C++ .NET DLL hangs when calling unmanaged C++ DLL

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4.50/5 (2 votes)

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Hi All,

This is a long shot but after 2 days of coding I figure I'm unable to fix my problem, which is...

I have a C# GUI that references a C++ .NET DLL (managed) that makes calls to underlying C++ (unmanaged) DLL's. One of the methods in my C++ .NET DLL starts a thread that calls to my underlying C++ DLL, that code looks like this:

MIDL

m_oThread = gcnew Thread(gcnew ParameterizedThreadStart(this, &MyClass::MethodToRunOnThread));
m_oThread->Start(MyParameter);

MIDL

void MyClass::MethodToRunOnThread(Object^ step)
{ //Call to underlying C++ DLL (unmanaged).
}

The problem is my underlying DLL calls CWinThread::PumpMessage, and my app crashes when I hit that line. It feels like some kind of timing/locking problems because if I step the code as opposed to running it it sometimes gets past that line.

To add more confusion, if I make the call to my underlying C++ DLL without putting it on it's on thread it runs fine, i.e.

MIDL

MethodToRunOnThread(MyParameter);

I hope I've provided enough info to get you peeps thinking!
Regards,
Chris

Posted 10-Feb-11 21:24pm

cr101037

Updated 14-Feb-11 4:26am

Nish Nishant

v3

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Rajesh R Subramanian 11-Feb-11 9:22am

Please modify your question to include the following two pieces of information.

1. Can you show me how exactly are you making the call to the native dll?
2. What exactly is the DLL doing?

Stephen Hewitt 11-Feb-11 9:44am

What is the crash? What type of exception is generated?

Sergey Alexandrovich Kryukov 11-Feb-11 12:41pm

Do the following: 1) catch exception (in managed code), in the catch make exception dump, include exception stack and all inner exceptions (recursive), 2) output the dump in some text file and post it here, 3) in exception stack find relevant line numbers and code files; 4) figure out part of code related to the exception and post it.

--SA

cr101037 14-Feb-11 6:37am

Rajesh R Subramanian:
1. This is how I make the call to my unmanaged DLL...

void MyClass::MethodToRunOnThread(Object^ step)
{
MyUnmanagedClassInDLL* pointer = NULL;

BOOL bReturn = TRUE;
bReturn = pointer->MyMethod(); //this method eventually calls CWinThread::PumpMessage where it hangs!
}

2. The DLL communicates with another piece of software (through sockets).

Stephen Hewitt:
The app doesn't crash and there is no exception.

SAKryukov:
There is no exception to catch.

Just to recap, my app only hangs when I call my managed DLL on a seprearte thread! i.e. this hangs...

m_oThread = gcnew Thread(gcnew ParameterizedThreadStart(this, &MyClass::MethodToRunOnThread));
m_oThread->Start(MyParameter);

...and this doesn't...

MethodToRunOnThread(MyParameter);

Nish Nishant 14-Feb-11 10:26am

Added C++/CLI tag to thread.

2 solutions

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Nish Nishant · Answer 1 · 2011-02-14T03:25:00

Solution 2

I suspect the problem is that your worker thread does not have a message loop. So you should not be calling PumpMessage from it. One thing you could try, if you can't clean up the code easily, is to call PeekMessage first to check for messages before calling PumpMessage.

[Edit]
~~~~~~~

Instead of creating a managed worker thread, try creating a UI thread using MFC's threading options. It seems the MFC code you call rely on the assumption that it's being called in a UI thread. That may fix your problem.

[Edit 2]
~~~~~~~~~

In response to your comment: Yes, but create the MFC thread from an unmanaged block (using #pragma unmanaged) and then have a managed method call the unmanaged function that creates the MFC thread. That should work for you.

Posted 14-Feb-11 3:25am

Nish Nishant

Updated 14-Feb-11 4:06am

v3

Comments

cr101037 14-Feb-11 9:49am

Hi Nishant Sivakumar, thanks for your response.
Something I've just noticed, if I put a breakpoint on my PumpMessage() line and step into, it takes me here:

//thrdcore.cpp
BOOL CWinThread::PumpMessage()
{
return AfxInternalPumpMessage();
}

If I step over the "return AfxInternalPumpMessage();" line my app runs fine!?
It feels like some kind of timing/locking problem!

Nish Nishant 14-Feb-11 9:53am

No, it's not a timing issue here. PumpMessage should not be called on a worker thread that does not have a message loop. You are creating a non-UI thread but the unmanaged code you call expects to be running on an UI thread.

Nish Nishant 14-Feb-11 9:54am

Instead of creating a managed worker thread, try creating a UI thread using MFC's threading options. It seems the MFC code you call rely on the assumption that it's being called in a UI thread. That may fix your problem.

cr101037 14-Feb-11 10:00am

Like the idea but is it possible to create an MFC UI Thread from managed code?

