The EvtSerCx2CustomReceiveTransactionStart event callback function is called by version 2 of the serial framework extension (SerCx2) to start a custom-receive transaction.



VOID EvtSerCx2CustomReceiveTransactionStart(
  _In_ WDFREQUEST                     Request,
  _In_ PMDL                           Mdl,
  _In_ ULONG                          Offset,
  _In_ ULONG                          Length
{ ... }


CustomReceiveTransaction [in]

A SERCX2CUSTOMRECEIVETRANSACTION handle to a custom-receive-transaction object. The serial controller driver previously called the SerCx2CustomReceiveTransactionCreate method to create this object.

Request [in]

A handle to the framework request object associated with the custom-receive transaction. The driver is responsible for completing this request. This request might not be the IRP_MJ_READ request sent by the client, and thus the serial controller driver should not try to use this request to access the read buffer. This request is primarily used for cancellation, completion, and queue forwarding (if needed). To access the read buffer for the client's read request, use the Mdl, Offset, and Length parameters.

Mdl [in]

A pointer to an MDL that describes the memory pages that are spanned by the read buffer for the custom-receive transaction. The scatter/gather list for the data transfer will use the region of this memory that is specified by the Offset and Length parameters.

Offset [in]

The starting offset for the data transfer. This parameter is a byte offset from the start of the buffer region described by the MDL. If the MDL specifies a total of N bytes of buffer space, possible values of Offset are in the range 0 to N–1.

Length [in]

The size, in bytes, of the data transfer. If the MDL specifies a total of N bytes of buffer space, possible values of Length are in the range 1 to N–Offset.

Return value



Your serial controller driver must implement this function if it creates a custom-receive-transaction object. The driver registers the function in the SerCx2CustomReceiveTransactionCreate call that creates this object.

After SerCx2 calls the EvtSerCx2CustomReceiveTransactionStart function, the serial controller driver initiates the transaction by programming the custom data-transfer mechanism to move data from the receive FIFO in the serial controller hardware to the buffer in the read request. Unless the request can be completed immediately, before the EvtSerCx2CustomReceiveTransactionStart function returns, the driver must call a method such as WdfRequestMarkCancelableEx to mark the request as cancelable.

After the EvtSerCx2CustomReceiveTransactionStart function starts the transaction, SerCx2 periodically calls the EvtSerCx2CustomReceiveTransactionQueryProgress event callback function to monitor progress made by the serial controller driver in performing this transaction. After the transaction finishes and the driver completes the pending read request, SerCx2 calls the EvtSerCx2CustomReceiveTransactionCleanup event callback function, if the driver implements this function.

If the serial controller driver implements an EvtSerCx2CustomReceiveTransactionInitialize event callback function, SerCx2 calls this function before calling the EvtSerCx2CustomReceiveTransactionStart function. Just before the EvtSerCx2CustomReceiveTransactionStart call, and after the EvtSerCx2CustomReceiveTransactionInitialize call returns, SerCx2 starts the timer that detects whether the read request times out. For more information, see the discussion of total time-outs in SERIAL_TIMEOUTS.

If the custom data-transfer mechanism is a bus-master DMA device, the EvtSerCx2CustomReceiveTransactionStart function can call a method such as WdfDmaTransactionInitializeUsingOffset to initiate a DMA transaction that uses the read buffer described by the Mdl, Offset, and Length parameters.

If the request object identified by the Request parameter contains storage for a private context, this storage might be uninitialized the first time the serial controller driver accesses the context. On first access, the driver typically should fill the context with zeros, and then, if needed, explicitly set any fields in the context that require nonzero initial values.

For more information, see SerCx2 Custom-Receive Transactions.


To define an EvtSerCx2CustomReceiveTransactionStart callback function, you must first provide a function declaration that identifies the type of callback function you're defining. Windows provides a set of callback function types for drivers. Declaring a function using the callback 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 an EvtSerCx2CustomReceiveTransactionStart callback function that is named MyCustomReceiveTransactionStart, use the EVT_SERCX2_CUSTOM_RECEIVE_TRANSACTION_START function type, as shown in this code example:


Then, implement your callback function as follows:

    WDFREQUEST  Request,
    PMDL  Mdl,
    ULONG  Offset,
    ULONG  Length

The EVT_SERCX2_CUSTOM_RECEIVE_TRANSACTION_START function type is defined in the Sercx.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 EVT_SERCX2_CUSTOM_RECEIVE_TRANSACTION_START 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 KMDF Drivers. For more information about _Use_decl_annotations_, see Annotating Function Behavior.


Target platform



Available starting with Windows 8.1.





See also




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