][$kFull] : '') _GUICtrlTreeView_SetItemParam($g_hTV,$hItem,$pStr) To retrieve a string I coded:
Local $ptr = _GUICtrlTreeView_GetItemParam($g_hTV,$hItem) Local $s = _PointerToStringW($ptr) . . . Func _PointerToStringW($ptr) Return DllStructGetData(DllStructCreate("wchar[" & _WinAPI_StringLenW($ptr) & "]", $ptr), 1) EndFunc I also keep track of these pointers in an array, and when the script is ending I free the memory:
OnAutoItExitRegister('FreePointersToParamStrings') Func FreePointersToParamStrings() For $i = 0 To $g_iqPointers _WinAPI_FreeMemory($g_pointers[$i]) Next EndFunc This works in x86 AutoIt. It may not work in AutoIt x64.

Fantastic explaination, thank you so much @pixelsearch
Also thanks a lot for the linked topic, it contains very helpful informations for me!