Update parent window using thread

Question

3.00/5 (1 vote)

See more:

Hi,
I am trying to print random numbers on a window using a thread named MyThread1 through C - Win32 programming.
But it does not update the main window and i see the main window being blank always.
Below is the pseudo code.

C++

VOID MyThread1( PVOID pvoid )
{
   HDC hdc;

   TCHAR szBuff[10];
   hdc = GetDC( hwnd );
   wsprintf( szBuff, TEXT("%d"), rand() );
   TextOut( hdc, rand(), rand(), szBuff, lstrlen( szBuff ) );
   ReleaseDC( hwnd );

}

LRESULT CALLBACK WndProc( HWND hwnd, UINT message, WPARAM, wParam, LPARAM lParam )
{
   switch( message )
   {
     case WM_CREATE:
          for( int i = 0; i < 10; i++ )
             _beginthread( MyThread1, 0, NULL );
     case WM_DESTROY:
          PostQuitMessage( 0 );
          break;
   }
   return DefWindowProc( hwnd, message, wParam, lParam );
}

Instead of using multiple threads i m using single thread to print random chars at random position on the main window.
Please let me know is this the right way or am i going wrong somewhere in between.

Thanks

Posted 2-Oct-11 6:43am

AshakiranBhatter

Updated 2-Oct-11 6:50am

André Kraak

v2

Add a Solution

Comments

André Kraak 2-Oct-11 12:51pm

Edited question:
Added pre tags

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Sergey Alexandrovich Kryukov · Accepted Answer · 2011-10-02T16:56:00

Writing on DC from the other thread is absolutely useless and even dangerous. All rendering should occur in the handler of a Windows message WM_PAINT. It always happens in your UI thread. So, the question is: how to make the rendered graphics changing with time and trigger this message? Well, not be sending or posting it. It's done via the Windows API Invalidate. In order to improve performance you can also call InvalidateRect or InvalidateRect with parameter (rectangle or region) to invalidate only the part of the scene.
See http://msdn.microsoft.com/en-us/library/dd145005%28v=vs.85%29.aspx[^].

Now, how to change graphics from a separate thread to create some animation effect? First of all, your thread makes no sense as it writes graphical data only once, but you could make the thread running in cycle with some delay, generating different text in each iteration. To do this, have a shared memory holding, say, the string you output. As this memory should be used by two threads (the string will be written by the thread you created and read by your UI thread), addressing to this memory should be inter-locked using critical section (this is the most light-weight method), via the methods EnterCriticalSection/LeaveCriticalSection, see:
http://en.wikipedia.org/wiki/Critical_section[^],
http://msdn.microsoft.com/en-us/library/windows/desktop/ms682608%28v=vs.85%29.aspx[^],
http://msdn.microsoft.com/en-us/library/windows/desktop/ms684169(v=vs.85).aspx[^].

You additional (non-UI) thread should only modify this data and notify your UI thread about the change by invalidating the client area of the window.

—SA