IoCompletion routine

The IoCompletion routine completes the processing of I/O operations.



NTSTATUS IoCompletion(
  _In_     PDEVICE_OBJECT DeviceObject,
  _In_     PIRP           Irp,
  _In_opt_ PVOID          Context
{ ... }


DeviceObject [in]

Caller-supplied pointer to a DEVICE_OBJECT structure. This is the device object for the target device, previously created by the driver's AddDevice routine.

Irp [in]

Caller-supplied pointer to an IRP structure that describes the I/O operation.

Context [in, optional]

Caller-supplied pointer to driver-specific context information, previously supplied when calling IoSetCompletionRoutine or IoSetCompletionRoutineEx. Context information must be stored in nonpaged memory, since an IoCompletion routine can be called at DISPATCH_LEVEL. For more information, see the following Remarks section.

Return value

If the IoCompletion routine determines that additional processing is required for the IRP, it must return STATUS_MORE_PROCESSING_REQUIRED. For more information, see the following Remarks section. Otherwise, it should return STATUS_SUCCESS. (The I/O manager only checks for the presence or absence of STATUS_MORE_PROCESSING_REQUIRED.)


A driver's IoCompletion routine executes in an arbitrary thread or DPC context, and at an IRQL that is less than or equal to DISPATCH_LEVEL. Because code written to execute at DISPATCH_LEVEL will also execute at lower levels, IoCompletion routines should be designed for execution at DISPATCH_LEVEL. However, because these routines are not guaranteed to run at DISPATCH_LEVEL, they must not call system routines that actually require execution at DISPATCH_LEVEL. (For more information about IRQLs, see Managing Hardware Priorities.)

To register an IoCompletion routine for a specific IRP, a driver must call IoSetCompletionRoutine or IoSetCompletionRoutineEx, which stores the IoCompletion routine's address in the next-lower driver's I/O stack location. (Thus, a lowest-level driver cannot register an IoCompletion routine.) A driver typically calls IoSetCompletionRoutine or IoSetCompletionRoutineEx from one of its dispatch routines, each time an IRP is received. Most drivers, including all PnP drivers, can use IoSetCompletionRoutine to register their IoCompletion routine. Non-PnP drivers that may be unloaded before their IoCompletion routine executes should use IoSetCompletionRoutineEx instead.

When any driver completes an IRP, it calls IoCompleteRequest, which in turn calls the IoCompletion routine of each higher-level driver, from the next-highest to the highest, until all higher IoCompletion routines have been called or until one routine returns STATUS_MORE_PROCESSING_REQUIRED.

When you create the IRP, allocate a stack location for the current driver as well as any lower drivers. If you do not allocate sufficient stack locations, the DeviceObject pointer might be set to NULL when the completion routine is called. You can avoid allocating extra stack location for the current driver if you use the Context field to pass information to IoCompletion rather then relying on the DeviceObject parameter.

If an IoCompletion routine returns STATUS_MORE_PROCESSING_REQUIRED, the lower driver's call to IoCompleteRequest immediately returns. In this case, a higher-level driver will have to call IoCompleteRequest to complete the IRP.

For more information about implementing IoCompletion routines, see Completing IRPs.


To define an IoCompletion callback routine, you must first provide a function declaration that identifies the type of callback routine 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 IoCompletion callback routine that is named MyIoCompletion, use the IO_COMPLETION_ROUTINE type as shown in this code example:


Then, implement your callback routine as follows:

    PDEVICE_OBJECT  DeviceObject,
    PIRP  Irp,
    PVOID  Context
      // Function body

The IO_COMPLETION_ROUTINE function type is defined in the Wdm.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 IO_COMPLETION_ROUTINE 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 WDM Drivers. For information about _Use_decl_annotations_, see Annotating Function Behavior.


Target platform



Wdm.h (include Wdm.h, Ntddk.h, or Ntifs.h)


Called at IRQL <= DISPATCH_LEVEL (see Remarks section).



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