Posts tagged handle

How to find the GdiObject type from handle.

0
1445 days
by in Codeproject, GDI & GDI+


Gdi Objects are represented by handles. Basically all handles are just an integer inside. For instance, if you’re writing a function with accepts HFONT as parameter, its quite possible to pass a typecasted handle of other Gdi object. So how can you determine the real Gdi object type from the handle?


You can use the function – GetObjectType(). See the following code snippet.

// Get the Black Brush.
HGDIOBJ hGdiObj = GetStockObject( BLACK_BRUSH );

// Check the object type with handle.
if( OBJ_BRUSH == GetObjectType( hGdiObj ))
{
    // Yes! The handle points to Gdi Brush.
}

So now add better error checking in your gdi routines. ;)


The values that GetObjectType() returns are,

  • OBJ_BITMAP – Bitmap
  • OBJ_BRUSH – Brush
  • OBJ_COLORSPACE – Color space
  • OBJ_DC Device – Context
  • OBJ_ENHMETADC – Enhanced metafile DC
  • OBJ_ENHMETAFILE – Enhanced metafile
  • OBJ_EXTPEN – Extended pen
  • OBJ_FONT – Font
  • OBJ_MEMDC – Memory DC
  • OBJ_METAFILE – Metafile
  • OBJ_METADC – Metafile DC
  • OBJ_PAL – Palette
  • OBJ_PEN – Pen
  • OBJ_REGION – Region


Targeted Audience – Beginners.

GetCurrentThread() returns pseudo handle, not the real handle.

0
1519 days
by in Codeproject, Debugging Tips

Icon Description
For getting our thread handle, we usually call GetCurrentThread() api. But actually it returns a pseudo handle. A pseudo handle, is a special handle which represents the handle of current thread which uses it. Please see the following e.g. for more clarify.

1) Thread A gets its threadID by calling GetCurrentThread and Passed to Thread B.
2) Thread B needs to terminate Thread A. So it calls TerminateThread( handle passed by Thread A ).
3) But instead of terminating Thread A, Thread B will be terminated, because the handle passed by Thread A was pseudo and it will become Thread B’s handle when Thread B uses it.

Debug one GetCurrentThread() call and GetCurrentProcess() call and watch its return values. You can see what they returns are follows,

  • GetCurrentThread() – 0xfffffffe
  • GetCurrentProcess() – 0xffffffff

Ofcourse pseudo… isn’t it ?

Icon How Can I Do It?
For getting a real handle which represents your thread, create a duplicate handle by calling DuplicateHandle(). Please see the code block below.

HANDLE hRealHandle = 0;
DuplicateHandle( GetCurrentProcess(), // Source Process Handle.
                 GetCurrentThread(),  // Source Handle to dup.
                 GetCurrentProcess(), // Target Process Handle.
                 &hRealHandle,        // Target Handle pointer.
                 0,                   // Options flag.
                 TRUE,                // Inheritable flag
                 DUPLICATE_SAME_ACCESS );// Options
// Now the hRealHandle contains a real handle for your thread.
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