IoAllocateIrp

The IoAllocateIrp routine allocates an IRP, given the number of I/O stack locations for each driver layered under the caller, and, optionally, for the caller.

PIRP 
  IoAllocateIrp(
    IN CCHAR  StackSize,
    IN BOOLEAN  ChargeQuota
    );

Parameters

StackSize

Specifies the number of I/O stack locations to be allocated for the IRP. This value must be at least equal to the StackSize of the next-lower driver’s device object, but can be one greater than this value. The calling driver need not allocate a stack location in the IRP for itself.

ChargeQuota

Should be set to FALSE by intermediate drivers. This can be set to TRUE only by highest-level drivers that are called in the context of the thread that originates the I/O request for which the driver is allocating another IRP.

Return Value

IoAllocateIrp returns a pointer to an IRP, which was allocated from nonpaged system space, or NULL if an IRP could not be allocated.

Headers

Declared in wdm.h and ntddk.h. Include wdm.h or ntddk.h.

Comments

An intermediate or highest-level driver can call IoAllocateIrp to create IRPs for requests it sends to lower-level drivers. Such a driver must initialize the IRP and must set its IoCompletion routine in the IRP it creates so the caller can dispose of the IRP when lower-level drivers have completed processing of the request.

IoAllocateIrp automatically initializes the IRP's members. Do not use IoInitializeIrp to initialize the IRP before its first use. (You can use IoInitializeIrp to reuse an IRP that you have already used under certain special circumstances. See Reusing IRPs for details.)

An intermediate or highest-level driver also can call IoBuildDeviceIoControlRequest, IoBuildAsynchronousFsdRequest or IoBuildSynchronousFsdRequest to set up requests it sends to lower-level drivers. Only a highest-level driver can call IoMakeAssociatedIrp.

Callers of IoAllocateIrp must be running at IRQL <= DISPATCH_LEVEL.

See Also

IO_STACK_LOCATION, IoBuildAsynchronousFsdRequest, IoBuildDeviceIoControlRequest, IoBuildSynchronousFsdRequest, IoFreeIrp, IoMakeAssociatedIrp, IoReuseIrp, IoSetCompletionRoutine, IRP