FsRtlMdlReadDev routine

The FsRtlMdlReadDev routine returns a memory descriptor list (MDL) that points directly to the specified byte range in the file cache.


BOOLEAN FsRtlMdlReadDev(
  _In_     PFILE_OBJECT     FileObject,
  _In_     PLARGE_INTEGER   FileOffset,
  _In_     ULONG            Length,
  _In_     ULONG            LockKey,
  _Out_    PMDL             *MdlChain,
  _Out_    PIO_STATUS_BLOCK IoStatus,
  _In_opt_ PDEVICE_OBJECT   DeviceObject


FileObject [in]

A pointer to a file object for the cached file.

FileOffset [in]

A pointer to a variable that specifies the starting byte offset within the cached file 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 nonexclusively, 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).

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.

DeviceObject [in, optional]

The device object for the device that holds the file data.

Return value

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


The FsRtlMdlReadDev routine bypasses the usual IRP mechanism and returns a linked list of memory descriptor lists (MDL) that the caller can use to directly access the cached file data. This operation does not copy or buffer data and therefore is much faster than a normal read.

The routine is similar to CcMdlRead. Both FsRtlMdlReadDev and CcMdlRead lock the pages that contain the cached file data to prevent the system from swapping these pages to the page file. The pages remain locked in memory until the caller invokes the FsRtlMdlReadCompleteDev routine.


Target platform



Ntifs.h (include Ntifs.h)







See also




Send comments about this topic to Microsoft