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CreateFileA function (fileapi.h)

  • 2023-02-09
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In this article

Creates or opens a file or I/O device. The most commonly used I/O devices are as follows: file, filestream, directory, physical disk, volume, console buffer, tape drive, communications resource, mailslot, andpipe. The function returns a handle that can be used to access the file or device for various types ofI/O depending on the file or device and the flags and attributes specified.

To perform this operation as a transacted operation, which results in a handle that can be used for transactedI/O, use theCreateFileTransacted function.

Syntax

HANDLE CreateFileA(  [in]           LPCSTR                lpFileName,  [in]           DWORD                 dwDesiredAccess,  [in]           DWORD                 dwShareMode,  [in, optional] LPSECURITY_ATTRIBUTES lpSecurityAttributes,  [in]           DWORD                 dwCreationDisposition,  [in]           DWORD                 dwFlagsAndAttributes,  [in, optional] HANDLE                hTemplateFile);

Parameters

[in] lpFileName

The name of the file or device to be created or opened. You may use either forward slashes (/) or backslashes (\) in this name.

By default, the name is limited to MAX_PATH characters. To extend this limit to 32,767 wide characters, prepend "\\?\" to the path. For more information, seeNaming Files, Paths, and Namespaces.

Tip

Starting with Windows 10, Version 1607, you can opt-in to remove the MAX_PATH limitation without prepending "\\?\". See the "Maximum Path Length Limitation" section ofNaming Files, Paths, and Namespaces for details.

For information on special device names, seeDefining an MS-DOS Device Name.

To create a file stream, specify the name of the file, a colon, and then the name of the stream. For moreinformation, seeFile Streams.

[in] dwDesiredAccess

The requested access to the file or device, which can be summarized as read, write, both or 0 to indicate neither).

The most commonly used values areGENERIC_READ,GENERIC_WRITE, or both(GENERIC_READ | GENERIC_WRITE). For more information, seeGeneric Access Rights,File Security and Access Rights,File Access Rights Constants, andACCESS_MASK.

If this parameter is zero, the application can query certain metadata such as file, directory, or deviceattributes without accessing that file or device, even ifGENERIC_READ access wouldhave been denied.

You cannot request an access mode that conflicts with the sharing mode that is specified by thedwShareMode parameter in an open request that already has an open handle.

For more information, see the Remarks section of this topic andCreating and Opening Files.

[in] dwShareMode

The requested sharing mode of the file or device, which can be read, write, both, delete, all of these, ornone (refer to the following table). Access requests to attributes or extended attributes are not affected bythis flag.

If this parameter is zero andCreateFile succeeds, thefile or device cannot be shared and cannot be opened again until the handle to the file or device is closed.For more information, see the Remarks section.

You cannot request a sharing mode that conflicts with the access mode that is specified in an existingrequest that has an open handle.CreateFile would fail andtheGetLastError function would returnERROR_SHARING_VIOLATION.

To enable a process to share a file or device while another process has the file or device open, use acompatible combination of one or more of the following values. For more information about valid combinations ofthis parameter with thedwDesiredAccess parameter, seeCreating and Opening Files.

Note  The sharing options for each open handle remain in effect until that handle is closed, regardless of process context.
 
ValueMeaning
0
0x00000000
Prevents other processes from opening a file or device if they request delete, read, or write access.
FILE_SHARE_DELETE
0x00000004
Enables subsequent open operations on a file or device to request delete access.

Otherwise, other processes cannot open the file or device if they request delete access.

If this flag is not specified, but the file or device has been opened for delete access, the functionfails.

Note  Delete access allows both delete and rename operations.
 
FILE_SHARE_READ
0x00000001
Enables subsequent open operations on a file or device to request read access.

Otherwise, other processes cannot open the file or device if they request read access.

If this flag is not specified, but the file or device has been opened for read access, the functionfails.

FILE_SHARE_WRITE
0x00000002
Enables subsequent open operations on a file or device to request write access.

Otherwise, other processes cannot open the file or device if they request write access.

If this flag is not specified, but the file or device has been opened for write access or has a file mappingwith write access, the function fails.

