FltReadFile function (fltkernel.h)

Article
11/15/2023

FltReadFile reads data from an open file, stream, or device.

Syntax

NTSTATUS FLTAPI FltReadFile(
  [in]            PFLT_INSTANCE                    InitiatingInstance,
  [in]            PFILE_OBJECT                     FileObject,
  [in, optional]  PLARGE_INTEGER                   ByteOffset,
  [in]            ULONG                            Length,
  [out]           PVOID                            Buffer,
  [in]            FLT_IO_OPERATION_FLAGS           Flags,
  [out, optional] PULONG                           BytesRead,
  [in, optional]  PFLT_COMPLETED_ASYNC_IO_CALLBACK CallbackRoutine,
  [in, optional]  PVOID                            CallbackContext
);

Parameters

[in] InitiatingInstance

Opaque instance pointer for the minifilter driver instance that the operation is to be sent to. The instance must be attached to the volume where the file resides. This parameter is required and cannot be NULL.

[in] FileObject

Pointer to a FILE_OBJECT for the file that the data is to be read from. This file object must be currently open. Calling FltReadFile 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.

[in, optional] ByteOffset

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

If ByteOffset is specified, the I/O is performed at that offset, regardless of the current value of the file object’s CurrentByteOffset field.

If the file was opened for synchronous I/O (FO_SYNCHRONOUS_IO is set in the file object’s Flags field), the caller can set ByteOffset->LowPart to FILE_USE_FILE_POINTER_POSITION and ByteOffset->HighPart to -1 to have the I/O performed at the file object’s CurrentByteOffset field. If the file wasn't opened for synchronous I/O, using FILE_USE_FILE_POINTER_POSITION is an error.

If ByteOffset isn't specified:

If the file wasn't opened for synchronous I/O, that's an error.
Otherwise, the I/O is performed at the file object’s CurrentByteOffset.

If the file object was opened for synchronous I/O, the CurrentByteOffset field gets updated unless the caller passes the FLTFL_IO_OPERATION_DO_NOT_UPDATE_BYTE_OFFSET flag.

Note: the file system still updates CurrentByteOffset in this case. Filter Manager saves the CurrentByteOffset value before sending the I/O down the stack and restores it when the I/O returns. From the perspective of the caller of FltReadFile (and filters at higher altitudes) the CurrrentByteOffset isn't updated. But filters below the caller see the updated CurrentByteOffset value in their post-read/write callbacks.

If the file object wasn't opened for synchronous I/O, the CurrentByteOffset field isn't updated regardless of the state of the ByteOffset parameter.

[in] Length

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

[out] Buffer

Pointer to a caller-allocated buffer that receives the data that is read from the file.

[in] Flags

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

Flag	Meaning
FLTFL_IO_OPERATION_DO_NOT_UPDATE_BYTE_OFFSET	Minifilter drivers can set this flag to specify that FltReadFile should not update the file object's CurrentByteOffset field.
FLTFL_IO_OPERATION_NON_CACHED	Minifilter drivers can set this flag to specify a noncached read, even if the file object was not opened with FILE_NO_INTERMEDIATE_BUFFERING.
FLTFL_IO_OPERATION_PAGING	Minifilter drivers can set this flag to specify a paging read.
FLTFL_IO_OPERATION_SYNCHRONOUS_PAGING	Minifilter drivers can set this flag to specify a synchronous paging I/O read. Minifilter drivers that set this flag must also set the FLTFL_IO_OPERATION_PAGING flag. Available starting in Windows Vista.

[out, optional] BytesRead

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

[in, optional] CallbackRoutine

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

[in, optional] 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

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

Remarks

A minifilter driver calls FltReadFile to read data from an open file.

FltReadFile creates a read request and sends it 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 read request.

FltReadFile 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 FltReadFile:

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 an attempt is made to read beyond the end of the file, FltReadFile returns an error.

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

If the value of the CallbackRoutine parameter is NULL, the read operation is performed synchronously. That is, FltReadFile waits until the read 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 FltReadFile 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, FltReadFile differs from ZwReadFile.

Requirements


Target Platform	Universal
Header	fltkernel.h (include Fltkernel.h)
Library	FltMgr.lib
DLL	Fltmgr.sys
IRQL	PASSIVE_LEVEL