ZwReadFile routine

The ZwReadFile routine reads data from an open file.


  _In_     HANDLE           FileHandle,
  _In_opt_ HANDLE           Event,
  _In_opt_ PIO_APC_ROUTINE  ApcRoutine,
  _In_opt_ PVOID            ApcContext,
  _Out_    PIO_STATUS_BLOCK IoStatusBlock,
  _Out_    PVOID            Buffer,
  _In_     ULONG            Length,
  _In_opt_ PLARGE_INTEGER   ByteOffset,
  _In_opt_ PULONG           Key


FileHandle [in]

Handle to the file object. This handle is created by a successful call to ZwCreateFile or ZwOpenFile.

Event [in, optional]

Optionally, a handle to an event object to set to the signaled state after the read operation completes. Device and intermediate drivers should set this parameter to NULL.

ApcRoutine [in, optional]

This parameter is reserved. Device and intermediate drivers should set this pointer to NULL.

ApcContext [in, optional]

This parameter is reserved. Device and intermediate drivers should set this pointer to NULL.

IoStatusBlock [out]

Pointer to an IO_STATUS_BLOCK structure that receives the final completion status and information about the requested read operation. The Information member receives the number of bytes actually read from the file.

Buffer [out]

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

Length [in]

The size, in bytes, of the buffer pointed to by Buffer.

ByteOffset [in, optional]

Pointer to a variable that specifies the starting byte offset in the file where the read operation will begin. If an attempt is made to read beyond the end of the file, ZwReadFile returns an error.

If the call to ZwCreateFile set either of the CreateOptions flags FILE_SYNCHRONOUS_IO_ALERT or FILE_SYNCHRONOUS_IO_NONALERT, the I/O Manager maintains the current file position. If so, the caller of ZwReadFile can specify that the current file position offset be used instead of an explicit ByteOffset value. This specification can be made by using one of the following methods:

  • Specify a pointer to a LARGE_INTEGER value with the HighPart member set to -1 and the LowPart member set to the system-defined value FILE_USE_FILE_POINTER_POSITION.

  • Pass a NULL pointer for ByteOffset.

ZwReadFile updates the current file position by adding the number of bytes read when it completes the read operation, if it is using the current file position maintained by the I/O Manager.

Even when the I/O Manager is maintaining the current file position, the caller can reset this position by passing an explicit ByteOffset value to ZwReadFile. Doing this automatically changes the current file position to that ByteOffset value, performs the read operation, and then updates the position according to the number of bytes actually read. This technique gives the caller atomic seek-and-read service.

Key [in, optional]

Device and intermediate drivers should set this pointer to NULL.

Return value

ZwReadFile returns either STATUS_SUCCESS or the appropriate NTSTATUS error code.


Callers of ZwReadFile must have already called ZwCreateFile with the FILE_READ_DATA or GENERIC_READ value set in the DesiredAccess parameter.

If the preceding call to ZwCreateFile set the FILE_NO_INTERMEDIATE_BUFFERING flag in the CreateOptions parameter to ZwCreateFile, the Length and ByteOffset parameters to ZwReadFile must be multiples of the sector size. For more information, see ZwCreateFile.

ZwReadFile begins reading from the given ByteOffset or the current file position into the given Buffer. It terminates the read operation under one of the following conditions:

  • The buffer is full because the number of bytes specified by the Length parameter has been read. Therefore, no more data can be placed into the buffer without an overflow.

  • The end of file is reached during the read operation, so there is no more data in the file to be transferred into the buffer.

If the caller opened the file with the SYNCHRONIZE flag set in DesiredAccess, the calling thread can synchronize to the completion of the read operation by waiting on the file handle, FileHandle. The handle is signaled each time that an I/O operation that was issued on the handle completes. However, the caller must not wait on a handle that was opened for synchronous file access (FILE_SYNCHRONOUS_IO_NONALERT or FILE_SYNCHRONOUS_IO_ALERT). In this case, ZwReadFile waits on behalf of the caller and does not return until the read operation is complete. The caller can safely wait on the file handle only if all three of the following conditions are met:

  • The handle was opened for asynchronous access (that is, neither FILE_SYNCHRONOUS_IO_XXX flag was specified).

  • The handle is being used for only one I/O operation at a time.

  • ZwReadFile returned STATUS_PENDING.

A driver should call ZwReadFile in the context of the system process if any of the following conditions exist:

  • The driver created the file handle that it passes to ZwReadFile.

  • ZwReadFile will notify the driver of I/O completion by means of an event that the driver created.

  • ZwReadFile will notify the driver of I/O completion by means of an APC callback routine that the driver passes to ZwReadFile.

File and event handles are valid only in the process context where the handles are created. Therefore, to avoid security holes, the driver should create any file or event handle that it passes to ZwReadFile in the context of the system process rather than the context of the process that the driver is in.

Likewise, ZwReadFile should be called in the context of the system process if it notifies the driver of I/O completion by means of an APC, because APCs are always fired in the context of the thread that issues the I/O request. If the driver calls ZwReadFile in the context of a process other than the system one, the APC could be delayed indefinitely, or it might not fire at all.

For more information about working with files, see Using Files in a Driver.

Callers of ZwReadFile must be running at IRQL = PASSIVE_LEVEL and with special kernel APCs enabled.

Note  If the call to this function occurs in user mode, you should use the name "NtReadFile" instead of "ZwReadFile".


Target platform



Available starting with Windows 2000.


Wdm.h (include Wdm.h, Ntddk.h, or Ntifs.h)






PASSIVE_LEVEL (see Remarks section)

DDI compliance rules

PowerIrpDDis, BufAfterReqCompletedIntIoctlA, BufAfterReqCompletedIoctlA, BufAfterReqCompletedReadA, BufAfterReqCompletedWriteA, HwStorPortProhibitedDDIs

See also




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