Cannot get a BSTR in VB6 dll from a referenced C++ dll - c++

Let me start off by saying that VB is not my strong suit.
I am developing a C++ dll to be used in a VB6 application's dll.
I have successfully instantiated the (C++) classes in VB. I am trying to access data members of the class using this syntax: "vbCppObj.dataMemberName".
I can confirm that this works for boolean and enum types and it invokes the getter methods defined in my class.
I have to access a string from the (C++) class as well.
The getter function for the string is given below:
class MyCPPClass
{
private:
WCHAR* CPPErrorString = L"This is a string";
public:
HRESULT __stdcall get_CPPErrorString(BSTR* pVal)
{
BSTR str = ::SysAllocString(CPPErrorString);
if(str)
*pVal = str;
return S_OK;
}
};
I am unable to debug the C++ dll right now.
I access this value in the VB6 code as follows:
ErrorString = vbCppObj.CPPErrorString
Logger.log "[Log]:" & ErrorString
"ErrorString" is a String type in VB. When this line executes, the "ErrorString" object shows "<Out of memory>" (when I hover over it). If I step further, to the logging code, it gives me a "Error 14: Out of string space".
Also, I have typed this code in the browser so, it may not be 100% correct.

As it turns out, I had to convert the string into a "_b_str" and then to a "BSTR". That worked for me.
I had tried it earlier but I don't know why it didn't work at that time.

Why you just don't use LPCTSTR?
I'm not an advanced C/C++ programmer, but this should work
class MyCPPClass
{
private:
LPCTSTR CPPErrorString = "This is a string";
public:
HRESULT __stdcall get_CPPErrorString(LPCTSTR * pVal)
{
// copy the value
*pVal = CPPErrorString;
// return
return S_OK;
}
}

Related

Confused in used BSTR in com dll in c++ builder

I make an OCX in C++ Builder Borland. My OCX has these function as I describe it:
Login
Write
Read
DebugOutput
GetVal
I am using this OCX in my c# application. In c# side I have a button and list box. The button calls login method from OCX and the list box shows this method's output.
In OCX side, the login method creates a command for server (with socket programming) to get authentication. Then call Write function to writes on socket. Then gets response from socket and calls Read function to reads the socket response.
The Read method reading the result and send it to DebugOutput to debug the output stream and call GetVal to find Last main server response. Then they pass their parameters to each other. after all the C# side show the result (SUCCESS | FAIL ) for login methods.
I used BSTR. (I read the topics about BSTR in stackoverflow and on MSDN but I think I did not get it well in my solution). These are my code in OCX side:
BSTR STDMETHODCALLTYPE TVImpl::Login(BSTR PassWord)
{
wchar_t wcs1[500];
Var *var=new Var();
//here make the command
...................
//get the server response to show to user
BSTR read=::SysAllocString(Write(wcs1));
if(read!=NULL) ::SysFreeString(read);
return read;
}
BSTR STDMETHODCALLTYPE TVImpl::Read()
{
BSTR str ;
try
{
IdTCPClient1->ReadTimeout=100;
str =::SysAllocString( IdTCPClient1->IOHandler->ReadLn().c_str());
}
catch(Exception &e)
{
str= e.Message.c_str();
}
str=(DebugOutput(str));
return str;
}
BSTR STDMETHODCALLTYPE TVImpl::Write(BSTR str)
{
IdTCPClient1->IOHandler->WriteLn(str) ;
BSTR str2=::SysAllocString(L"TST");
str2=Read();
return str2;
}
BSTR STDMETHODCALLTYPE TVImpl::GetVal(BSTR st,BSTR ValTyp)
{
BSTR res;
AnsiString stAnsi;
//Do some thing with st and save it to stAnsi
.................
res=(BSTR)WideString(stAnsi);
return ::SysAllocString(res);
}
BSTR STDMETHODCALLTYPE TVImpl::DebugOutput(BSTR st)
{
Var *val=new Var();
BSTR res;
res=GetVal(st,val->CMD_CMD);
if(res==val->CMD_AUTHENTICATE)
res=GetVal(st,val->XPassword);
return res;
}
In C3 code it was hanging. I know my problem is to use sysAllocString. but I when I used ::sysFreestring for each one in each method again my C# code hanging.
This is my C# code :
private void button4_Click(object sender, EventArgs e)
{
VinSockCmplt.Vin vin = new Vin();
listBox1.Items.Add(vin.Login("1234"));
}
You should not return BSTR (or any "complex" type that various compilers compile in various ways). The recommended return type for COM methods is HRESULT (a 32-bit integer). Also, you must not release a BSTR you just allocated and return it to the caller.
Here is how you could layout your methods:
HRESULT STDMETHODCALLTYPE TVImpl::Login(BSTR PassWord, /*out*/ BSTR *pRead)
{
...
*pRead = ::SysAllocString(L"blabla"); // allocate a BSTR
...
// don't SysFreeString here, the caller should do it
...
return S_OK;
}
If you were defining your interface in a .idl file, it would be something like:
interface IMyInterface : IUnknown
{
...
HRESULT Login([in] BSTR PassWord, [out, retval] BSTR *pRead);
...
}
retval is used to indicate assignment semantics are possible for languages that support it, like what you were trying to achieve initially with code like this var read = obj.Login("mypass")
There are lots of mistakes in your code:
if(read!=NULL) ::SysFreeString(read); return read; frees a string and then returns it
str= e.Message.c_str(); ... return str; returns a pointer to something that isn't even a BSTR
BSTR str2=::SysAllocString(L"TST"); str2=Read(); allocates a string and then leaks that string immediately
res=(BSTR)WideString(stAnsi); creates a dangling pointer
All of these are undefined behaviour (except the leak) and might cause a crash.
Also, I'm not sure if it is valid to have your functions return BSTR; the normal convention in COM programming is that functions return HRESULT and any "return values" go back via [out] parameters. This is compatible with all languages that have COM bindings; whereas actually returning BSTR limits your code compatibility. (I could be wrong - would welcome corrections here). You can see this technique used in the other question you linked.
To get help with a question like "Why is my code crashing?", see How to Ask and How to create a Minimal, Complete, and Verifiable example.

