NtCreateFile function (winternl.h)
Creates a new file or directory, or opens an existing file, device, directory, or volume.
This function is the user-mode equivalent to the ZwCreateFile function documented in the Windows Driver Kit (WDK).
__kernel_entry NTSTATUS NtCreateFile( [out] PHANDLE FileHandle, [in] ACCESS_MASK DesiredAccess, [in] POBJECT_ATTRIBUTES ObjectAttributes, [out] PIO_STATUS_BLOCK IoStatusBlock, [in, optional] PLARGE_INTEGER AllocationSize, [in] ULONG FileAttributes, [in] ULONG ShareAccess, [in] ULONG CreateDisposition, [in] ULONG CreateOptions, [in] PVOID EaBuffer, [in] ULONG EaLength );
A pointer to a variable that receives the file handle if the call is successful.
The ACCESS_MASK value that expresses 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.
Do not specify FILE_READ_DATA, FILE_WRITE_DATA, FILE_APPEND_DATA, or FILE_EXECUTE when you create or open a directory.
Callers of NtCreateFile can specify one or a combination of the following, possibly using a bitwise-OR with additional compatible flags from the preceding DesiredAccess flags list, for any file object that does not represent a directory file.
The FILE_GENERIC_EXECUTE value is irrelevant for device and intermediate drivers.
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 NtCreateFile can specify one or a combination of the following, possibly using a bitwise-OR with one or more compatible flags from the preceding DesiredAccess flags list.
||Files in the directory can be listed.|
||The directory can be traversed: that is, it can be part of the pathname of a file.|
A pointer to a structure already initialized with InitializeObjectAttributes. Members of this structure for a file object include the following.
||Specifies the number of bytes of ObjectAttributes data supplied. This value must be at least sizeof(OBJECT_ATTRIBUTES).|
||Optionally specifies a handle to a directory obtained by a preceding call to NtCreateFile. If this value is NULL, the ObjectName member must be a fully qualified file specification that includes the full path to the target file. If this value is non-NULL, the ObjectName member specifies a file name relative to this directory.|
||Points to a buffered Unicode string that names the file to be created or opened. This value must be a fully qualified file specification or the name of a device object, unless it is the name of a file relative to the directory specified by RootDirectory. For example, \Device\Floppy1\myfile.dat or \??\B:\myfile.dat could be the fully qualified file specification, provided that the floppy driver and overlying file system are already loaded. For more information, see File Names, Paths, and Namespaces.|
||Is a set of flags that controls the file object attributes. This value can be zero or OBJ_CASE_INSENSITIVE, which indicates that name-lookup code should ignore the case of the ObjectName member rather than performing an exact-match search. The value OBJ_INHERIT is irrelevant to device and intermediate drivers.|
||Optionally specifies a security descriptor to be applied to a file. ACLs specified by such a security descriptor are applied to the file only when it is created. If the value is NULL when a file is created, the ACL placed on the file is file-system-dependent; most file systems propagate some part of such an ACL from the parent directory file combined with the caller's default ACL. Device and intermediate drivers can set this member to NULL.|
||Specifies the access rights a server should be given to the client's security context. This value is non-NULL only when a connection to a protected server is established, allowing the caller to control which parts of the caller's security context are made available to the server and whether the server is allowed to impersonate the caller.|
A pointer to a variable that receives the final completion status and information about the requested operation. On return from NtCreateFile, the Information member contains one of the following values:
[in, optional] AllocationSize
The initial allocation size in bytes for the file. A nonzero value has no effect unless the file is being created, overwritten, or superseded.
The file attributes. Explicitly specified attributes are applied only when the file is created, superseded, or, in some cases, overwritten. By default, this value is a FILE_ATTRIBUTE_NORMAL, which can be overridden by an ORed combination of one or more FILE_ATTRIBUTE_xxxx flags, which are defined in Wdm.h and NtDdk.h. For a list of flags that can be used with NtCreateFile, see CreateFile.
The type of share access that the caller would like to use in the file, as zero, or as one or a combination of the following values.
For more information, see the Windows SDK.
Specifies what to do, depending on whether the file already exists, as one of the following values.
