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FltReadFileEx function

FltReadFileEx reads data from an open file, stream, or device. This function extends FltReadFile to allow the optional use of an MDL for read data instead of a mapped buffer address.

Syntax


NTSTATUS WINAPI FltReadFileEx(
  _In_       PFLT_INSTANCE InitiatingInstance,
  _In_       PFILE_OBJECT FileObject,
  _In_opt_   PLARGE_INTEGER ByteOffset,
  _In_       ULONG Length,
  _Out_      PVOID Buffer,
  _In_       FLT_IO_OPERATION_FLAGS Flags,
  _Out_opt_  PULONG BytesRead,
  _In_opt_   PFLT_COMPLETED_ASYNC_IO_CALLBACK CallbackRoutine,
  _In_opt_   PVOID CallbackContext,
  _In_opt_   PULONG Key,
  _In_opt_   PMDL Mdl
);

Parameters

InitiatingInstance [in]

An opaque instance pointer for the minifilter driver instance that is initiating the read request. This parameter is required and cannot be NULL.

FileObject [in]

A pointer to a file object for the file that the data is to be read from. This file object must be currently open. Calling FltReadFileEx 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 [in, optional]

A pointer to a caller-allocated variable that specifies the starting byte offset within the file where the read 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, FltReadFileEx 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 FltReadFileEx 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, FltReadFileEx updates the file object's CurrentByteOffset field by adding the number of bytes read when it completes the read operation.

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

Length [in]

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

Buffer [out]

A pointer to a caller-allocated buffer that receives the data that is read from the file. If an MDL is provided in Mdl, Buffer must be NULL.

Flags [in]

A bitmask of flags that specify the type of read operation to be performed.

FlagMeaning

FLTFL_IO_OPERATION_DO_NOT_UPDATE_BYTE_OFFSET

Minifilter drivers can set this flag to specify that FltReadFileEx 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.

This flag is available for Windows Vista and later versions of the Windows operating system.

 

BytesRead [out, optional]

A 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.

CallbackRoutine [in, optional]

A 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.

CallbackContext [in, optional]

A 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.

Key [in, optional]

An optional key associated with a byte range lock.

Mdl [in, optional]

An optional MDL that describes the memory where the data is read. If a buffer is provided in Buffer , then Mdl must be NULL.

Return value

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

Remarks

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

FltReadFileEx 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.

FltReadFileEx 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 FltReadFileEx:

  • 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, FltReadFileEx 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, FltReadFileEx 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 FltReadFileEx 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, FltReadFileEx differs from ZwReadFile.

The Mdl parameter is provided as a convenience when a minifilter already has an MDL available. The MDL is used directly and the additional step of mapping an address for Buffer can be avoided.

Requirements

Version

The FltReadFileEx function is available starting with Windows 8.

Header

Fltkernel.h (include Fltkernel.h)

Library

FltMgr.lib

IRQL

PASSIVE_LEVEL

See also

FltAllocatePoolAlignedWithTag
FltCreateFile
FltCreateFileEx
FltWriteFile
FltWriteFileEx
ObReferenceObjectByHandle
PFLT_COMPLETED_ASYNC_IO_CALLBACK
ZwCreateFile
ZwReadFile

 

 

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