Nish Nishant 14-Feb-11 10:05am

Yes, but create the MFC thread from an unmanaged block (using #pragma unmanaged) and then have a managed method call the unmanaged function that creates the MFC thread. That should work for you.

Espen Harlinn 14-Feb-11 10:18am

Good effort as usual, a 5

Nish Nishant 14-Feb-11 10:25am

Thank you.

cr101037 14-Feb-11 10:54am

So why does it work when I step the code as opposed to running it?

Nish Nishant 14-Feb-11 11:01am

Could be due to many factors, it's in debug mode, you are in a break point scenario (which itself creates an artificial context for the executing app). The debugger may have its own message pumping in place, etc.

cr101037 16-Feb-11 3:50am

Nishant Sivakumar:
I'm fairly new to C++, not sure how to do the MFC thread stuff, this is what I've tried but it doesn't work.
I've added a new class definition into my existing .h file, like so...

//MyHeader.h

public ref class ManagedClass //this was already here
{
public:
void MethodWhichWillCallMyUnmanagedClass();
};

public class SomeUnmanagedClass //this is new
{
public:
UINT ThreadProc(LPVOID param);
};

And then in my cpp file

//MyCpp.cpp

void ManagedClass::MethodWhichWillCallMyUnmanagedClass()
{
::AfxBeginThread(SomeUnmanagedClass::ThreadProc, this);
}

#pragma unmanaged
UINT SomeUnmanagedClass::ThreadProc(LPVOID pArg)
{
//some work
return TRUE;
}

error C3867: 'SomeUnmanagedClass::ThreadProc': function call missing argument list; use '&SomeUnmanagedClass::ThreadProc' to create a pointer to member

Any ideas?
Chris

Nish Nishant 16-Feb-11 8:31am

Make that a static method or a global method. AfxBeginThread will not take a member function.

cr101037 16-Feb-11 8:49am

Hi Nishant Sivakumar, thanks again for your posts. I've managed to get the code to compile (by making the method static [thanks for that!]). The good news is it spawns the new thread which calls my underlying unmanaged C++ dll. The bad new is when the execution gets to the PumpMessage line it still hangs.

I've tried MFC UI thread, MFC Worker thread and Managed thread... all of which hang on PumpMessage!

Nish Nishant 16-Feb-11 8:52am

Well, I should have picked that from your last post, but what you created is an MFC worker thread, NOT an MFC UI thread.

Read this : http://www.codeproject.com/KB/threads/usinguithreads.aspx

You need to specifically create a UI thread so your thread will have a message loop.

cr101037 16-Feb-11 9:32am

Hi Nishant Sivakumar,
I've updated my code to use an MFC UI thread, like this.

//MyHeader.h

public class SomeUnmanagedClass : public CWinThread
{
public:
virtual BOOL InitInstance() { return TRUE; };
virtual int Run();

private:
UINT MethodToRunOnMFCUIThread();
};

And in my cpp...

//MyCpp.cpp

#pragma unmanaged
int SomeUnmanagedClass::Run()
{
MethodToRunOnMFCUIThread();
return (0);
}

UINT SomeUnmanagedClass::MethodToRunOnMFCUIThread()
{
//some work
return TRUE;
}

Finally, from my managed code I make use of it like this...

SomeUnmanagedClass* trd = new SomeUnmanagedClass();
trd->CreateThread();

The call to CreateThread() will in turn call Run() which calls 'MethodToRunOnMFCUIThread' that does the work.
Does this look ok? Looks ok to me but yet it still hangs on PumpMessage!

Nish Nishant 16-Feb-11 11:41am

Ok, assuming you have not made a careless mistake somewhere, I am out of ideas. If I get time this evening, I'll try and recreate your scenario by creating an MFC DLL, and then having a C++/CLI DLL wrap it and call it through a thread, and then have a C# caller call the C++/CLI DLL.

Hopefully I will get your same error which will give me a chance to figure out how to fix it.

cr101037 21-Feb-11 7:32am

any luck?

Olivier Levrey · Answer 2 · 2011-02-14T01:53:00

Solution 1

If everything is related to the PumpMessage call, then can't you just remove the call?

Are you doing the call on the current thread or the main app's thread? In the former case, there shouldn't be any problem, but in the later you would be pumping the main thread's messages from 2 different threads... it would lead to some unexpected results.

Posted 14-Feb-11 1:53am

Olivier Levrey

Comments

cr101037 14-Feb-11 9:35am

Hi Olivier Levrey, the underlying unmanaged code is part of an existing system and I'm not able to modify it. My task was to simply remove the MFC UI and replace it with managed code.