ZwCreateFile

NTSTATUS
ZwCreateFile(
OUT PHANDLE FileHandle,
IN ACCESS_MASK DesiredAccess,
IN POBJECT_ATTRIBUTES ObjectAttributes,
OUT PIO_STATUS_BLOCK IoStatusBlock,
IN PLARGE_INTEGER AllocationSize,/* optional */
IN ULONG FileAttributes,
IN ULONG ShareAccess,
IN ULONG CreateDisposition,
IN ULONG CreateOptions,
IN PVOID EaBuffer,/* optional */
IN ULONG EaLength
);

ZwCreateFile either causes a new file or directory to be created, or it opens an existing file, device, directory, or volume, giving the caller a handle for the file object. This handle can be used by subsequent calls to manipulate data within the file or the file object's state or attributes. For example, a driver might call this routine during initialization to open a file of microcode for its device.

Parameters

FileHandle

Points to a variable that receives the file handle if the call is successful.

DesiredAccess

Specifies the type of access that the caller requires to the file or directory. The set of system-defined DesiredAccess flags determines the following specific access rights for file objects:

Callers of ZwCreateFile can specify one or a combination of the following, possibly ORed with additional compatible flags from the preceding DesiredAccess Flags list, for any file object that does not represent a directory file:

The STANDARD_RIGHTS_XXX are predefined system values used to enforce security on system objects.

To open or create a directory file, as also indicated with the CreateOptions parameter, callers of ZwCreateFile can specify one or a combination of the following, possibly ORed with one or more compatible flags from the preceding DesiredAccess Flags list:

The FILE_READ_DATA, FILE_WRITE_DATA, FILE_EXECUTE, and FILE_APPEND_DATA DesiredAccess flags are incompatible with creating or opening a directory file.

ObjectAttributes

Points to a structure already initialized with InitializeObjectAttributes. Members of this structure for a file object include the following:

IoStatusBlock

Points to a variable that receives the final completion status and information about the requested operation. On return from ZwCreateFile, the Information member contains one of the following values:

FILE_CREATED
FILE_OPENED
FILE_OVERWRITTEN
FILE_SUPERSEDED
FILE_EXISTS
FILE_DOES_NOT_EXIST

AllocationSize

Optionally specifies the initial allocation size in bytes for the file. A nonzero value has no effect unless the file is being created, overwritten, or superseded.

FileAttributes

Explicitly specified attributes are applied only when the file is created, superseded, or, in some cases, overwritten. By default, this value is FILE_ATTRIBUTE_NORMAL, which can be overridden by any other flag or by an ORed combination of compatible flags. Possible FileAttributes flags include the following:

ShareAccess

Specifies the type of share access that the caller would like to the file, as zero, or as one or a combination of the following:

Device and intermediate drivers usually set ShareAccess to zero, which gives the caller exclusive access to the open file.

CreateDisposition

Specifies what to do, depending on whether the file already exists, as one of the following:

CreateOptions

Specifies the options to be applied when creating or opening the file, as a compatible combination of the following flags:

EaBuffer

For device and intermediate drivers, this parameter must be a NULL pointer.

EaLength

For device and intermediate drivers, this parameter must be zero.

Return Value

ZwCreateFile either returns STATUS_SUCCESS or an appropriate error status. If it returns an error status, the caller can find more information about the cause of the failure by checking the IoStatusBlock.

Comments

There are two alternate ways to specify the name of the file to be created or opened with ZwCreateFile:

1.As a fully qualified pathname, supplied in the ObjectName member of the input ObjectAttributes

2.As pathname relative to the directory file represented by the handle in the RootDirectory member of the input ObjectAttributes

Certain DesiredAccess flags and combinations of flags have the following effects:

·For a caller to synchronize an I/O completion by waiting on the returned FileHandle, the SYNCHRONIZE flag must be set. Otherwise, a caller that is a device or intermediate driver must synchronize an I/O completion by using an event object.

·If only the FILE_APPEND_DATA and SYNCHRONIZE flags are set, the caller can write only to the end of the file, and any offset information on writes to the file is ignored. However, the file will automatically be extended as necessary for this type of write operation.

·Setting the FILE_WRITE_DATA flag for a file also allows writes beyond the end of the file to occur. The file is automatically extended for this type of write, as well.

·If only the FILE_EXECUTE and SYNCHRONIZE flags are set, the caller cannot directly read or write any data in the file using the returned FileHandle: that is, all operations on the file occur through the system pager in response to instruction and data accesses. Device and intermediate drivers should not set the FILE_EXECUTE flag in DesiredAccess.