[in, optional] lpSecurityAttributes

A pointer to aSECURITY_ATTRIBUTESstructure that contains two separate but related data members: an optional security descriptor, and a Booleanvalue that determines whether the returned handle can be inherited by child processes.

This parameter can beNULL.

If this parameter isNULL, the handle returned byCreateFile cannot be inherited by any child processes theapplication may create and the file or device associated with the returned handle gets a default securitydescriptor.

ThelpSecurityDescriptor member of the structure specifies aSECURITY_DESCRIPTOR for a file or device. Ifthis member isNULL, the file or device associated with the returned handle isassigned a default security descriptor.

CreateFile ignores thelpSecurityDescriptor member when opening an existing file or device, but continuesto use thebInheritHandle member.

ThebInheritHandle member of the structure specifies whether the returned handlecan be inherited.

For more information, see the Remarks section.

[in] dwCreationDisposition

An action to take on a file or device that exists or does not exist.

For devices other than files, this parameter is usually set toOPEN_EXISTING.

For more information, see the Remarks section.

This parameter must be one of the following values, which cannot be combined:

ValueMeaning
CREATE_ALWAYS
2
Creates a new file, always.

If the specified file exists and is writable, the function truncates the file, the function succeeds, andlast-error code is set toERROR_ALREADY_EXISTS (183).

If the specified file does not exist and is a valid path, a new file is created, the function succeeds, andthe last-error code is set to zero.

For more information, see the Remarks section of this topic.

CREATE_NEW
1
Creates a new file, only if it does not already exist.

If the specified file exists, the function fails and the last-error code is set toERROR_FILE_EXISTS (80).

If the specified file does not exist and is a valid path to a writable location, a new file is created.

OPEN_ALWAYS
4
Opens a file, always.

If the specified file exists, the function succeeds and the last-error code is set toERROR_ALREADY_EXISTS (183).

If the specified file does not exist and is a valid path to a writable location, the function creates afile and the last-error code is set to zero.

OPEN_EXISTING
3
Opens a file or device, only if it exists.

If the specified file or device does not exist, the function fails and the last-error code is set toERROR_FILE_NOT_FOUND (2).

For more information about devices, see the Remarks section.

TRUNCATE_EXISTING
5
Opens a file and truncates it so that its size is zero bytes, only if it exists.

If the specified file does not exist, the function fails and the last-error code is set toERROR_FILE_NOT_FOUND (2).

The calling process must open the file with theGENERIC_WRITE bit set as part ofthedwDesiredAccess parameter.

[in] dwFlagsAndAttributes

The file or device attributes and flags,FILE_ATTRIBUTE_NORMAL being the mostcommon default value for files.

This parameter can include any combination of the available file attributes(FILE_ATTRIBUTE_*). All other file attributes overrideFILE_ATTRIBUTE_NORMAL.

This parameter can also contain combinations of flags (FILE_FLAG_*) for control offile or device caching behavior, access modes, and other special-purpose flags. These combine with anyFILE_ATTRIBUTE_* values.

This parameter can also contain Security Quality of Service (SQOS) information by specifying theSECURITY_SQOS_PRESENT flag. Additional SQOS-related flags information is presented inthe table following the attributes and flags tables.

Note  WhenCreateFile opens an existing file, it generally combines the file flags with the file attributes of the existing file, and ignores any file attributes supplied as part ofdwFlagsAndAttributes. Special cases are detailed inCreating and Opening Files.
 
Some of the following file attributes and flags may only apply to files and not necessarily all other types of devices thatCreateFile can open. For additional information, see the Remarks section of this topic andCreating and Opening Files.

For more advanced access to file attributes, seeSetFileAttributes. For a complete listof all file attributes with their values and descriptions, seeFile Attribute Constants.

AttributeMeaning
FILE_ATTRIBUTE_ARCHIVE
32 (0x20)
The file should be archived. Applications use this attribute to mark files for backup or removal.
FILE_ATTRIBUTE_ENCRYPTED
16384 (0x4000)
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. For more information, seeFile Encryption.

