FltWriteFile function

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NTSTATUS FLTAPI FltWriteFile(
  PFLT_INSTANCE                    InitiatingInstance,
  PFILE_OBJECT                     FileObject,
  PLARGE_INTEGER                   ByteOffset,
  ULONG                            Length,
  PVOID                            Buffer,
  FLT_IO_OPERATION_FLAGS           Flags,
  PULONG                           BytesWritten,
  PFLT_COMPLETED_ASYNC_IO_CALLBACK CallbackRoutine,
  PVOID                            CallbackContext
);

Parameters

InitiatingInstance

Opaque instance pointer for the minifilter driver instance that is initiating the write request. This parameter is required and cannot be NULL.

FileObject

Pointer to a file object for the file that the data is to be written to. This file object must be currently open. Calling FltWriteFile when the file object is not yet open or is no longer open (for example, in a pre-create or post-cleanup callback routine) causes the system to ASSERT on a checked build. This parameter is required and cannot be NULL.

ByteOffset

Pointer to a caller-allocated variable that specifies the starting byte offset within the file where the write operation is to begin.

If this offset is supplied, or if the FLTFL_IO_OPERATION_DO_NOT_UPDATE_BYTE_OFFSET flag is specified in the Flags parameter, FltWriteFile does not update the file object's CurrentByteOffset field.

If the file object that FileObject points to was opened for synchronous I/O, the caller of FltWriteFile can specify that the current file position offset be used instead of an explicit ByteOffset value by setting this parameter to NULL. If the current file position is used, FltWriteFile updates the file object's CurrentByteOffset field by adding the number of bytes written when it completes the write operation.

If the file object that FileObject points to was opened for asynchronous I/O, this parameter is required and cannot be NULL.

Length

Size, in bytes, of the buffer that the Buffer parameter points to.

Buffer

Pointer to a buffer that contains the data to be written to the file. If the file is opened for noncached I/O, this buffer be must be aligned in accordance with the alignment requirement of the underlying storage device. Minifilter drivers can allocate such an aligned buffer by calling FltAllocatePoolAlignedWithTag.

Flags

Bitmask of flags specifying the type of write operation to be performed.

BytesWritten

Pointer to a caller-allocated variable that receives the number of bytes written to the file. If CallbackRoutine is not NULL, this parameter is ignored. Otherwise, this parameter is optional and can be NULL.

CallbackRoutine

Pointer to a PFLT_COMPLETED_ASYNC_IO_CALLBACK-typed callback routine to call when the write operation is complete. This parameter is optional and can be NULL.

CallbackContext

Context pointer to be passed to the CallbackRoutine if one is present. This parameter is optional and can be NULL. If CallbackRoutine is NULL, this parameter is ignored.

Return value

FltWriteFile returns the NTSTATUS value that was returned by the file system.

Remarks

A minifilter driver calls FltWriteFile to write data to an open file.

FltWriteFile causes a write request to be sent to the minifilter driver instances attached below the initiating instance and to the file system. The specified instance and the instances attached above it do not receive the write request.

FltWriteFile performs noncached I/O if either of the following is true:

• The caller set the FLTFL_IO_OPERATION_NON_CACHED flag in the Flags parameter.
• The file object was opened for noncached I/O. Usually, this is done by specifying the FILE_NO_INTERMEDIATE_BUFFERING CreateOptions flag in the preceding call to FltCreateFile, FltCreateFileEx, or ZwCreateFile.

Noncached I/O imposes the following restrictions on the parameter values passed to FltWriteFile:

• The buffer that the Buffer parameter points to must be aligned in accordance with the alignment requirement of the underlying storage device. To allocate such an aligned buffer, call FltAllocatePoolAlignedWithTag.
• The byte offset that the ByteOffset parameter points to must be a nonnegative multiple of the volume's sector size.
• The length specified in the Length parameter must be a nonnegative multiple of the volume's sector size.

If the value of the CallbackRoutine parameter is not NULL, the write operation is performed asynchronously.

If the value of the CallbackRoutine parameter is NULL, the write operation is performed synchronously. That is, FltWriteFile waits until the write operation is complete before returning. This is true even if the file object that FileObject points to was opened for asynchronous I/O.

If multiple threads call FltWriteFile for the same file object, and the file object was opened for synchronous I/O, the Filter Manager does not attempt to serialize I/O on the file. In this respect, FltWriteFile differs from ZwWriteFile.

Requirements