CreateFile

The CreateFile function creates or opens the following objects and returns a handle that can be used to access the object:

• Consoles
• Communications resources
• Directories (open only)
• Disk devices
• Files
• Mailslots
• Pipes

HANDLE CreateFile(
  LPCTSTR lpFileName,                         // file name
  DWORD dwDesiredAccess,                      // access mode
  DWORD dwShareMode,                          // share mode
  LPSECURITY_ATTRIBUTES lpSecurityAttributes, // SD
  DWORD dwCreationDisposition,                // how to create
  DWORD dwFlagsAndAttributes,                 // file attributes
  HANDLE hTemplateFile                        // handle to template file
);

Parameters

lpFileName

[in] Pointer to a null-terminated string that specifies the name of the object to create or open.

Windows NT/2000/XP: In the ANSI version of this function, the name is limited to MAX_PATH characters. To extend this limit to nearly 32,000 wide characters, call the Unicode version of the function and prepend "//?/" to the path. For more information, see File Name Conventions.

Windows 95/98/Me: This string must not exceed MAX_PATH characters.

dwDesiredAccess

[in] Specifies the type of access to the object. An application can obtain read access, write access, read/write access, or device query access. This parameter can be any combination of the following values.

Value	Meaning
0	Specifies device query access to the object. An application can query device attributes without accessing the device.
GENERIC_READ	Specifies read access to the object. Data can be read from the file and the file pointer can be moved. Combine with GENERIC_WRITE for read/write access.
GENERIC_WRITE	Specifies write access to the object. Data can be written to the file and the file pointer can be moved. Combine with GENERIC_READ for read/write access.

In addition, you can specify the following access flags.

Value	Documented
DELETE	Standard Access Rights
READ_CONTROL	Standard Access Rights
WRITE_DAC	Standard Access Rights
WRITE_OWNER	Standard Access Rights
SYNCHRONIZE	Standard Access Rights
STANDARD_RIGHTS_REQUIRED	Standard Access Rights
STANDARD_RIGHTS_READ	Standard Access Rights
STANDARD_RIGHTS_WRITE	Standard Access Rights
STANDARD_RIGHTS_EXECUTE	Standard Access Rights
STANDARD_RIGHTS_ALL	Standard Access Rights
SPECIFIC_RIGHTS_ALL	ACCESS_MASK
ACCESS_SYSTEM_SECURITY	ACCESS_MASK
MAXIMUM_ALLOWED	ACCESS_MASK
GENERIC_READ	ACCESS_MASK
GENERIC_WRITE	ACCESS_MASK
GENERIC_EXECUTE	ACCESS_MASK
GENERIC_ALL	ACCESS_MASK

dwShareMode

[in] Specifies how the object can be shared. If dwShareMode is 0, and CreateFile succeeds, the object cannot be shared and cannot be opened again until the handle is closed. Please refer the information concerning sharing violations in the Remarks section.

To share the object, use a combination of one or more of the following values.

Value	Meaning
FILE_SHARE_DELETE	Windows NT/2000/XP: Subsequent open operations on the object will succeed only if delete access is requested.
FILE_SHARE_READ	Subsequent open operations on the object will succeed only if read access is requested.
FILE_SHARE_WRITE	Subsequent open operations on the object will succeed only if write access is requested.

lpSecurityAttributes

[in] Pointer to a SECURITY_ATTRIBUTES structure that determines whether the returned handle can be inherited by child processes. If lpSecurityAttributes is NULL, the handle cannot be inherited.

Windows NT/2000/XP: The lpSecurityDescriptor member of the structure specifies a security descriptor for the object. If lpSecurityAttributes is NULL, the object gets a default security descriptor. The target file system must support security on files and directories for this parameter to have an effect on files.

dwCreationDisposition

[in] Specifies which action to take on files that exist, and which action to take when files do not exist. For more information about this parameter, see the Remarks section. This parameter must be one of the following values.

Value	Meaning
CREATE_NEW	Creates a new file. The function fails if the specified file already exists.
CREATE_ALWAYS	Creates a new file. If the file exists, the function overwrites the file, clears the existing attributes, and combines the file attributes and flags specified by dwFlagsAndAttributes with FILE_ATTRIBUTE_ARCHIVE.
OPEN_EXISTING	Opens the file. The function fails if the file does not exist. For a discussion of why you should use the OPEN_EXISTING flag if you are using the CreateFile function for devices, see Remarks.
OPEN_ALWAYS	Opens the file, if it exists. If the file does not exist, the function creates the file as if dwCreationDisposition were CREATE_NEW.
TRUNCATE_EXISTING	Opens the file. Once opened, the file is truncated so that its size is zero bytes. The calling process must open the file with at least GENERIC_WRITE access. The function fails if the file does not exist.

dwFlagsAndAttributes

[in] Specifies the file attributes and flags for the file.

