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The QFile class provides an interface for reading from and writing to files. More...
Inherits QIODevice.
Inherited by QTemporaryFile.
The QFile class provides an interface for reading from and writing to files.
QFile is an I/O device for reading and writing text and binary files and resources. A QFile may be used by itself or, more conveniently, with a QTextStream or QDataStream.
The file name is usually passed in the constructor, but it can be set at any time using setFileName(). QFile expects the file separator to be '/' regardless of operating system. The use of other separators (e.g., '\') is not supported.
You can check for a file's existence using exists(), and remove a file using remove(). (More advanced file system related operations are provided by QFileInfo and QDir.)
The file is opened with open(), closed with close(), and flushed with flush(). Data is usually read and written using QDataStream or QTextStream, but you can also call the QIODevice-inherited functions read(), readLine(), readAll(), write(). QFile also inherits getChar(), putChar(), and ungetChar(), which work one character at a time.
The size of the file is returned by size(). You can get the current file position using pos(), or move to a new file position using seek(). If you've reached the end of the file, atEnd() returns true.
The following example reads a text file line by line:
QFile file("in.txt");
if (!file.open(QIODevice.ReadOnly | QIODevice.Text))
return;
while (!file.atEnd()) {
QByteArray line = file.readLine();
process_line(line);
}
The QIODevice.Text flag passed to open() tells Qt to convert Windows-style line terminators ("\r\n") into C++-style terminators ("\n"). By default, QFile assumes binary, i.e. it doesn't perform any conversion on the bytes stored in the file.
The next example uses QTextStream to read a text file line by line:
QFile file("in.txt");
if (!file.open(QIODevice.ReadOnly | QIODevice.Text))
return;
QTextStream in(&file);
while (!in.atEnd()) {
QString line = in.readLine();
process_line(line);
}
QTextStream takes care of converting the 8-bit data stored on disk into a 16-bit Unicode QString. By default, it assumes that the user system's local 8-bit encoding is used (e.g., ISO 8859-1 for most of Europe; see QTextCodec.codecForLocale() for details). This can be changed using setCodec().
To write text, we can use operator<<(), which is overloaded to take a QTextStream on the left and various data types (including QString) on the right:
QFile file("out.txt");
if (!file.open(QIODevice.WriteOnly | QIODevice.Text))
return;
QTextStream out(&file);
out << "The magic number is: " << 49 << "\n";
QDataStream is similar, in that you can use operator<<() to write data and operator>>() to read it back. See the class documentation for details.
When you use QFile, QFileInfo, and QDir to access the file system with Qt, you can use Unicode file names. On Unix, these file names are converted to an 8-bit encoding. If you want to use standard C++ APIs (<cstdio> or <iostream>) or platform-specific APIs to access files instead of QFile, you can use the encodeName() and decodeName() functions to convert between Unicode file names and 8-bit file names.
On Unix, there are some special system files (e.g. in /proc) for which size() will always return 0, yet you may still be able to read more data from such a file; the data is generated in direct response to you calling read(). In this case, however, you cannot use atEnd() to determine if there is more data to read (since atEnd() will return true for a file that claims to have size 0). Instead, you should either call readAll(), or call read() or readLine() repeatedly until no more data can be read. The next example uses QTextStream to read /proc/modules line by line:
QFile file("/proc/modules");
if (!file.open(QIODevice.ReadOnly | QIODevice.Text))
return;
QTextStream in(&file);
QString line = in.readLine();
while (!line.isNull()) {
process_line(line);
line = in.readLine();
}
Unlike other QIODevice implementations, such as QTcpSocket, QFile does not emit the aboutToClose(), bytesWritten(), or readyRead() signals. This implementation detail means that QFile is not suitable for reading and writing certain types of files, such as device files on Unix platforms.
File permissions are handled differently on Linux/Mac OS X and Windows. In a non writable directory on Linux, files cannot be created. This is not always the case on Windows, where, for instance, the 'My Documents' directory usually is not writable, but it is still possible to create files in it.
