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.