This flag has no effect ifFILE_ATTRIBUTE_SYSTEM is also specified.

This flag is not supported on Home, Home Premium, Starter, or ARM editions of Windows.

FILE_ATTRIBUTE_HIDDEN
2 (0x2)
The file is hidden. Do not include it in an ordinary directory listing.
FILE_ATTRIBUTE_NORMAL
128 (0x80)
The file does not have other attributes set. This attribute is valid only if used alone.
FILE_ATTRIBUTE_OFFLINE
4096 (0x1000)
The data of a file is not immediately available. This attribute indicates that file data is 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
1 (0x1)
The file is read only. Applications can read the file, but cannot write to or delete it.
FILE_ATTRIBUTE_SYSTEM
4 (0x4)
The file is part of or used exclusively by an operating system.
FILE_ATTRIBUTE_TEMPORARY
256 (0x100)
The file is being used for temporary storage.

For more information, see theCaching Behavior section of thistopic.

 
FlagMeaning
FILE_FLAG_BACKUP_SEMANTICS
0x02000000
The file is being opened or created for a backup or restore operation. The system ensures that the calling process overrides file security checks when the process hasSE_BACKUP_NAME andSE_RESTORE_NAME privileges. For more information, seeChanging Privileges in a Token.

You must set this flag to obtain a handle to a directory. A directory handle can be passed to somefunctions instead of a file handle. For more information, see the Remarks section.

FILE_FLAG_DELETE_ON_CLOSE
0x04000000
The file is to be deleted immediately after all of its handles are closed, which includes the specified handle and any other open or duplicated handles.

If there are existing open handles to a file, the call fails unless they were all opened with theFILE_SHARE_DELETE share mode.

Subsequent open requests for the file fail, unless theFILE_SHARE_DELETE sharemode is specified.

FILE_FLAG_NO_BUFFERING
0x20000000
The file or device is being opened with no system caching for data reads and writes. This flag does not affect hard disk caching or memory mapped files.

There are strict requirements for successfully working with files opened withCreateFile using theFILE_FLAG_NO_BUFFERING flag, for details seeFile Buffering.

FILE_FLAG_OPEN_NO_RECALL
0x00100000
The file data is requested, but it should continue to be located in remote storage. It should not be transported back to local storage. This flag is for use by remote storage systems.
FILE_FLAG_OPEN_REPARSE_POINT
0x00200000
Normalreparse point processing will not occur;CreateFile will attempt to open the reparse point. When a file is opened, a file handle is returned, whether or not the filter that controls the reparse point is operational.

This flag cannot be used with theCREATE_ALWAYS flag.

If the file is not a reparse point, then this flag is ignored.

For more information, see the Remarks section.

FILE_FLAG_OVERLAPPED
0x40000000
The file or device is being opened or created for asynchronous I/O.

When subsequent I/O operations are completed on this handle, the event specified in theOVERLAPPED structure will be set to thesignaled state.

If this flag is specified, the file can be used for simultaneous read and write operations.

If this flag is not specified, then I/O operations are serialized, even if the calls to the read and writefunctions specify anOVERLAPPED structure.

For information about considerations when using a file handle created with this flag, see theSynchronous and Asynchronous I/O Handlessection of this topic.

FILE_FLAG_POSIX_SEMANTICS
0x01000000
Access will occur according to POSIX rules. This includes allowing multiple files with names, differing only in case, for file systems that support that naming. Use care when using this option, because files created with this flag may not be accessible by applications that are written for MS-DOS or 16-bit Windows.
FILE_FLAG_RANDOM_ACCESS
0x10000000
Access is intended to be random. The system can use this as a hint to optimize file caching.

This flag has no effect if the file system does not support cached I/O andFILE_FLAG_NO_BUFFERING.

For more information, see theCaching Behavior section of thistopic.

FILE_FLAG_SESSION_AWARE
0x00800000
The file or device is being opened with session awareness. If this flag is not specified, then per-session devices (such as a device using RemoteFX USB Redirection) cannot be opened by processes running in session 0. This flag has no effect for callers not in session 0. This flag is supported only on server editions of Windows.

