FltFastIoPrepareMdlWrite function (fltkernel.h)

Article
02/22/2024

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.

Syntax

BOOLEAN FLTAPI FltFastIoPrepareMdlWrite(
        PFLT_INSTANCE    InitiatingInstance,
  [in]  PFILE_OBJECT     FileObject,
  [in]  PLARGE_INTEGER   FileOffset,
  [in]  ULONG            Length,
  [in]  ULONG            LockKey,
  [out] PMDL             *MdlChain,
  [out] PIO_STATUS_BLOCK IoStatus
);

Parameters

InitiatingInstance

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

[in] FileObject

A pointer to the file object.

[in] FileOffset

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

[in] Length

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

[in] LockKey

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.

[out] MdlChain

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

[out] IoStatus

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.

Remarks

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.

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.

Requirements

Requirement	Value
Target Platform	Universal
Header	fltkernel.h (include Ntifs.h)
Library	NtosKrnl.lib
DLL	NtosKrnl.exe
IRQL	<= APC_LEVEL