This enum describes the errors that may be returned by the error() function.
| Constant | Value | Description |
|---|---|---|
| QFile.NoError | 0 | No error occurred. |
| QFile.ReadError | 1 | An error occurred when reading from the file. |
| QFile.WriteError | 2 | An error occurred when writing to the file. |
| QFile.FatalError | 3 | A fatal error occurred. |
| QFile.ResourceError | 4 | |
| QFile.OpenError | 5 | The file could not be opened. |
| QFile.AbortError | 6 | The operation was aborted. |
| QFile.TimeOutError | 7 | A timeout occurred. |
| QFile.UnspecifiedError | 8 | An unspecified error occurred. |
| QFile.RemoveError | 9 | The file could not be removed. |
| QFile.RenameError | 10 | The file could not be renamed. |
| QFile.PositionError | 11 | The position in the file could not be changed. |
| QFile.ResizeError | 12 | The file could not be resized. |
| QFile.PermissionsError | 13 | The file could not be accessed. |
| QFile.CopyError | 14 | The file could not be copied. |
This enum is used when opening a file to specify additional options which only apply to files and not to a generic QIODevice.
| Constant | Value | Description |
|---|---|---|
| QFile.AutoCloseHandle | 0x0001 | The file handle passed into open() should be closed by close(), the default behaviour is that close just flushes the file and the application is responsible for closing the file handle. When opening a file by name, this flag is ignored as Qt always "owns" the file handle and must close it. |
| QFile.DontCloseHandle | 0 | The file handle passed into open() will not be closed by Qt. The application must ensure that close() is called. |
This enum was introduced or modified in Qt 4.8.
The FileHandleFlags type is a typedef for QFlags<FileHandleFlag>. It stores an OR combination of FileHandleFlag values.
This enum describes special options that may be used by the map() function.
| Constant | Value | Description |
|---|---|---|
| QFile.NoOptions | 0 | No options. |
This enum was introduced or modified in Qt 4.4.
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.
| Constant | Value | Description |
|---|---|---|
| QFile.ReadOwner | 0x4000 | The file is readable by the owner of the file. |
| QFile.WriteOwner | 0x2000 | The file is writable by the owner of the file. |
| QFile.ExeOwner | 0x1000 | The file is executable by the owner of the file. |
| QFile.ReadUser | 0x0400 | The file is readable by the user. |
| QFile.WriteUser | 0x0200 | The file is writable by the user. |
| QFile.ExeUser | 0x0100 | The file is executable by the user. |
| QFile.ReadGroup | 0x0040 | The file is readable by the group. |
| QFile.WriteGroup | 0x0020 | The file is writable by the group. |
| QFile.ExeGroup | 0x0010 | The file is executable by the group. |
| QFile.ReadOther | 0x0004 | The file is readable by anyone. |
| QFile.WriteOther | 0x0002 | The file is writable by anyone. |
| QFile.ExeOther | 0x0001 | The file is executable by anyone. |
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 that Qt does not by default check for permissions on NTFS file systems, as this may decrease the performance of file handling considerably. It is possible to force permission checking on NTFS by including the following code in your source:
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 on qt_ntfs_permission_lookup--; // turn it off again
The Permissions type is a typedef for QFlags<Permission>. It stores an OR combination of Permission values.
Constructs a new file object to represent the file with the given name.
The parent argument, if not None, causes self to be owned by Qt instead of PyQt.
Constructs a new file object with the given parent.
The parent argument, if not None, causes self to be owned by Qt instead of PyQt.
Constructs a new file object with the given parent to represent the file with the specified name.
Reimplemented from QIODevice.atEnd().
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.
Reimplemented from QIODevice.close().
Calls QFile.flush() and closes the file. Errors from flush are ignored.
See also QIODevice.close().
Copies the file currently specified by fileName() to a file called newName. Returns true if successful; otherwise returns false.
Note that if a file with the name newName already exists, copy() returns false (i.e. QFile will not overwrite it).
The source file is closed before it is copied.
See also setFileName().
This is an overloaded function.
Copies the file fileName to newName. Returns true if successful; otherwise returns false.