Windows Server 2008 R2 and Windows Server 2008:  This flag is not supported before Windows Server 2012.

FILE_FLAG_SEQUENTIAL_SCAN
0x08000000
Access is intended to be sequential from beginning to end. The system can use this as a hint to optimize file caching.

This flag should not be used if read-behind (that is, reverse scans) will be used.

This flag has no effect if the file system does not support cached I/O andFILE_FLAG_NO_BUFFERING.

For more information, see theCaching Behavior section of thistopic.

FILE_FLAG_WRITE_THROUGH
0x80000000
Write operations will not go through any intermediate cache, they will go directly to disk.

For additional information, see theCaching Behavior section of thistopic.

 

ThedwFlagsAndAttributes parameter can also specify SQOS information. For moreinformation, seeImpersonation Levels. When thecalling application specifies theSECURITY_SQOS_PRESENT flag as part ofdwFlagsAndAttributes, it can also contain one or more of the following values.

Security flagMeaning
SECURITY_ANONYMOUS
Impersonates a client at the Anonymous impersonation level.
SECURITY_CONTEXT_TRACKING
The security tracking mode is dynamic. If this flag is not specified, the security tracking mode is static.
SECURITY_DELEGATION
Impersonates a client at the Delegation impersonation level.
SECURITY_EFFECTIVE_ONLY
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 allows the client to limit the groups and privileges that a server can use while impersonating theclient.

SECURITY_IDENTIFICATION
Impersonates a client at the Identification impersonation level.
SECURITY_IMPERSONATION
Impersonate a client at the impersonation level. This is the default behavior if no other flags are specified along with theSECURITY_SQOS_PRESENT flag.

[in, optional] hTemplateFile

A valid handle to a template file with theGENERIC_READ access right. The templatefile supplies file attributes and extended attributes for the file that is being created.

This parameter can beNULL.

When opening an existing file,CreateFile ignores thisparameter.

When opening a new encrypted file, the file inherits the discretionary access control list from its parentdirectory. For additional information, seeFile Encryption.

Return value

If the function succeeds, the return value is an open handle to the specified file, device, named pipe, ormail slot.

If the function fails, the return value isINVALID_HANDLE_VALUE. To get extendederror information, callGetLastError.

Remarks

CreateFile was originally developed specifically for fileinteraction but has since been expanded and enhanced to include most other types of I/O devices and mechanismsavailable to Windows developers. This section attempts to cover the varied issues developers may experience whenusingCreateFile in different contexts and with different I/Otypes. The text attempts to use the wordfile only when referring specifically to data stored in anactual file on a file system. However, some uses offile may be referring more generally to an I/Oobject that supports file-like mechanisms. This liberal use of the termfile is particularlyprevalent in constant names and parameter names because of the previously mentioned historical reasons.

When an application is finished using the object handle returned byCreateFile, use theCloseHandle function to close the handle. This not onlyfrees up system resources, but can have wider influence on things like sharing the file or device and committingdata to disk. Specifics are noted within this topic as appropriate.

Windows Server 2003 and Windows XP:  A sharing violation occurs if an attempt is made to open a file or directory for deletion on a remotecomputer when the value of thedwDesiredAccess parameter is theDELETE access flag (0x00010000)OR'ed with any other access flag, and the remote fileor directory has not been opened withFILE_SHARE_DELETE. To avoid the sharing violationin this scenario, open the remote file or directory with theDELETE access right only,or callDeleteFile without first opening the file ordirectory for deletion.

Some file systems, such as the NTFS file system, support compression or encryption for individual files anddirectories. On volumes that have a mounted file system with this support, a new file inherits the compressionand encryption attributes of its directory.

You cannot useCreateFile to control compression,decompression, or decryption on a file or directory. For more information, seeCreating and Opening Files,File Compression and Decompression,andFile Encryption.