Call Delphi CLASS exported in DLL from C++ code

i have a problem to use delphi class from C++ code. delphi dll demo that export a function that return an object.
my delphi Dll code is as follow:
library DelphiTest;
// uses part....
type
IMyObject = interface
procedure DoThis( n: Integer );
function DoThat : PWideChar;
end;
TMyObject = class(TInterfacedObject,IMyObject)
procedure DoThis( n: Integer );
function DoThat: PChar;
end;
// TMyObject implementation go here ...
procedure TMyObject.DoThis( n: Integer );
begin
showmessage('you are calling the DoThis methode with '+intToStr(n) +'parameter');
end;
function TMyObject.DoThat: PChar;
begin
showmessage('you are calling the DoThat function');
Result := Pchar('Hello im Dothat');
end;
// exporting DLL function :
function CreateMyObject : IMyObject; stdcall;export;
var
txt : TextFile;
begin
AssignFile(txt,'C:\log.log');
Reset(txt);
Writeln(txt,'hello');
Result := TMyObject.Create;
end;
exports CreateMyObject;
in my C++ project i declared the IMyObject interface as follow :
class IMyObject
{
public:
IMyObject();
virtual ~IMyObject();
virtual void DoThis(int n) = 0;
virtual char* DoThat() = 0;
};
and my main function as follow :
typedef IMyObject* (__stdcall *CreateFn)();
int main()
{
HMODULE hLib;
hLib = LoadLibrary(L"DelphiTest.dll");
assert(hLib != NULL); // pass !!
CreateFn pfnCreate;
pfnCreate = (CreateFn)GetProcAddress((HINSTANCE)hLib, "CreateMyObject");
if (pfnCreate == NULL)
{
DWORD errc = GetLastError();
printf("%u\n", errc); // it gets error 127
}
else{
printf("success load\n");
}
IMyObject* objptr = pfnCreate();
objptr->DoThis(5);
FreeLibrary(hLib);
int in;
scanf_s("%i", &in);
return 0;
}
with this example i got an error at run time when i try to access the exported function. the errors is at line :
IMyObject* objptr = pfnCreate();
can you tell me what is wrong about my example.
and if possible any working example to access Delphi class (in DLL) from C++ code.
The first problem is calling convention of the methods. The Delphi interface uses register which is a Delphi specific calling convention. Use stdcall, for example, for the methods of the interface.
The next problem is in the C++. Your C++ interface must derive from IUnknown. Further, it should not declare a constructor or destructor.
Beyond that your Delphi code exports PWideChar which does not map to char*. It maps to wchar_t*.
Looking further ahead, returning a PChar works fine here because your implementation returns a literal. But more serious code will want to use a dynamically allocated string presumably and at that point your design is flawed.
Do note that you need to be an elevate administrator to create a file at the root of the system drive. So that's yet another potential failure point.
I expect there are other mistakes, but that's all I've found so far.

What is the difference between COM property methods and regular interface methods?