The ShareAccess parameter determines whether separate threads can access the same file, possibly simultaneously. Provided that both file openers have the privilege to access a file in the specified manner, the file can be successfully opened and shared. If the original caller of ZwCreateFile does not specify FILE_SHARE_READ, FILE_SHARE_WRITE, or FILE_SHARE_DELETE, no other open operations can be performed on the file: that is, the original caller is given exclusive access to the file.

In order for a shared file to be successfully opened, the requested DesiredAccess to the file must be compatible with both the DesiredAccess and ShareAccess specifications of all preceding opens that have not yet been released with ZwClose. That is, the DesiredAccess specified to ZwCreateFile for a given file must not conflict with the accesses that other openers of the file have disallowed.

The CreateDispostion value FILE_SUPERSEDE requires that the caller have DELETE access to a existing file object. If so, a successful call to ZwCreateFile with FILE_SUPERSEDE on an existing file effectively deletes that file, and then recreates it. This implies that, if the file has already been opened by another thread, it opened the file by specifying a ShareAccess parameter with the FILE_SHARE_DELETE flag set. Note that this type of disposition is consistent with the POSIX style of overwriting files.

The CreateDisposition values FILE_OVERWRITE_IF and FILE_SUPERSEDE are similar. If ZwCreateFile is called with a existing file and either of these CreateDisposition values, the file will be replaced.

Overwriting a file is semantically equivalent to a supersede operation, except for the following:

·The caller must have write access to the file, rather than delete access. This implies that, if the file has already been opened by another thread, it opened the file with the FILE_SHARE_WRITE flag set in the input ShareAccess.

·The specified file attributes are logically ORed with those already on the file. This implies that, if the file has already been opened by another thread, a subsequent caller of ZwCreateFile cannot disable existing FileAttributes flags but can enable additional flags for the same file. Note that this style of overwriting files is consistent with MS-DOS™, with DOS-based Windows™, and with OS/2®.

The CreateOptions FILE_DIRECTORY_FILE value specifies that the file to be created or opened is a directory file. When a directory file is created, the file system creates an appropriate structure on the disk to represent an empty directory for that particular file system's on-disk structure. If this option was specified and the given file to be opened is not a directory file, or if the caller specified an inconsistent CreateOptions or CreateDispostion value, the call to ZwCreateFile will fail.

The CreateOptions FILE_NO_INTERMEDIATE_BUFFERING flag prevents the file system from performing any intermediate buffering on behalf of the caller. Specifying this value places certain restrictions on the caller's parameters to other Zw..File routines, including the following:

·Any optional ByteOffset passed to ZwReadFile or ZwWriteFile must be an integral of the sector size.

·The Length passed to ZwReadFile or ZwWriteFile, must be an integral of the sector size. Note that specifying a read operation to a buffer whose length is exactly the sector size might result in a lesser number of significant bytes being transferred to that buffer if the end of the file was reached during the transfer.

·Buffers must be aligned in accordance with the alignment requirement of the underlying device. This information can be obtained by calling ZwCreateFile to get a handle for the file object that represents the physical device, and, then, calling ZwQueryInformationFile with that handle. For a list of the system FILE_XXX_ALIGNMENT values, see DEVICE_OBJECT in Chapter 12.

·Calls to ZwSetInformationFile with the FileInformationClass parameter set to FilePositionInformation must specify an offset that is an integral of the sector size.

The CreateOptions FILE_SYNCHRONOUS_IO_ALERT and FILE_SYNCHRONOUS_IO_NONALERT, which are mutually exclusive as their names suggest, specify that all I/O operations on the file are to be synchronous as long as they occur through the file object referred to by the returned FileHandle. All I/O on such a file is serialized across all threads using the returned handle. With either of these CreateOptions, the DesiredAccess SYNCHRONIZE flag must be set so that the I/O Manager will use the file object as a synchronization object. With either of these CreateOptions set, the I/O Manager maintains the "file position context" for the file object, an internal, current file position offset. This offset can be used in calls to ZwReadFile and ZwWriteFile. Its position also can be queried or set with ZwQueryInformationFile and ZwSetInformationFile.

Callers of ZwCreateFile must be running at IRQL PASSIVE_LEVEL.

See Also

InitializeObjectAttributes, DEVICE_OBJECT, IO_STATUS_BLOCK, ZwClose, ZwReadFile, ZwQueryInformationFile, ZwSetInformationFile, ZwWriteFile