BSTR from C Dll to VB

Author: Dave
Date: 11.16.20 - 3:29pm

So lets say you have a C function prototype such as the following:

BSTR __stdcall vbcurl_string_escape(wchar_t* ws, int size)

How do you write your API declaration in VB6 to handle this? We all know VB6 deals with BSTRs this should be ok right?

Mmm not really. The Api declare mechanism was written to allow vb6 to interact with standard C dlls. So the runtime does all kinds of automagic translations to meet that expectation.

If your C dll is returning a SysAlloc* BSTR, technically you would need to write a tlb file to include in your VB6. what if you dont want to write and reference a tlb file because they are inconvenient? Can VB6 work with this BSTR anyway?

Now I know what your thinking and the answer is NO. If you try to declare the vb api def with an As String return type, you will get an double unicode string and a leaked BSTR that never gets free'd.

We can however screw with things a little bit and make it work.

Public Declare Function vbcurl_string_escape Lib "vblibcurl.dll" ( _
    ByVal strPtr As Long, _
    ByVal sz As Long _
) As Long

Function escape(ByVal buf As String) As String
    Dim tmp As Long, s As String
    tmp = vbcurl_string_escape(strPtr(buf), Len(buf))
    CopyMemory ByVal VarPtr(s), tmp, 4 'steal a ref to an existing BSTR so we now own it
    escape = s
End Function

If you look at the varptr(s) before it has been set to anything, it value at that address is 0. So VB6 knows it is not a active BSTR and does not need to free it.

This means there is no memory leak if we manually stuff it with a reference to an existing BSTR which no one else owns and no one else will free.

Another example:

'[entry(0x60000027), helpstring("Get supported protocols")]
'void _stdcall vbcurl_version_protocols(
'                [in] long ver,
'                [out] SAFEARRAY(BSTR)* ppsa);
Public Declare Sub vbcurl_version_protocols Lib "vblibcurl.dll" (ByVal hVerInfo As Long, ByRef ary() As Long)

Function libcurlProtocols() As String()
    Dim ptr() As Long, vd As Long, i As Long, s() As String, tmp As String
    vd = vbcurl_version_info(CURLVERSION_NOW)
    vbcurl_version_protocols vd, ptr
    If AryIsEmpty(ptr) Then Exit Function
    ReDim s(UBound(ptr))
    For i = 0 To UBound(ptr)
        CopyMemory ByVal VarPtr(tmp), ptr(i), 4 'steal a ref to an existing BSTR so we now own it
        s(i) = tmp
        'Debug.Print ptr(i) & " " & s(i)
    libcurlProtocols = s()
End Function

Comments: (1)

On 03.23.22 - 2:40pm Dave wrote:
this gave me some problems today see the zlib2 post

Leave Comment:
Email: (not shown)
Message: (Required)
Math Question: 12 + 58 = ? followed by the letter: O 

About Me
More Blogs
Main Site
Posts: (year)
2024 (2)
     ffmpeg voodoo
     RegJump Vb
2023 (9)
     VB6 Virtual Files
     File handles across dlls
     python abort / script timeout
     VB6 Python embed w/debugger
     python embedding
     VB6 IDE Enhancements
     No Sleep
     A2W no ATL
2022 (4)
     More VB6 - C data passing
     Vb6 Asm listing
     Byte Array C to VB6
     Planet Source Code DVDs
2021 (2)
     Obscure VB
     VB6 IDE SP6
2020 (4)
     BSTR from C Dll to VB
     Cpp Memory Manipulation
     ActiveX Binary Compatability
2019 (5)
     Console tricks
     FireFox temp dir
     OCX License
     Extract substring
     VB6 Console Apps
2018 (6)
     VB6 UDTs
     VB6 Debugger View As Hex tooltips
     VB6 - C Share registry data
     VB6 Addin Missing Menus
     VB6 Class Init Params
     VB6 isIn function
2017 (6)
     Python and VB6
     Python pros and cons
     download web Dir
     vc rand in python
     VB6 Language Enhancement
     Register .NET as COM
2016 (22)
     VB6 CDECL
     UDT Tricks pt2
     Remote Data Extraction
     Collection Extender
     VB6 FindResource
     DirList Single Click
     Reset CheckPoint VPN Policy
     VB6 BSTR Oddities Explained
     SafeArrays in C
     BSTR and Variant in C++
     Property let optional args
     Misc Libs
     Enum Named Pipes
     Vb6 Collection in C++
     VB6 Overloaded Methods
     VB6 Syncronous Socket
     Simple IPC
     VB6 Auto Resize Form Elements
     Mach3 Automation
     Exit For in While
2015 (15)
     C# self register ocx
     VB6 Class Method Pointers
     Duktape Debug Protocol
     QtScript 4 VB
     Vb6 Named Args
     vb6 Addin Part 2
     VB6 Addin vrs Toolbars
     OpenFile Dialog MultiSelect
     Duktape Example
     DukTape JS
     VB6 Unsigned
     .Net version
     TitleBar Height
     .NET again
     VB6 Self Register OCXs
2014 (25)
     Query Last 12 Mos
     Progid from Interface ID
     VB6 to C Array Examples
     Human Readable Variant Type
     ScriptBasic COM Integration
     CodeView Addin
     ScriptBasic - Part 2
     Script Env
     MSCOMCTL Win7 Error
     printf override
     History Combo
     Disable IE
     API Hooking in VB6
     Addin Hook Events
     FastBuild Addin
     VB6 MemoryWindow
     Link C Obj Files into VB6
     Vb6 Standard Dlls
     CStr for Pascal
     Lazarus Review
     asprintf for VS
     VB6 GlobalMultiUse
     Scintilla in VB6
     Dynamic Highlight
     WinVerifyTrust, CryptMsgGetParam VB6
2013 (4)
     MS GLEE Graphing
     printf for VB6
     C# App Config
     Tero DES C# Test
2012 (10)
     VC 2008 Bit Fields
     Speed trap
     C# Db Class Generator
     VB6 vrs .NET (again)
     FireFox Whois Extension
     git and vb6
     Code Additions
     Compiled date to string
     C# ListView Sorter
     VB6 Wish List
2011 (7)
     C# Process Injection
     CAPTCHA Bots
     C# PE Offset Calculator
     VB6 Async Download
     Show Desktop
     coding philosophy
     Code release
2010 (11)
     Dll Not Found in IDE
     Advanced MSScript Control
     random tip
     Clipart / Vector Art
     VB6 Callback from C#
     Binary data from VB6 to C#
     CSharp and MsScriptControl
     HexDumper functions
     Js Beautify From VB6 or C#
     vb6 FormPos
     Inline Asm w VB6
2009 (3)
     The .NET Fiasco
     One rub on computers
     Universal extractor