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Debugging DirectShow Filters

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[The feature associated with this page, DirectShow, is a legacy feature. It has been superseded by MediaPlayer, IMFMediaEngine, and Audio/Video Capture in Media Foundation. Those features have been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaPlayer, IMFMediaEngine and Audio/Video Capture in Media Foundation instead of DirectShow, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

Many of the debugging facilities described in this topic are implemented in the DirectShow base class library. For more information, see DirectShow Base Classes.

Assertion Checking

Use assertion checking liberally. Assertions can verify assumptions that you make in your code about preconditions and postconditions. DirectShow provides several assertion macros. For more information, see Assert and Breakpoint Macros.

Object Names

In debug builds, there is a global list of objects that derive from the CBaseObject class. As objects are created, they are added to the list. When they are destroyed, they are removed from the list. To display a list of those objects, call the DbgDumpObjectRegister function.

The constructor method for the CBaseObject base class—and most classes derived from it—includes a parameter for the name of the object. Give your objects unique names to identify them. Use the NAME macro when you declare the name, so that the name is allocated only in debug builds. In retail builds, the name becomes NULL.

Debug Logging

The DbgLog function displays debugging messages as your program executes. Use this function to trace the execution of your application or filter. You can log return codes, the values of variables, and any other relevant information.

Every debug message has a type, such as LOG_TRACE or LOG_ERROR, and a debug level, which indicates the importance of the message. Messages with lower debug levels are more important than those with higher levels.

In the following example, a hypothetical filter disconnects a pin from an array of pins. If the disconnect attempt succeeds, the filter displays a LOG_TRACE message. If the attempt fails, it displays a LOG_ERROR message:

hr = m_PinArray[iPin]->Disconnect();
if (FAILED(hr))
{
    DbgLog((
        LOG_ERROR, 
        1, 
        TEXT("Could not disconnect pin %d. HRESULT = %#x", 
        iPin, 
        hr
        ));
 
   // Error handling code (not shown).
}
DbgLog((LOG_TRACE, 3, TEXT("Disconnected pin %d"), iPin));

When you are debugging, you can set the debug level for each message type. Also, each module (DLL or executable) maintains its own debugging levels. If you are testing a filter, raise the debug levels for the DLL that contains the filter.

For more information, see Debug Output Functions.

Critical Sections

To make deadlocks easier to track, include assertions that determine whether the calling code owns a given critical section. The CritCheckIn and CritCheckOut functions indicate whether the calling thread owns a critical section. Typically, you would call these functions from inside an assert macro.

You can also use the DbgLockTrace function to trace when critical sections are held or released.

Debugging in DirectShow