QFileDevice Class | Qt Core 6.7.2 (2024)

Member Type Documentation

enum QFileDevice::FileError

This enum describes the errors that may be returned by the error() function.

enum QFileDevice::FileHandleFlag
flags QFileDevice::FileHandleFlags

This enum is used when opening a file to specify additional options which only apply to files and not to a generic QIODevice.

The FileHandleFlags type is a typedef for QFlags<FileHandleFlag>. It stores an OR combination of FileHandleFlag values.

enum QFileDevice::FileTime

This enum is used by the fileTime() and setFileTime() functions.

See also setFileTime(), fileTime(), and QFileInfo::fileTime().

enum QFileDevice::MemoryMapFlag
flags QFileDevice::MemoryMapFlags

This enum describes special options that may be used by the map() function.

The MemoryMapFlags type is a typedef for QFlags<MemoryMapFlag>. It stores an OR combination of MemoryMapFlag values.

enum QFileDevice::Permission
flags QFileDevice::Permissions

This enum is used by the permission() function to report the permissions and ownership of a file. The values may be OR-ed together to test multiple permissions and ownership values.

Warning: Because of differences in the platforms supported by Qt, the semantics of ReadUser, WriteUser and ExeUser are platform-dependent: On Unix, the rights of the owner of the file are returned and on Windows the rights of the current user are returned. This behavior might change in a future Qt version.

Note: On NTFS file systems, ownership and permissions checking is disabled by default for performance reasons. To enable it, include the following line:

extern Q_CORE_EXPORT int qt_ntfs_permission_lookup;

Permission checking is then turned on and off by incrementing and decrementing qt_ntfs_permission_lookup by 1.

qt_ntfs_permission_lookup++; // turn checking onqt_ntfs_permission_lookup--; // turn it off again

Note: Since this is a non-atomic global variable, it is only safe to increment or decrement qt_ntfs_permission_lookup before any threads other than the main thread have started or after every thread other than the main thread has ended.

Note: From Qt 6.6 the variable qt_ntfs_permission_lookup is deprecated. Please use the following alternatives.

The safe and easy way to manage permission checks is to use the RAII class QNtfsPermissionCheckGuard.

void complexFunction(){ QNtfsPermissionCheckGuard permissionGuard; // check is enabled // do complex things here that need permission check enabled} // as the guard goes out of scope the check is disabled

If you need more fine-grained control, it is possible to manage the permission with the following functions instead:

qAreNtfsPermissionChecksEnabled(); // check statusqEnableNtfsPermissionChecks(); // turn checking onqDisableNtfsPermissionChecks(); // turn it off again

The Permissions type is a typedef for QFlags<Permission>. It stores an OR combination of Permission values.

Member Function Documentation

[virtual noexcept] QFileDevice::~QFileDevice()

Destroys the file device, closing it if necessary.

[override virtual] bool QFileDevice::atEnd() const

Reimplements: QIODevice::atEnd() const.

Returns true if the end of the file has been reached; otherwise returns false.

For regular empty files on Unix (e.g. those in /proc), this function returns true, since the file system reports that the size of such a file is 0. Therefore, you should not depend on atEnd() when reading data from such a file, but rather call read() until no more data can be read.

[override virtual] void QFileDevice::close()

Reimplements: QIODevice::close().

Calls QFileDevice::flush() and closes the file. Errors from flush are ignored.

See also QIODevice::close().

QFileDevice::FileError QFileDevice::error() const

Returns the file error status.

The I/O device status returns an error code. For example, if open() returns false, or a read/write operation returns -1, this function can be called to find out the reason why the operation failed.

See also unsetError().

[virtual] QString QFileDevice::fileName() const

Returns the name of the file. The default implementation in QFileDevice returns a null string.

QDateTime QFileDevice::fileTime(QFileDevice::FileTime time) const

Returns the file time specified by time. If the time cannot be determined return QDateTime() (an invalid date time).