The options to be applied when creating or opening the file, as a compatible combination of the following flags.
||The file being created or opened is a directory file. With this flag, the CreateDisposition parameter must be set to FILE_CREATE, FILE_OPEN, or FILE_OPEN_IF. With this flag, other compatible CreateOptions flags include only the following: FILE_SYNCHRONOUS_IO_ALERT, FILE_SYNCHRONOUS_IO _NONALERT, FILE_WRITE_THROUGH, FILE_OPEN_FOR_BACKUP_INTENT, and FILE_OPEN_BY_FILE_ID.|
||The file being opened must not be a directory file or this call fails. The file object being opened can represent a data file, a logical, virtual, or physical device, or a volume.|
||Applications that write data to the file must actually transfer the data into the file before any requested write operation is considered complete. This flag is automatically set if the CreateOptions flag FILE_NO_INTERMEDIATE _BUFFERING is set.|
||All accesses to the file are sequential.|
||Accesses to the file can be random, so no sequential read-ahead operations should be performed on the file by FSDs or the system.|
||The file cannot be cached or buffered in a driver's internal buffers. This flag is incompatible with the DesiredAccess FILE_APPEND_DATA flag.|
||All operations on the file are performed synchronously. Any wait on behalf of the caller is subject to premature termination from alerts. This flag also causes the I/O system to maintain the file position context. If this flag is set, the DesiredAccess SYNCHRONIZE flag also must be set.|
||All operations on the file are performed synchronously. Waits in the system to synchronize I/O queuing and completion are not subject to alerts. This flag also causes the I/O system to maintain the file position context. If this flag is set, the DesiredAccess SYNCHRONIZE flag also must be set.|
||Create a tree connection for this file in order to open it over the network. This flag is not used by device and intermediate drivers.|
||If the extended attributes on an existing file being opened indicate that the caller must understand EAs to properly interpret the file, fail this request because the caller does not understand how to deal with EAs. This flag is irrelevant for device and intermediate drivers.|
||Open a file with a reparse point and bypass normal reparse point processing for the file. For more information, see the Remarks section.|
||Delete the file when the last handle to it is passed to NtClose. If this flag is set, the DELETE flag must be set in the DesiredAccess parameter.|
||The file name that is specified by the ObjectAttributes parameter includes the 8-byte file reference number for the file. This number is assigned by and specific to the particular file system. If the file is a reparse point, the file name will also include the name of a device. Note that the FAT file system does not support this flag. This flag is not used by device and intermediate drivers.|
||The file is being opened for backup intent. Therefore, the system should check for certain access rights and grant the caller the appropriate access to the file before checking the DesiredAccess parameter against the file's security descriptor. This flag not used by device and intermediate drivers.|
||This flag allows an application to request a filter opportunistic lock (oplock) to prevent other applications from getting share violations. If there are already open handles, the create request will fail with STATUS_OPLOCK_NOT_GRANTED. For more information, see the Remarks section.|
The file is being opened and an opportunistic lock (oplock) on the file is being requested as a single atomic operation. The file system checks for oplocks before it performs the create operation and will fail the create with a return code of STATUS_CANNOT_BREAK_OPLOCK if the result would be to break an existing oplock. For more information, see the Remarks section.Windows Server 2008, Windows Vista, Windows Server 2003 and Windows XP: This flag is not supported.
This flag is supported on the following file systems: NTFS, FAT, and exFAT.
||Complete this operation immediately with an alternate success code of STATUS_OPLOCK_BREAK_IN_PROGRESS if the target file is oplocked, rather than blocking the caller's thread. If the file is oplocked, another caller already has access to the file. This flag is not used by device and intermediate drivers.|
Pointer to an EA buffer used to pass extended attributes.
Length of the EA buffer.
NtCreateFile returns either 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. To simplify this check, an application can use the NT_SUCCESS, NT_ERROR, and NT_WARNING macros.
The handle, given by NtCreateFile, can be used by subsequent calls to manipulate data within the file or the file object's state or attributes.
There are two alternate ways to specify the name of the file to be created or opened with NtCreateFile:
- As a fully qualified pathname, supplied in the ObjectName member of the input ObjectAttributes
- As a pathname relative to the directory file represented by the handle in the RootDirectory member of the input ObjectAttributes
- For a caller to synchronize an I/O completion by waiting on the returned FileHandle, the SYNCHRONIZE flag must be set.
- Passing a zero to DesiredFlags is not valid.
- 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 is automatically 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.
For a shared file to be successfully opened, the requested DesiredAccess parameter to the file must be compatible with both the DesiredAccess and ShareAccess specifications of all preceding opens that have not yet been released with NtClose. That is, the DesiredAccess parameter specified to NtCreateFile for a given file must not conflict with the accesses that other openers of the file have disallowed.