Any combination of the following attributes is acceptable for the dwFlagsAndAttributes parameter, except all other file attributes override FILE_ATTRIBUTE_NORMAL.

Attribute	Meaning
FILE_ATTRIBUTE_ARCHIVE	The file should be archived. Applications use this attribute to mark files for backup or removal.
FILE_ATTRIBUTE_ENCRYPTED	The file or directory is encrypted. For a file, this means that all data in the file is encrypted. For a directory, this means that encryption is the default for newly created files and subdirectories. This flag has no effect if FILE_ATTRIBUTE_SYSTEM is also specified.
FILE_ATTRIBUTE_HIDDEN	The file is hidden. It is not to be included in an ordinary directory listing.
FILE_ATTRIBUTE_NORMAL	The file has no other attributes set. This attribute is valid only if used alone.
FILE_ATTRIBUTE_NOT_CONTENT_INDEXED	The file will not be indexed by the content indexing service.
FILE_ATTRIBUTE_OFFLINE	The data of the file is not immediately available. This attribute indicates that the file data has been physically moved to offline storage. This attribute is used by Remote Storage, the hierarchical storage management software. Applications should not arbitrarily change this attribute.
FILE_ATTRIBUTE_READONLY	The file is read only. Applications can read the file but cannot write to it or delete it.
FILE_ATTRIBUTE_SYSTEM	The file is part of or is used exclusively by the operating system.
FILE_ATTRIBUTE_TEMPORARY	The file is being used for temporary storage. File systems attempt to keep all of the data in memory for quicker access rather than flushing the data back to mass storage. A temporary file should be deleted by the application as soon as it is no longer needed.

Any combination of the following flags is acceptable for the dwFlagsAndAttributes parameter.

Flag	Meaning
FILE_FLAG_WRITE_THROUGH	Instructs the system to write through any intermediate cache and go directly to disk. The system can still cache write operations, but cannot lazily flush them.
FILE_FLAG_OVERLAPPED	Instructs the system to initialize the object, so that operations that take a significant amount of time to process return ERROR_IO_PENDING. When the operation is finished, the specified event is set to the signaled state. When you specify FILE_FLAG_OVERLAPPED, the file read and write functions must specify an OVERLAPPED structure. That is, when FILE_FLAG_OVERLAPPED is specified, an application must perform overlapped reading and writing. When FILE_FLAG_OVERLAPPED is specified, the system does not maintain the file pointer. The file position must be passed as part of the lpOverlapped parameter (pointing to an OVERLAPPED structure) to the file read and write functions. This flag also enables more than one operation to be performed simultaneously with the handle (a simultaneous read and write operation, for example).
FILE_FLAG_NO_BUFFERING	Instructs the system to open the file with no intermediate buffering or caching. When combined with FILE_FLAG_OVERLAPPED, the flag gives maximum asynchronous performance, because the I/O does not rely on the synchronous operations of the memory manager. However, some I/O operations will take longer, because data is not being held in the cache. An application must meet certain requirements when working with files opened with FILE_FLAG_NO_BUFFERING: File access must begin at byte offsets within the file that are integer multiples of the volume's sector size. File access must be for numbers of bytes that are integer multiples of the volume's sector size. For example, if the sector size is 512 bytes, an application can request reads and writes of 512, 1024, or 2048 bytes, but not of 335, 981, or 7171 bytes. Buffer addresses for read and write operations should be sector aligned (aligned on addresses in memory that are integer multiples of the volume's sector size). Depending on the disk, this requirement may not be enforced. One way to align buffers on integer multiples of the volume sector size is to use VirtualAlloc to allocate the buffers. It allocates memory that is aligned on addresses that are integer multiples of the operating system's memory page size. Because both memory page and volume sector sizes are powers of 2, this memory is also aligned on addresses that are integer multiples of a volume's sector size. An application can determine a volume's sector size by calling the GetDiskFreeSpace function.
FILE_FLAG_RANDOM_ACCESS	Indicates that the file is accessed randomly. The system can use this as a hint to optimize file caching.
FILE_FLAG_SEQUENTIAL_SCAN	Indicates that the file is to be accessed sequentially from beginning to end. The system can use this as a hint to optimize file caching. If an application moves the file pointer for random access, optimum caching may not occur; however, correct operation is still guaranteed. Specifying this flag can increase performance for applications that read large files using sequential access. Performance gains can be even more noticeable for applications that read large files mostly sequentially, but occasionally skip over small ranges of bytes.
FILE_FLAG_DELETE_ON_CLOSE	Indicates that the operating system is to delete the file immediately after all of its handles have been closed, not just the handle for which you specified FILE_FLAG_DELETE_ON_CLOSE. Subsequent open requests for the file will fail, unless FILE_SHARE_DELETE is used.
FILE_FLAG_BACKUP_SEMANTICS	Windows NT/2000/XP: Indicates that the file is being opened or created for a backup or restore operation. The system ensures that the calling process overrides file security checks, provided it has the necessary privileges. The relevant privileges are SE_BACKUP_NAME and SE_RESTORE_NAME. You can also set this flag to obtain a handle to a directory. A directory handle can be passed to some functions in place of a file handle.
FILE_FLAG_POSIX_SEMANTICS	Indicates that the file is to be accessed according to POSIX rules. This includes allowing multiple files with names, differing only in case, for file systems that support such naming. Use care when using this option because files created with this flag may not be accessible by applications written for MS-DOS or 16-bit Windows.
FILE_FLAG_OPEN_REPARSE_POINT	Specifying this flag inhibits the reparse behavior of NTFS reparse points. When the file is opened, a file handle is returned, whether the filter that controls the reparse point is operational or not. This flag cannot be used with the CREATE_ALWAYS flag.
FILE_FLAG_OPEN_NO_RECALL	Indicates that the file data is requested, but it should continue to reside in remote storage. It should not be transported back to local storage. This flag is intended for use by remote storage systems or the Hierarchical Storage Management system.