See also setFileTime(), FileTime, and QDateTime::isValid().

bool QFileDevice::flush()

Flushes any buffered data to the file. Returns true if successful; otherwise returns false.

int QFileDevice::handle() const

Returns the file handle of the file.

This is a small positive integer, suitable for use with C library functions such as fdopen() and fcntl(). On systems that use file descriptors for sockets (i.e. Unix systems, but not Windows) the handle can be used with QSocketNotifier as well.

If the file is not open, or there is an error, handle() returns -1.

See also QSocketNotifier.

[override virtual] bool QFileDevice::isSequential() const

Reimplements: QIODevice::isSequential() const.

Returns true if the file can only be manipulated sequentially; otherwise returns false.

Most files support random-access, but some special files may not.

See also QIODevice::isSequential().

uchar *QFileDevice::map(qint64 offset, qint64 size, QFileDevice::MemoryMapFlags flags = NoOptions)

Maps size bytes of the file into memory starting at offset. A file should be open for a map to succeed but the file does not need to stay open after the memory has been mapped. When the QFile is destroyed or a new file is opened with this object, any maps that have not been unmapped will automatically be unmapped.

The mapping will have the same open mode as the file (read and/or write), except when using MapPrivateOption, in which case it is always possible to write to the mapped memory.

Any mapping options can be passed through flags.

Returns a pointer to the memory or nullptr if there is an error.

See also unmap().

[virtual] QFileDevice::Permissions QFileDevice::permissions() const

Returns the complete OR-ed together combination of QFile::Permission for the file.

See also setPermissions().

[override virtual] qint64 QFileDevice::pos() const

Reimplements: QIODevice::pos() const.

[override virtual protected] qint64 QFileDevice::readData(char *data, qint64 len)

Reimplements: QIODevice::readData(char *data, qint64 maxSize).

[override virtual protected] qint64 QFileDevice::readLineData(char *data, qint64 maxlen)

Reimplements: QIODevice::readLineData(char *data, qint64 maxSize).

[virtual] bool QFileDevice::resize(qint64 sz)

Sets the file size (in bytes) sz. Returns true if the resize succeeds; false otherwise. If sz is larger than the file currently is, the new bytes will be set to 0; if sz is smaller, the file is simply truncated.

Warning: This function can fail if the file doesn't exist.

See also size().

[override virtual] bool QFileDevice::seek(qint64 pos)

Reimplements: QIODevice::seek(qint64 pos).

For random-access devices, this function sets the current position to pos, returning true on success, or false if an error occurred. For sequential devices, the default behavior is to do nothing and return false.

Seeking beyond the end of a file: If the position is beyond the end of a file, then seek() will not immediately extend the file. If a write is performed at this position, then the file will be extended. The content of the file between the previous end of file and the newly written data is UNDEFINED and varies between platforms and file systems.

bool QFileDevice::setFileTime(const QDateTime &newDate, QFileDevice::FileTime fileTime)

Sets the file time specified by fileTime to newDate, returning true if successful; otherwise returns false.

Note: The file must be open to use this function.

See also fileTime() and FileTime.

[virtual] bool QFileDevice::setPermissions(QFileDevice::Permissions permissions)

Sets the permissions for the file to the permissions specified. Returns true if successful, or false if the permissions cannot be modified.

Warning: This function does not manipulate ACLs, which may limit its effectiveness.

See also permissions().

[override virtual] qint64 QFileDevice::size() const

Reimplements: QIODevice::size() const.

Returns the size of the file.

For regular empty files on Unix (e.g. those in /proc), this function returns 0; the contents of such a file are generated on demand in response to you calling read().

bool QFileDevice::unmap(uchar *address)

Unmaps the memory address.

Returns true if the unmap succeeds; false otherwise.

See also map().

void QFileDevice::unsetError()

Sets the file's error to QFileDevice::NoError.

See also error().

[override virtual protected] qint64 QFileDevice::writeData(const char *data, qint64 len)

Reimplements: QIODevice::writeData(const char *data, qint64 maxSize).