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How to Complete an IRP in a Dispatch Routine

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If an input IRP can be completed immediately, a dispatch routine does the following:

  1. Sets the Status and Information members of the IRP's I/O status block with appropriate values, in general:

    • The dispatch routine sets Status either to STATUS_SUCCESS or to an appropriate error (STATUS_XXX), which can be the value returned by a call to a support routine or, for certain synchronous requests, by a lower driver.

      If a lower-level driver returns STATUS_PENDING, a higher-level driver should not call IoCompleteRequest for the IRP, with one exception: The higher-level driver can use an event to synchronize between its IoCompletion routine and its dispatch routine, in which case the IoCompletion routine signals the event and returns STATUS_MORE_PROCESSING_REQUIRED. The dispatch routine waits for the event and then calls IoCompleteRequest to complete the IRP.

    • It sets Information to the number of bytes successfully transferred if a request to transfer data, such as a read or write request, was satisfied.

    • It sets Information to a value that varies according to the specific request for other IRPs that it completes with STATUS_SUCCESS.

    • It sets Information to a value that varies according to the specific request for IRPs that it completes with a warning STATUS_XXX. For example, it would set Information to the number of bytes transferred for such a warning as STATUS_BUFFER_OVERFLOW.

    • Usually, it sets Information to zero for requests that it completes with an error STATUS_XXX.

  2. Calls IoCompleteRequest with the IRP and with PriorityBoost = IO_NO_INCREMENT.

  3. Returns the appropriate STATUS_XXX that it already set in the I/O status block. Note that a call to IoCompleteRequest makes the given IRP inaccessible by the caller, so the return value from a dispatch routine cannot be set from the I/O status block of an already completed IRP.

Follow this implementation guideline for calling IoCompleteRequest with an IRP:

Always release any spin lock(s) the driver is holding before calling IoCompleteRequest.

It takes an indeterminate amount of time to complete an IRP, particularly in a chain of layered drivers. Moreover, a deadlock can occur if a higher-level driver's IoCompletion routine sends an IRP back down to a lower driver that is holding a spin lock.