If the CreateFile function opens the client side of a named pipe, the dwFlagsAndAttributes parameter can also contain Security Quality of Service information. For more information, see Impersonation Levels. When the calling application specifies the SECURITY_SQOS_PRESENT flag, the dwFlagsAndAttributes parameter can contain one or more of the following values.

Value	Meaning
SECURITY_ANONYMOUS	Specifies to impersonate the client at the Anonymous impersonation level.
SECURITY_IDENTIFICATION	Specifies to impersonate the client at the Identification impersonation level.
SECURITY_IMPERSONATION	Specifies to impersonate the client at the Impersonation impersonation level.
SECURITY_DELEGATION	Specifies to impersonate the client at the Delegation impersonation level.
SECURITY_CONTEXT_TRACKING	Specifies that the security tracking mode is dynamic. If this flag is not specified, Security Tracking Mode is static.
SECURITY_EFFECTIVE_ONLY	Specifies that only the enabled aspects of the client's security context are available to the server. If you do not specify this flag, all aspects of the client's security context are available. This flag allows the client to limit the groups and privileges that a server can use while impersonating the client.

hTemplateFile

[in] Specifies a handle with GENERIC_READ access to a template file. The template file supplies file attributes and extended attributes for the file being created.

Windows 95/98/Me: The hTemplateFile parameter must be NULL. If you supply a handle, the call fails and GetLastError returns ERROR_NOT_SUPPORTED.

Return Values

If the function succeeds, the return value is an open handle to the specified file. If the specified file exists before the function call and dwCreationDisposition is CREATE_ALWAYS or OPEN_ALWAYS, a call to GetLastError returns ERROR_ALREADY_EXISTS (even though the function has succeeded). If the file does not exist before the call, GetLastError returns zero.

If the function fails, the return value is INVALID_HANDLE_VALUE. To get extended error information, call GetLastError.

Remarks

Use the CloseHandle function to close an object handle returned by CreateFile.

As noted above, specifying zero for dwDesiredAccess allows an application to query device attributes without actually accessing the device. This type of querying is useful, for example, if an application wants to determine the size of a floppy disk drive and the formats it supports without having a floppy in the drive.

Windows 95/98/Me: CreateFileW is supported by the Microsoft Layer for Unicode. To use this, you must add certain files to your application, as outlined in Microsoft Layer for Unicode on Windows 95/98/Me Systems.

