On a receive queue, this is the number of PD_BUFFER structures that are available for placing incoming packets into. For example, if 10 buffers were posted, and 3 of the 10 have already been fetched by the provider for placing incoming packets into, this value should be 7. On a transmit queue, this is the number of PD_BUFFER structures that the provider has not yet fetched from the queue for transmitting.
This callback function does not return a value.
Note For IHV partners: The ability to monitor the queue depth is very important for clients to access congestion build-up and take precautionary action. An increasing queue depth for a transmit queue is a sign of increasing congestion on the outbound link. A decreasing queue depth for an RX queue is a sign of the client not being able to process incoming packets fast enough on the inbound link. Clients may need to monitor the queue depth status with frequently (For example, getting the queue depth for each post and operation, therefore the cost (in CPU cycles) of querying the queue depth must not be noticeable in comparison to the cost of a single PDPostAndDrainBufferList call.
To define a PDQueryQueueDepth function, you must first provide a function declaration that identifies the type of function you're defining. Windows provides a set of function types for drivers. Declaring a function using the function types helps Code Analysis for Drivers, Static Driver Verifier (SDV), and other verification tools find errors, and it's a requirement for writing drivers for the Windows operating system.
For example, to define a PDQueryQueueDepth function that is named "MyPDQueryQueueDepth", use the NDIS_PD_QUERY_QUEUE_DEPTH type as shown in this code example:
The NDIS_PD_QUERY_QUEUE_DEPTH function type is defined in the Ntddndis.h header file. To more accurately identify errors when you run the code analysis tools, be sure to add the _Use_decl_annotations_ annotation to your function definition. The _Use_decl_annotations_ annotation ensures that the annotations that are applied to the NDIS_PD_QUERY_QUEUE_DEPTH function type in the header file are used. For more information about the requirements for function declarations, see Declaring Functions by Using Function Role Types for NDIS Drivers.