I have been poking around with WRL at the ABI layer for the last couple of weeks and have run into this problem.
I have an interface defined in IDL as follows:
namespace Async{
[uuid(f469e110-7ef5-41df-a237-9ddef9aed55c), version(1.0)]
interface IDownloader : IInspectable
{
HRESULT GetFeed([in] HSTRING url,[out, retval] Windows.Web.Syndication.SyndicationFeed ** feed);
[propget]HRESULT Feed([out, retval]Windows.Web.Syndication.SyndicationFeed ** feed);
}
[version(1.0), activatable(1.0)]
runtimeclass Downloader
{
[default] interface IDownloader;
}
}
Which I have defined in my header file like so:
#pragma once
#include "Async_h.h"
namespace ABI {
namespace Async {
class Downloader : public Microsoft::WRL::RuntimeClass<ABI::Async::IDownloader>
{
InspectableClass(L"Async.Downloader", BaseTrust);
public:
Downloader();
STDMETHOD(GetFeed)(HSTRING url, ABI::Windows::Web::Syndication::ISyndicationFeed ** feed);
STDMETHOD(get_Feed)(ABI::Windows::Web::Syndication::ISyndicationFeed ** feed);
private:
//Microsoft::WRL::ComPtr<ABI::Windows::Foundation::Uri> feedUrl;
Microsoft::WRL::ComPtr<ABI::Windows::Web::Syndication::ISyndicationFeed> m_feed;
};
ActivatableClass(Downloader);
}
}
In my cpp file I implement the functions:
STDMETHODIMP Downloader::GetFeed(HSTRING url, ISyndicationFeed** feed)
{
HRESULT hr;
RoInitializeWrapper ro(RO_INIT_MULTITHREADED);
ComPtr<IUriRuntimeClass> uri;
ComPtr<IUriRuntimeClassFactory> uriFactory;
hr = GetActivationFactory(HStringReference(RuntimeClass_Windows_Foundation_Uri).Get(), &uriFactory);
hr = uriFactory->CreateUri(url, uri.GetAddressOf());
ComPtr<ISyndicationClient> client;
ComPtr<IInspectable> inspectable;
RoActivateInstance(HStringReference(RuntimeClass_Windows_Web_Syndication_SyndicationClient).Get(), &inspectable);
hr = inspectable.As(&client);
Event timerCompleted(CreateEventEx(nullptr, nullptr, CREATE_EVENT_MANUAL_RESET, WRITE_OWNER | EVENT_ALL_ACCESS));
auto callback = Callback<IAsyncOperationWithProgressCompletedHandler<SyndicationFeed*,RetrievalProgress>>([&](IAsyncOperationWithProgress<SyndicationFeed*,RetrievalProgress> *op, AsyncStatus status) ->HRESULT
{
auto error = GetLastError();
if (status == AsyncStatus::Completed)
{
hr = op->GetResults(m_feed.GetAddressOf());
*feed = m_feed.Get();
}
return S_OK;
});
ComPtr<IAsyncOperationWithProgress<SyndicationFeed*,RetrievalProgress>> operation;
hr = client->RetrieveFeedAsync(uri.Get(), operation.GetAddressOf());
operation->put_Completed(callback.Get());
return S_OK;
}
STDMETHODIMP Downloader::get_Feed(ISyndicationFeed** feed)
{
*feed = m_feed.Get();
return S_OK;
}
The property works as expected it is projected to c++/cx as it should be. However,in the GetFeed method, when I attempt to set the feed parameter to the retrieved feed I get an access violation. Obviously I know that the memory is bad but the way I understand COM properties, they are essentially function calls and the property method and the GetFeed method are doing exactly the same thing minus the retrieval part.
Here are my questions:
What is the difference between COM property methods and regular interface methods in terms of the projected return value if any?
Why is the parameter to the property method initialized to nullptr and the parameter to the GetFeed Method not when they are described exactly the same in IDL?
If the out parameters in property methods are initialized, what part of the COM runtime is doing that for me and is that controllable? IE is there a way to get memory that I can write to passed to me?
I know that I could probably design that away but that is not the point. I am just trying to learn how it all works.
Thanks.
In your lambda you are capturing by reference with [&]. You need to capture the feed parameter by value, since the stack frame is long gone by the time your lambda executes.
The bigger issue is that the client has no idea when they can retrieve the results since you don't provide that information. (I see you create an unused Win32 Event object, so maybe there's some other code to make that work that you've deleted).