Windows 2000/XP: A sharing violation will occur if an attempt is made to open a file or directory for deletion on a remote machine when the value of the dwDesiredAccess parameter is the DELETE access flag OR'ed with any other access flag, and the remote file or directory has not been opened with FILE_SHARE_DELETE share access. To avoid the sharing violation in this scenario, open the remote file or directory with DELETE access only or call DeleteFile without first opening the file or directory for deletion.

Windows 2000/XP: If CREATE_ALWAYS is specified as the value of the dwCreationDisposition parameter and FILE_ATTRIBUTE_NORMAL is specified as the value of the dwFlagsAndAttributes parameter, CreateFile will fail and set the error GetLastError reports to ACCESS_DENIED. In this case, set the value of the dwFlagsAndAttributes parameter to the ORed value of the FILE_ATTRIBUTE_HIDDEN and FILE_ATTRIBUTE_NORMAL attributes to avoid this error.

Files

If you are attempting to create a file on a floppy drive that does not have a floppy disk or a CD-ROM drive that does not have a CD, the system displays a message box asking the user to insert a disk or a CD, respectively. To prevent the system from displaying this message box, call the SetErrorMode function with SEM_FAILCRITICALERRORS.

When creating a new file, the CreateFile function performs the following actions:

• Clears the existing file attributes.
• Combines the file attributes and flags specified by dwFlagsAndAttributes with FILE_ATTRIBUTE_ARCHIVE.
• Sets the file length to zero.
• Copies the extended attributes supplied by the template file to the new file if the hTemplateFile parameter is specified.

When opening an existing file, CreateFile performs the following actions:

• Combines the file flags specified by dwFlagsAndAttributes with existing file attributes. CreateFile ignores the file attributes specified by dwFlagsAndAttributes.
• Sets the file length according to the value of dwCreationDisposition.
• Ignores the hTemplateFile parameter.
• Ignores the lpSecurityDescriptor member of the SECURITY_ATTRIBUTES structure if the lpSecurityAttributes parameter is not NULL. The other structure members are used. The bInheritHandle member is the only way to indicate whether the file handle can be inherited.

Windows NT/2000/XP: If you rename or delete a file, then restore it shortly thereafter, the system searches the cache for file information to restore. Cached information includes its short/long name pair and creation time.

Windows NT/2000/XP: Some file systems, such as NTFS, support compression or encryption for individual files and directories. On volumes formatted for such a file system, a new file inherits the compression and encryption attributes of its directory.

You cannot use the CreateFile function to set a file's compression state. Use the DeviceIoControl function to set a file's compression state.

When multiple processes try to open the same file, or a single process tries to open the file several times, a sharing violation can occur after the first time the file is opened if the second and subsequent open operations specify either a sharing mode that is different from the access mode specified in the first open operation, or the second and subsequent open operations specify an access mode that is different from the sharing mode specified in the first open operation. In other words, Windows cannot provide for conflicting sharing-mode requests in multiple open operations of a single file.

Two examples of this sharing violation scenario would be the following:

• If you specify the GENERIC_READ and GENERIC_WRITE access modes along with the FILE_SHARE_READ sharing mode in your first call to CreateFile. If you specify the GENERIC_WRITE access mode in your second call to CreateFile, then the function will fail with a sharing violation because the first call opened the file in read-only mode.
• If you specify the GENERIC_READ and GENERIC_WRITE access modes along with the FILE_SHARE_READ and FILE_SHARE_WRITE sharing modes in your first call to CreateFile. If you specify the GENERIC_READ and GENERIC_WRITE access modes and the FILE_SHARE_READ sharing mode only in your second call to CreateFile, the function will fail with a sharing violation because the read-only sharing mode specified in the second call conflicts with the read/write access that has been granted in the first call.

Pipes

If CreateFile opens the client end of a named pipe, the function uses any instance of the named pipe that is in the listening state. The opening process can duplicate the handle as many times as required but, once opened, the named pipe instance cannot be opened by another client. The access specified when a pipe is opened must be compatible with the access specified in the dwOpenMode parameter of the CreateNamedPipe function. For more information about pipes, see Pipes.

Mailslots

If CreateFile opens the client end of a mailslot, the function returns INVALID_HANDLE_VALUE if the mailslot client attempts to open a local mailslot before the mailslot server has created it with the CreateMailSlot function. For more information about mailslots, see Mailslots.