Windows Server 2003 and Windows XP:  For backward compatibility purposes,CreateFile doesnot apply inheritance rules when you specify a security descriptor inlpSecurityAttributes. To support inheritance, functions that later query the securitydescriptor of this file may heuristically determine and report that inheritance is in effect. For moreinformation, seeAutomatic Propagation of Inheritable ACEs.

As stated previously, if thelpSecurityAttributes parameter isNULL, the handle returned byCreateFile cannot be inherited by any child processes yourapplication may create. The following information regarding this parameter also applies:

  • If thebInheritHandle member variable is notFALSE, which is any nonzero value, then the handle can be inherited. Therefore it is critical this structure member be properly initialized toFALSE if you do not intend the handle to be inheritable.
  • The access control lists (ACL) in the default security descriptor for a file or directory are inherited from its parent directory.
  • The target file system must support security on files and directories for thelpSecurityDescriptor member to have an effect on them, which can be determined by usingGetVolumeInformation.
In Windows 8 and Windows Server 2012, this function is supported by the following technologies.
TechnologySupported
Server Message Block (SMB) 3.0 protocolYes
SMB 3.0 Transparent Failover (TFO)See remarks
SMB 3.0 with Scale-out File Shares (SO)See remarks
Cluster Shared Volume File System (CsvFS)Yes
Resilient File System (ReFS)Yes
 

Note thatCreateFile with supersede disposition will fail if performed on a file where there is already an open alternate data stream.

Symbolic Link Behavior

If the call to this function creates a file, there is no change in behavior. Also, consider the following information regardingFILE_FLAG_OPEN_REPARSE_POINT:
  • IfFILE_FLAG_OPEN_REPARSE_POINT is specified:
    • If an existing file is opened and it is a symbolic link, the handle returned is a handle to the symbolic link.
    • IfTRUNCATE_EXISTING orFILE_FLAG_DELETE_ON_CLOSE are specified, the file affected is a symbolic link.
  • IfFILE_FLAG_OPEN_REPARSE_POINT is not specified:
    • If an existing file is opened and it is a symbolic link, the handle returned is a handle to the target.
    • IfCREATE_ALWAYS,TRUNCATE_EXISTING, orFILE_FLAG_DELETE_ON_CLOSE are specified, the file affected is the target.

Caching Behavior

Several of the possible values for thedwFlagsAndAttributes parameter are used byCreateFile to control or affect how the data associated with the handle is cached by the system. They are:
  • FILE_FLAG_NO_BUFFERING
  • FILE_FLAG_RANDOM_ACCESS
  • FILE_FLAG_SEQUENTIAL_SCAN
  • FILE_FLAG_WRITE_THROUGH
  • FILE_ATTRIBUTE_TEMPORARY
If none of these flags is specified, the system uses a default general-purpose caching scheme. Otherwise, the system caching behaves as specified for each flag.

Some of these flags should not be combined. For instance, combiningFILE_FLAG_RANDOM_ACCESS withFILE_FLAG_SEQUENTIAL_SCAN isself-defeating.

Specifying theFILE_FLAG_SEQUENTIAL_SCAN flag can increase performance forapplications that read large files using sequential access. Performance gains can be even more noticeable forapplications that read large files mostly sequentially, but occasionally skip forward over small ranges ofbytes. If an application moves the file pointer for random access, optimum caching performance most likely willnot occur. However, correct operation is still guaranteed.

The flagsFILE_FLAG_WRITE_THROUGH andFILE_FLAG_NO_BUFFERING are independent and may be combined.

IfFILE_FLAG_WRITE_THROUGH is used butFILE_FLAG_NO_BUFFERING is not also specified, so that system caching is in effect,then the data is written to the system cache but is flushed to disk without delay.

IfFILE_FLAG_WRITE_THROUGH andFILE_FLAG_NO_BUFFERING areboth specified, so that system caching is not in effect, then the data is immediately flushed to disk withoutgoing through the Windows system cache. The operating system also requests a write-through of the hard disk'slocal hardware cache to persistent media.

Note  Not all hard disk hardware supports this write-through capability.
 
Proper use of theFILE_FLAG_NO_BUFFERING flag requires special application considerations. For more information, seeFile Buffering.

