CreateFile

CreateFile

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

·files

·pipes

·mailslots

·communications resources

·disk devices (Windows NT only)

·consoles

·directories (open only)

HANDLE CreateFile(

LPCTSTR lpFileName,	// pointer to name of the file
DWORD dwDesiredAccess,	// access (read-write) mode
DWORD dwShareMode,	// share mode
LPSECURITY_ATTRIBUTES lpSecurityAttributes,	// pointer to security attributes
DWORD dwCreationDistribution,	// how to create
DWORD dwFlagsAndAttributes,	// file attributes
HANDLE hTemplateFile	// handle to file with attributes to copy
);

Parameters

lpFileName

Points to a null-terminated string that specifies the name of the object (file, pipe, mailslot, communications resource, disk device, console, or directory) to create or open.

If *lpFileName is a path, there is a default string size limit of MAX_PATH characters. This limit is related to how the CreateFile function parses paths.

Windows NT: You can use paths longer than MAX_PATH characters by calling the wide (W) version of CreateFile and prepending "\\?\" to the path. The "\\?\" tells the function to turn off path parsing. This lets you use paths that are nearly 32,000 Unicode characters long. You must use fully-qualified paths with this technique. This also works with UNC names. The "\\?\" is ignored as part of the path. For example, "\\?\C:\myworld\private" is seen as "C:\myworld\private", and "\\?\UNC\tom_1\hotstuff\coolapps" is seen as "\\tom_1\hotstuff\coolapps".

dwDesiredAccess

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.

dwShareMode

Set of bit flags that specifies how the object can be shared. If dwShareMode is 0, the object cannot be shared. Subsequent open operations on the object will fail, until the handle is closed.

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

Value	Meaning
FILE_SHARE_DELETE	Windows NT only: 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

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: 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.

Windows 95: The lpSecurityDescriptor member of the structure is ignored.

dwCreationDistribution

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. The function overwrites the file if it exists.
OPEN_EXISTING	Opens the file. The function fails if the file does not exist.
	See the Remarks section for a discussion of why you should use the OPEN_EXISTING flag if you are using the CreateFile function for devices, including the console.
OPEN_ALWAYS	Opens the file, if it exists. If the file does not exist, the function creates the file as if dwCreationDistribution 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

Specifies the file attributes and flags for the file.

Any combination of the following attributes is acceptable, 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_COMPRESSED	The file or directory is compressed. For a file, this means that all of the data in the file is compressed. For a directory, this means that compression is the default for newly created files and subdirectories.
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_OFFLINE	The data of the file is not immediately available. Indicates that the file data has been physically moved to offline storage.
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.

Flag

Meaning

FILE_FLAG_WRITE_THROUGH

Instructs the operating system to write through any intermediate cache and go directly to disk. The operating system can still cache write operations, but cannot lazily flush them.

FILE_FLAG_OVERLAPPED

Instructs the operating system to initialize the object, so ReadFile, WriteFile, ConnectNamedPipe, and TransactNamedPipe operations that take a significant amount of time to process return ERROR_IO_PENDING. When the operation is finished, an event is set to the signaled state.

When you specify FILE_FLAG_OVERLAPPED, the ReadFile and WriteFile 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 operating 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 ReadFile and WriteFile 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 operating system to open the file with no intermediate buffering or caching. This can provide performance gains in some situations.

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 must be aligned on addresses in memory that are integer multiples of the volume's sector size.

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. Since 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. Windows 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. Windows 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 only: Indicates that the file is being opened or created for a backup or restore operation. The operating system ensures that the calling process overrides file security checks, provided it has the necessary permission to do so. The relevant permissions 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 Win32 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, Windows, or Windows NT.

If the CreateFile function opens the client side of a named pipe, the dwFlagsAndAttributes parameter can also contain Security Quality of Service information. 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.

For more information, see Security.

hTemplateFile

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: This value must be NULL. If you supply a handle under Windows 95, 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 dwCreationDistribution 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.

Files

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

·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 dwCreationDistribution.

·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.

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.

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 dwCreationDistribution 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

Windows NT: 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:

String	Meaning
\\.\PHYSICALDRIVE2	Obtains a handle to the third physical drive on the user's computer.

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

String	Meaning
\\.\A:	Obtains a handle to drive A on the user's computer.
\\.\C:	Obtains a handle to drive C on the user's computer.

Windows 95: This technique does not work for opening a logical drive. In Windows 95, specifying a string in this form causes CreateFile to return an error.

·The dwCreationDistribution 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.

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:

Parameters	Value
lpFileName	Use the CONIN$ value to specify console input and the CONOUT$ value to specify console output.
	CONIN$ gets a handle to the console's input buffer, even if the SetStdHandle function redirected the standard input handle. To get the standard input handle, use the GetStdHandle function.
	CONOUT$ gets a handle to the active screen buffer, even if SetStdHandle redirected the standard output handle. To get the standard output handle, use GetStdHandle.
dwDesiredAccess	GENERIC_READ \| GENERIC_WRITE is preferred, but either one can limit access.
dwShareMode	If the calling process inherited the console or if a child process should be able to access the console, this parameter must be FILE_SHARE_READ \| FILE_SHARE_WRITE.
lpSecurityAttributes	If you want the console to be inherited, the bInheritHandle member of the SECURITY_ATTRIBUTES structure must be TRUE.
dwCreationDistribution	You should specify OPEN_EXISTING when using CreateFile to open the console.
dwFlagsAndAttributes	Ignored.
hTemplateFile	Ignored.

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

lpFileName	fwdAccess	Result
CON	GENERIC_READ	Opens console for input.
CON	GENERIC_WRITE	Opens console for output.
CON	GENERIC_READ\ GENERIC_WRITE	Windows 95: Causes CreateFile to fail; GetLastError returns ERROR_PATH_NOT_FOUND. Windows NT: Causes CreateFile to fail; GetLastError returns ERROR_FILE_NOT_FOUND.

Directories

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

Windows NT:

You can obtain a handle to a directory by setting the FILE_FLAG_BACKUP_SEMANTICS flag. A directory handle can be passed to some Win32 functions in place of a file handle.

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