The CreateDisposition value FILE_SUPERSEDE requires that the caller have DELETE access to an existing file object. If so, a successful call to NtCreateFile with FILE_SUPERSEDE on an existing file effectively deletes that file, and then re-creates 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 an existing file and either of these CreateDisposition values, the file is 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 parameter.
- 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 NtCreateFile 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, Windows 3.1, and OS/2 operating systems.
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 NtXXXFile routines, including the following:
- Any optional ByteOffset passed to the NtReadFile or NtWriteFile function must be an integral of the sector size.
- The Length passed to NtReadFile or NtWriteFile, 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 adjusted in accordance with the alignment requirement of the underlying device. This information can be obtained by calling NtCreateFile to get a handle for the file object that represents the physical device, and then calling NtQueryInformationFile with that handle. For a list of the system FILE_XXX_ALIGNMENT values, see the Windows SDK documentation.
- Calls to NtSetInformationFile with the FileInformationClass parameter set to FilePositionInformation must specify an offset that is an integral of the sector size.
If the CreateOptions parameter specifies the FILE_OPEN_REPARSE_POINT flag and NtCreateFile opens a file with a reparse point, normal reparse processing does not occur and NtCreateFile attempts to directly open the reparse point file. If the FILE_OPEN_REPARSE_POINT flag is not specified, normal reparse point processing occurs for the file. In either case, if the open operation was successful, NtCreateFile returns STATUS_SUCCESS; otherwise, an error code. The NtCreateFile function never returns STATUS_REPARSE.
The CreateOptions parameter's FILE_OPEN_REQUIRING_OPLOCK flag eliminates the time between when you open the file and request an oplock that could potentially allow a third party to open the file, which would cause a sharing violation. An application can use the FILE_OPEN_REQUIRING_OPLOCK flag with NtCreateFile and then request any oplock. This ensures that an oplock owner will be notified of any subsequent open request that causes a sharing violation.
In Windows 7, if other handles exist on the file when an application uses this flag, the create operation will fail with STATUS_OPLOCK_NOT_GRANTED. This restriction no longer exists starting with Windows 8.
If this create operation would break an oplock that already exists on the file, then setting the FILE_OPEN_REQUIRING_OPLOCK flag will cause the create operation to fail with STATUS_CANNOT_BREAK_OPLOCK. The existing oplock will not be broken by this create operation.
An application that uses this flag must request an oplock after this call succeeds, or all subsequent attempts to open the file will be blocked without the benefit of normal oplock processing. Similarly, if this call succeeds but the subsequent oplock request fails, an application that uses this flag must close its handle after it detects that the oplock request has failed.
The CreateOptions parameter's FILE_RESERVE_OPFILTER flag allows an application to request a Level 1, Batch, or Filter oplock to prevent other applications from getting share violations. However, in practical terms, the FILE_RESERVE_OPFILTER is useful only for filter oplocks. To use it, you must complete the following steps:
- Issue a create request with CreateOptions of FILE_RESERVE_OPFILTER, DesiredAccess of exactly FILE_READ_ATTRIBUTES, and ShareAccess of exactly
(FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE). Possible failures are as follows:
- If there are already open handles, the create request fails with STATUS_OPLOCK_NOT_GRANTED, and the next requested oplock also fails.
- If you open with more access than FILE_READ_ATTRIBUTES or less sharing than
(FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE), the request fails with STATUS_OPLOCK_NOT_GRANTED.
- If the create request succeeds, request an oplock.
- Open another handle to the file to do I/O.
(FILE_READ_ATTRIBUTES | FILE_WRITE_ATTRIBUTES | FILE_READ_DATA | FILE_READ_EA | FILE_EXECUTE | SYNCHRONIZE | READ_CONTROL)and still not break a filter oplock. However, any DesiredAccess greater than
(FILE_READ_ATTRIBUTES | FILE_WRITE_ATTRIBUTES | SYNCHRONIZE)will break a Level 1 or Batch oplock and make the FILE_RESERVE_OPFILTER flag useless for those oplock types.
NTFS is the only Microsoft file system that implements FILE_RESERVE_OPFILTER.
For more information on oplocks, see Oplock Semantics.
Note that the WDK header file NtDef.h is necessary for many constant definitions as well as the InitializeObjectAttributes macro. The associated import library, NtDll.lib is available in the WDK. To obtain the WDK, see Download kits for Windows hardware development. You can also use the LoadLibrary and GetProcAddress functions to dynamically link to NtDll.dll.