A write-through request viaFILE_FLAG_WRITE_THROUGH also causes NTFS to flush anymetadata changes, such as a time stamp update or a rename operation, that result from processing the request.For this reason, theFILE_FLAG_WRITE_THROUGH flag is often used with theFILE_FLAG_NO_BUFFERING flag as a replacement for calling theFlushFileBuffers function after each write, which cancause unnecessary performance penalties. Using these flags together avoids those penalties. For generalinformation about the caching of files and metadata, seeFile Caching.

WhenFILE_FLAG_NO_BUFFERING is combined withFILE_FLAG_OVERLAPPED, the flags give maximum asynchronous performance, because the I/Odoes not rely on the synchronous operations of the memory manager. However, some I/O operations take more time,because data is not being held in the cache. Also, the file metadata may still be cached (for example, whencreating an empty file). To ensure that the metadata is flushed to disk, use theFlushFileBuffers function.

Specifying theFILE_ATTRIBUTE_TEMPORARY attribute causes file systems to avoidwriting data back to mass storage if sufficient cache memory is available, because an application deletes atemporary file after a handle is closed. In that case, the system can entirely avoid writing the data. Althoughit does not directly control data caching in the same way as the previously mentioned flags, theFILE_ATTRIBUTE_TEMPORARY attribute does tell the system to hold as much as possible inthe system cache without writing and therefore may be of concern for certain applications.

Files

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

If you callCreateFile on a file that is pending deletionas a result of a previous call toDeleteFile, the functionfails. The operating system delays file deletion until all handles to the file are closed.GetLastError returnsERROR_ACCESS_DENIED.

ThedwDesiredAccess parameter can be zero, allowing the application to query fileattributes without accessing the file if the application is running with adequate security settings. This isuseful to test for the existence of a file without opening it for read and/or write access, or to obtain otherstatistics about the file or directory. SeeObtaining and Setting File InformationandGetFileInformationByHandle.

IfCREATE_ALWAYS andFILE_ATTRIBUTE_NORMAL arespecified,CreateFile fails and sets the last error toERROR_ACCESS_DENIED if the file exists and has theFILE_ATTRIBUTE_HIDDEN orFILE_ATTRIBUTE_SYSTEM attribute.To avoid the error, specify the same attributes as the existing file.

When an application creates a file across a network, it is better to useGENERIC_READ | GENERIC_WRITE fordwDesiredAccess than to useGENERIC_WRITE alone. Theresulting code is faster, because the redirector can use the cache manager and send fewer SMBs with more data.This combination also avoids an issue where writing to a file across a network can occasionally returnERROR_ACCESS_DENIED.

For more information, seeCreating and Opening Files.

Synchronous and Asynchronous I/O Handles

CreateFile provides for creating a file or device handle that is either synchronous or asynchronous. A synchronous handle behaves such that I/O function calls using that handle are blocked until they complete, while an asynchronous file handle makes it possible for the system to return immediately from I/O function calls, whether they completed the I/O operation or not. As stated previously, this synchronous versus asynchronous behavior is determined by specifyingFILE_FLAG_OVERLAPPED within thedwFlagsAndAttributes parameter. There are several complexities and potential pitfalls when using asynchronous I/O; for more information, seeSynchronous and Asynchronous I/O.

File Streams

On NTFS file systems, you can useCreateFile to create separate streams within a file. For more information, seeFile Streams.

Directories

An application cannot create a directory by usingCreateFile, therefore only theOPEN_EXISTING value is valid fordwCreationDisposition for this use case. To create a directory, the application must callCreateDirectory orCreateDirectoryEx.

To open a directory usingCreateFile, specify theFILE_FLAG_BACKUP_SEMANTICS flag as part ofdwFlagsAndAttributes. Appropriate security checks still apply when this flag is usedwithoutSE_BACKUP_NAME andSE_RESTORE_NAME privileges.

When usingCreateFile to open a directory duringdefragmentation of a FAT or FAT32 file system volume, do not specify theMAXIMUM_ALLOWED access right. Access to the directory is denied if this is done.Specify theGENERIC_READ access right instead.

