FltFastIoPrepareMdlWrite routine

The FltFastIoPrepareMdlWrite routine returns a linked list of memory descriptor lists (MDLs) that point to the specified range of cached file data to write data directly to the cache.


BOOLEAN FsRtlPrepareMdlWriteDev(
        PFLT_INSTANCE    InitiatingInstance,
  _In_  PFILE_OBJECT     FileObject,
  _In_  PLARGE_INTEGER   FileOffset,
  _In_  ULONG            Length,
  _In_  ULONG            LockKey,
  _Out_ PMDL             *MdlChain,



Opaque instance pointer for the caller. This parameter is required and cannot be NULL.

FileObject [in]

A pointer to the file object.

FileOffset [in]

A pointer to a value that specifies the starting byte offset within the cache that holds the data.

Length [in]

The length in bytes of the data to read from the cache.

LockKey [in]

A value that is associated with the byte range to lock. If the range to lock overlaps another range that is already locked with a nonexclusive lock, or if the range to read is a subrange of another range that is already locked non-exclusively, the value in this parameter must be the key for that nonexclusive lock. The lock must be held by the parent process of the calling thread. Otherwise, this parameter has no effect.

MdlChain [out]

On output, a pointer to a linked list of memory descriptor lists (MDLs) that point to the byte range within the cached data.

IoStatus [out]

A pointer to an IO_STATUS_BLOCK structure that, on output, contains the status of the transfer. If the operation succeeds, IoStatus.Status is set to STATUS_SUCCESS. Otherwise, it is set to an appropriate NTSTATUS error code. IoStatus.Information is set to the actual number of bytes that the routine successfully locked.

Return value

The FltFastIoPrepareMdlWrite routine returns TRUE if the operation succeeds and FALSE if the operation fails.


FltFastIoPrepareMdlWrite is similar to FsRtlCopyWrite, except that FltFastIoPrepareMdlWrite does not copy data to the cache. Instead, the physical pages that the caller will overwrite are locked in memory, and FltFastIoPrepareMdlWrite returns one or more memory descriptor lists (MDLs) that point to the specified byte range. The locked pages remain locked until the caller calls FltFastIoMdlWriteComplete. Thus each call to FltFastIoPrepareMdlWrite must be followed by a call to FltFastIoMdlWriteComplete.

The pages that the MDLs point to are locked in memory, but are not mapped in system space. The caller can perform this mapping by calling MmGetSystemAddressForMdlSafe.

Even if the call to FltFastIoPrepareMdlWrite fails, one or more MDLs might have been allocated. The caller can examine the value of IoStatus.Information to determine if this has occurred. If it has, the caller must call FltFastIoMdlWriteComplete to free the allocated MDLs.


Target platform



Ntifs.h (include Ntifs.h)







See also




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