Communications Resources

The CreateFile function can create a handle to a communications resource, such as the serial port COM1. For communications resources, the dwCreationDisposition parameter must be OPEN_EXISTING, and the hTemplate parameter must be NULL. Read, write, or read/write access can be specified, and the handle can be opened for overlapped I/O. For more information about communications, see Communications.

Disk Devices

Volume handles may be opened as noncached at the discretion of the file system, even when the noncached option is not specified with CreateFile. You should assume that all Microsoft file systems open volume handles as noncached. The restrictions on noncached I/O for files apply to volumes as well.

A file system may or may not require buffer alignment even though the data is noncached. However, if the noncached option is specified when opening a volume, buffer alignment is enforced regardless of the file system on the volume. It is recommended on all file systems that you open volume handles as noncached and follow the noncached I/O restrictions.

Windows NT/2000/XP: You can use the CreateFile function to open a disk drive or a partition on a disk drive. The function returns a handle to the disk device; that handle can be used with the DeviceIOControl function. The following requirements must be met in order for such a call to succeed:

• The caller must have administrative privileges for the operation to succeed on a hard disk drive.
• The lpFileName string should be of the form //./PHYSICALDRIVEx to open the hard disk x. Hard disk numbers start at zero. For example:

For an example showing how to open a physical drive, see Calling DeviceIoControl on Windows NT/2000.

• The lpFileName string should be //./x: to open a floppy drive x or a partition x on a hard disk. For example:

There is no trailing backslash in a drive name. The string "//./c:/" refers to the root directory of drive C.

On Windows 2000 or later, you can also open a volume by referring to its unique volume name. In this case also, there should be no trailing backslash on the unique volume name.

Note that all I/O buffers should be sector aligned (aligned on addresses in memory that are integer multiples of the volume's sector size), even if the disk device is opened without the FILE_FLAG_NO_BUFFERING flag. Depending the disk, this requirement may not be enforced.

Windows 95/98/Me: This technique does not work for opening a logical drive. Specifying a string in this form causes CreateFile to return an error.

• The dwCreationDisposition parameter must have the OPEN_EXISTING value.
• When opening a floppy disk or a partition on a hard disk, you must set the FILE_SHARE_WRITE flag in the dwShareMode parameter.

Tape Drives

Windows NT/2000/XP: You can open tape drives using a file name of the form //./TAPEx where x is a number indicating which drive to open, starting with tape drive 0. To open tape drive 0 in C, use the file name "////.//TAPE0". For more information on manipulating tape drives for backup or other applications, see Tape Backup.

Consoles

The CreateFile function can create a handle to console input (CONIN$). If the process has an open handle to it as a result of inheritance or duplication, it can also create a handle to the active screen buffer (CONOUT$). The calling process must be attached to an inherited console or one allocated by the AllocConsole function. For console handles, set the CreateFile parameters as follows.

The following list shows the effects of various settings of fwdAccess and lpFileName.

Directories

An application cannot create a directory with CreateFile; it must call CreateDirectory or CreateDirectoryEx to create a directory.

Windows NT/2000/XP: You can obtain a handle to a directory by setting the FILE_FLAG_BACKUP_SEMANTICS flag. A directory handle can be passed to some functions in place of a file handle.

Some file systems, such as NTFS, support compression or encryption for individual files and directories. On volumes formatted for such a file system, a new directory inherits the compression and encryption attributes of its parent directory.

You cannot use the CreateFile function to set a directory's compression state. Use the DeviceIoControl function to set a directory's compression state.

When using CreateFile to open a directory during defragmentation of a FAT or FAT32 volume, do not specify the MAXIMUM_ALLOWED access mask value. Access to the directory will be denied if this is done. Specify the GENERIC_READ access mask value instead.

Example Code

For an example, see Creating and Opening Files.

Requirements

  Windows NT/2000/XP: Included in Windows NT 3.1 and later.
  Windows 95/98/Me: Included in Windows 95 and later.
  Header: Declared in Winbase.h; include Windows.h.
  Library: Use Kernel32.lib.
  Unicode: Implemented as Unicode and ANSI versions on Windows NT/2000/XP. Also supported by Microsoft Layer for Unicode.

BOOL WriteFile(
  HANDLE hFile,                    // handle to file
  LPCVOID lpBuffer,                // data buffer
  DWORD nNumberOfBytesToWrite,     // number of bytes to write
  LPDWORD lpNumberOfBytesWritten,  // number of bytes written
  LPOVERLAPPED lpOverlapped        // overlapped buffer
);