For more information, seeAbout Directory Management.

Physical Disks and Volumes

Direct access to the disk or to a volume is restricted.

Windows Server 2003 and Windows XP:  Direct access to the disk or to a volume is not restricted in this manner.

You can use theCreateFile function to open a physicaldisk drive or a volume, which returns a direct access storage device (DASD) handle that can be used with theDeviceIoControl function. This enables you to accessthe disk or volume directly, for example such disk metadata as the partition table. However, this type of accessalso exposes the disk drive or volume to potential data loss, because an incorrect write to a disk using thismechanism could make its contents inaccessible to the operating system. To ensure data integrity, be sure tobecome familiar withDeviceIoControl and how otherAPIs behave differently with a direct access handle as opposed to a file system handle.

The following requirements must be met for such a call to succeed:

  • The caller must have administrative privileges. For more information, seeRunning with Special Privileges.
  • ThedwCreationDisposition parameter must have theOPEN_EXISTING flag.
  • When opening a volume or floppy disk, thedwShareMode parameter must have theFILE_SHARE_WRITE flag.
Note  ThedwDesiredAccess parameter can be zero, allowing the application to query device attributes without accessing a device. This is useful for an application to determine the size of a floppy disk drive and the formats it supports without requiring a floppy disk in a drive, for instance. It can also be used for reading statistics without requiring higher-level data read/write permission.
 
When opening a physical drivex:, thelpFileName string should be the following form: "\\.\PhysicalDriveX". Hard disk numbers start at zero. The following table shows some examples of physical drive strings.
StringMeaning
"\\.\PhysicalDrive0"Opens the first physical drive.
"\\.\PhysicalDrive2"Opens the third physical drive.
 

To obtain the physical drive identifier for a volume, open a handle to the volume and call theDeviceIoControl function withIOCTL_VOLUME_GET_VOLUME_DISK_EXTENTS.This control code returns the disk number and offset for each of the volume's one or more extents; a volume canspan multiple physical disks.

For an example of opening a physical drive, seeCalling DeviceIoControl.

When opening a volume or removable media drive (for example, a floppy disk drive or flash memory thumb drive),thelpFileName string should be the following form:"\\.\X:". Do not use a trailing backslash(\), which indicates the root directory of a drive. The following table shows some examples of drive strings.

StringMeaning
"\\.\A:"Opens floppy disk drive A.
"\\.\C:"Opens the C: volume.
"\\.\C:\"Opens the file system of the C: volume.
 

You can also open a volume by referring to its volume name. For more information, seeNaming a Volume.

A volume contains one or more mounted file systems. Volume handles can be opened as noncached at thediscretion of the particular file system, even when the noncached option is not specified inCreateFile. You should assume that all Microsoft filesystems open volume handles as noncached. The restrictions on noncached I/O for files also apply to volumes.

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

Note  To read or write to the last few sectors of the volume, you must callDeviceIoControl and specifyFSCTL_ALLOW_EXTENDED_DASD_IO. This signals the file system driver not to perform any I/O boundary checks on partition read or write calls. Instead, boundary checks are performed by the device driver.
 

Changer Device

TheIOCTL_CHANGER_* control codes forDeviceIoControl accept a handle to a changer device. To open a changer device, use a file name of the following form: "\\.\Changerx" wherex is a number that indicates which device to open, starting with zero. To open changer device zero in an application that is written in C or C++, use the following file name: "\\.\Changer0".

Tape Drives

You can open tape drives by using a file name of the following form: "\\.\TAPEx" wherex is a number that indicates which drive to open, starting with tape drive zero. To open tape drive zero in an application that is written in C or C++, use the following file name: "\\.\TAPE0".

For more information, seeBackup.

Communications Resources

TheCreateFile function can create a handle to a communications resource, such as the serial port COM1. For communications resources, thedwCreationDisposition parameter must beOPEN_EXISTING, thedwShareMode parameter must be zero (exclusive access), and thehTemplateFile parameter must beNULL. Read, write, or read/write access can be specified, and the handle can be opened for overlapped I/O.