Using a passed by reference string array to runtime loaded dll function

I'm creating a DLL lib that should be used during runtime (i.e. loaded in Windows 7 with LoadLibrary, closed with FreeLibrary and function handles given by GetProcAddress). This is being done in C++ using Borland C++ Builder. All functions in the DLL should receive some value by reference as parameter (normally std::string).
By now the method I'm using to do this is the following (example) (summarized):
typedef void (*HIS_validity)(string &);
//LoadLibrary
HIS_validity fValidity = (HIS_validity) GetProcAddress(frMain->HIS_DLL.hisLibrary,"checkForValidity");
if (fValidity == NULL) return;
string testeValidade;
fValidity(testeValidade);
const AnsiString testeValidade2(testeValidade.c_str());
if (testeValidade2 != "...")
//etc...
In the DLL:
extern "C" void LIBRARY_API checkForValidity(string &str);
void checkForValidity(string &str)
{
str = "...";
}
Now this code is running fine. The problem is that in some functions I want to pass a whole array of strings by reference. Previously I discovered how to pass a string array by reference here and I though it would be just a matter of ajusting things accordingly:
typedef void (*HIS_patientData)(string (&)[32]);
HIS_patientData fPatientData = (HIS_patientData) GetProcAddress(frMain->HIS_DLL.hisLibrary,"patientDataFields");
string strDado2[32];
fPatientData(strDado2);
frMain->pluginData.patientData.numProntuario = AnsiString(strDado2[cont1++].c_str());
frMain->pluginData.patientData.pacNome = AnsiString(strDado2[cont1++].c_str());
In the DLL:
extern "C" void LIBRARY_API patientDataFields(string (&str)[32]);
void patientDataFields(string (&str)[32])
{
str[0] = "One";
str[1] = "Two";
str[2] = "Three";
//....
}
But here the problem appears. When I compile and run my application, the same problem always come up: if my function in the DLL has only two data attributed to 'str[]', the code goes one after 'fPatientData(strDado2);' but when I read the content of strDado2[0], it has the value of str[1] and strDado2[1] has NULL inside! By the other hand, if I add three or more attributions to 'str[]' in my DLL function, the software always crash when it comes to 'fPatientData(strDado2);' with a pop-up telling "access violation ... in module libstdc++-6.dll".
And I have no ideia what the problem is :T
Thanks for any help,
Momergil
Ok, it seems I found the answer to all such problems... Namely, I'm trying to return a C++ class (std::string) in a "extern "C"" function. It was just a matter of making it return a standart const char* that everything started to run just fine.
Thanks for the contributors,
Momergil

C++, COM and passing strings

I am debugging some other programmer's source code of a Windows Media Player plugin. This plugin causes WMP to crash sometimes, and sometimes it takes really long time to open plugin settings window. The problem occurs only when opening settings window while music is being played back. It opens without issues if the player is stopped.
While looking through the code and debugging, I have came to the line of code which seems to be the cause of the problems.
The property page has the following member variable:
CComPtr<IDsp_plugin> m_pDsp_plugin;
and the property page at initialization calls get_text method of the COM object:
unsigned char * txt = NULL;
//m_pDsp_plugin is a valid pointer to IDsp_plugin
HRESULT res = m_pDsp_plugin->get_text(&txt);
At this moment hres is "0x80010105: The server threw an exception." and Visual Studio Debug output shows "First-chance exception at 0x764efbae in wmplayer.exe: 0x80010105:
get_text method is defined as follows:
in Dsp_plugin.idl
interface IDsp_plugin : IUnknown
{
HRESULT get_text([out] unsigned char* *pVal);
...
in Dsp_plugin.h
class ATL_NO_VTABLE CDsp_plugin :
public CComObjectRootEx<CComMultiThreadModel>,
public CComCoClass<CDsp_plugin, &CLSID_Dsp_plugin>,
public IDsp_plugin,
public IMediaObject,
public IWMPPluginEnable,
public ISpecifyPropertyPages
{
STDMETHOD(get_text)(unsigned char* *txt);
...
and finally the method itself which throws this exception:
Dsp_plugin.cpp
STDMETHODIMP CDsp_plugin::get_text (unsigned char* *txt)
{
... // some code for copying a valid string from somewhere to char* y
// 11 bytes of memory for y was allocated using malloc(10+1);
// y contains a valid C string here, tested with debugger and passing to OutputDebugStringA
*txt = (unsigned char*)(y); // This line executes normally, but at the end the caller gets "The server threw an exception." and WMP starts behaving weirdly.
// If I comment it out, the caller gets S_OK and there are no any issues with WMP.
return S_OK;
}
The COM DLL is compiled with setting "Use Unicode Character Set".
I am not experienced COM programmer, but passing strings as unsigned char** seems unusual to me, I have seen mostly BSTR or VARIANT when dealing with COM.
Maybe some COM guru can explain, why this exception happens and can it be possibly fixed just by converting methods to using BSTR* and SysAllocString/SysfreeString instead of unsigned char**/malloc/free ?
Put simply, COM doesn't know how to pass around pointers of type unsigned char *. The default marshalling rules are applied (since the interface definition doesn't specify any parameter attributes), and, if I'm interpreting this correctly, COM marshals the outer pointer itself txt properly, but treats *txt as a pointer to a single unsigned char, not a string.
This may still work if the caller and callee happen to be in the same apartment; from the sounds of it, they're not.
The easiest solution is simply to make the parameter a BSTR *. COM has special handling for BSTR which will ensure it's passed correctly.