To specify a COM port number greater than 9, use the following syntax:"\\.\COM10". This syntax works for all port numbers and hardware thatallows COM port numbers to be specified.

For more information about communications, seeCommunications.

Consoles

TheCreateFile 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 theAllocConsole function. For console handles, set theCreateFile parameters as follows.
ParametersValue
lpFileNameUse the CONIN$ value to specify console input.

Use the CONOUT$ value to specify console output.

CONIN$ gets a handle to the console input buffer, even if theSetStdHandle function redirects the standard inputhandle. To get the standard input handle, use theGetStdHandle function.

CONOUT$ gets a handle to the active screen buffer, even ifSetStdHandle redirects the standard output handle. Toget the standard output handle, useGetStdHandle.

dwDesiredAccessGENERIC_READ | GENERIC_WRITE is preferred, but either one can limit access.
dwShareModeWhen opening CONIN$, specifyFILE_SHARE_READ. When opening CONOUT$, specifyFILE_SHARE_WRITE.

If the calling process inherits the console, or if a child process should be able to access the console,this parameter must beFILE_SHARE_READ | FILE_SHARE_WRITE.

lpSecurityAttributesIf you want the console to be inherited, thebInheritHandle member of theSECURITY_ATTRIBUTES structure must beTRUE.
dwCreationDispositionYou should specifyOPEN_EXISTING when usingCreateFile to open the console.
dwFlagsAndAttributesIgnored.
hTemplateFileIgnored.
 

The following table shows various settings ofdwDesiredAccess andlpFileName.

lpFileNamedwDesiredAccessResult
"CON"GENERIC_READOpens console for input.
"CON"GENERIC_WRITEOpens console for output.
"CON"GENERIC_READ | GENERIC_WRITECausesCreateFile to fail;GetLastError returnsERROR_FILE_NOT_FOUND.
 

Mailslots

IfCreateFile opens the client end of a mailslot, the function returnsINVALID_HANDLE_VALUE if the mailslot client attempts to open a local mailslot before the mailslot server has created it with theCreateMailSlot function.

For more information, seeMailslots.

Pipes

IfCreateFile 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 after it is opened, the named pipe instance cannot be opened by another client. The access that is specified when a pipe is opened must be compatible with the access that is specified in thedwOpenMode parameter of theCreateNamedPipe function.

If theCreateNamedPipe function was notsuccessfully called on the server prior to this operation, a pipe will not exist andCreateFile will fail withERROR_FILE_NOT_FOUND.

If there is at least one active pipe instance but there are no available listener pipes on the server, whichmeans all pipe instances are currently connected,CreateFile fails withERROR_PIPE_BUSY.

For more information, seePipes.

Examples

Example file operations are shown in the following topics:

Physical device I/O is demonstrated in the following topics:An example using named pipes is located atNamed Pipe Client.

Working with a mailslot is shown inWriting to a Mailslot.

A tape backup code snippet can found atCreating a Backup Application.

Note

The fileapi.h header defines CreateFile as an alias that automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that is not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, seeConventions for Function Prototypes.

Requirements

RequirementValue
Minimum supported clientWindows XP [desktop apps only]
Minimum supported serverWindows Server 2003 [desktop apps only]
Target PlatformWindows
Headerfileapi.h (include Windows.h)
LibraryKernel32.lib
DLLKernel32.dll

See also

About Directory Management

About Volume Management

Backup

CloseHandle

Communications

CreateDirectory

CreateDirectoryEx

CreateFileTransacted

CreateMailSlot

CreateNamedPipe

Creating, Deleting, and Maintaining Files

DeleteFile

Device Input and Output Control (IOCTL)

DeviceIoControl

File Compression and Decompression

File Encryption

File Management Functions

File Security and Access Rights

File Streams

Functions

GetLastError

I/O Completion Ports

I/O Concepts

Mailslots

Obtaining and Setting File Information

Overview Topics

Pipes

ReadFile

ReadFileEx

Running with Special Privileges

SetFileAttributes

WriteFile

WriteFileEx


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