If a file with the name newName already exists, copy() returns false (i.e., QFile will not overwrite it).
See also rename().
This does the reverse of QFile.encodeName() using localFileName.
See also setDecodingFunction() and encodeName().
This is an overloaded function.
Returns the Unicode version of the given localFileName. See encodeName() for details.
By default, this function converts fileName to the local 8-bit encoding determined by the user's locale. This is sufficient for file names that the user chooses. File names hard-coded into the application should only use 7-bit ASCII filename characters.
See also decodeName() and setEncodingFunction().
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().
Returns true if the file specified by fileName exists; otherwise returns false.
This is an overloaded function.
Returns true if the file specified by fileName() exists; otherwise returns false.
See also fileName() and setFileName().
Returns the name set by setFileName() or to the QFile constructors.
See also setFileName() and QFileInfo.fileName().
Flushes any buffered data to the file. Returns true if successful; otherwise returns false.
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.
This function is not supported on Windows CE.
On Symbian, this function returns -1 if the file was opened normally, as Symbian OS native file handles do not fit in an int, and are incompatible with C library functions that the handle would be used for. If the file was opened using the overloads that take an open C library file handle / file descriptor, then this function returns that same handle.
See also QSocketNotifier.
Reimplemented from QIODevice.isSequential().
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().
Creates a link named linkName that points to the file currently specified by fileName(). What a link is depends on the underlying filesystem (be it a shortcut on Windows or a symbolic link on Unix). Returns true if successful; otherwise returns false.
This function will not overwrite an already existing entity in the file system; in this case, link() will return false and set error() to return RenameError.
Note: To create a valid link on Windows, linkName must have a .lnk file extension.
Note: Symbian filesystem does not support links.
See also setFileName().
This is an overloaded function.
Creates a link named linkName that points to the file fileName. What a link is depends on the underlying filesystem (be it a shortcut on Windows or a symbolic link on Unix). Returns true if successful; otherwise returns false.
See also link().
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.
Any mapping options can be passed through flags.
Returns a pointer to the memory or 0 if there is an error.
Note: On Windows CE 5.0 the file will be closed before mapping occurs.
This function was introduced in Qt 4.4.
See also unmap() and QAbstractFileEngine.supportsExtension().
Reimplemented from QIODevice.open().
Opens the file using OpenMode mode, returning true if successful; otherwise false.
The mode must be QIODevice.ReadOnly, QIODevice.WriteOnly, or QIODevice.ReadWrite. It may also have additional flags, such as QIODevice.Text and QIODevice.Unbuffered.
Note: In WriteOnly or ReadWrite mode, if the relevant file does not already exist, this function will try to create a new file before opening it.
See also QIODevice.OpenMode and setFileName().
Returns the complete OR-ed together combination of QFile.Permission for the file.
See also setPermissions() and setFileName().
This is an overloaded function.
Returns the complete OR-ed together combination of QFile.Permission for fileName.
Reimplemented from QIODevice.pos().
Reimplemented from QIODevice.readData().
Reimplemented from QIODevice.readLineData().
Removes the file specified by fileName(). Returns true if successful; otherwise returns false.
The file is closed before it is removed.
See also setFileName().
This is an overloaded function.
Removes the file specified by the fileName given.
Returns true if successful; otherwise returns false.
See also remove().
Renames the file currently specified by fileName() to newName. Returns true if successful; otherwise returns false.
If a file with the name newName already exists, rename() returns false (i.e., QFile will not overwrite it).
The file is closed before it is renamed.
See also setFileName().
This is an overloaded function.
Renames the file oldName to newName. Returns true if successful; otherwise returns false.
If a file with the name newName already exists, rename() returns false (i.e., QFile will not overwrite it).
See also rename().
Sets the file size (in bytes) sz. Returns true if the file 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.
See also size() and setFileName().
This is an overloaded function.
Sets fileName to size (in bytes) sz. Returns true if the file if the resize succeeds; false otherwise. If sz is larger than fileName currently is the new bytes will be set to 0, if sz is smaller the file is simply truncated.
See also resize().
Reimplemented from QIODevice.seek().
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() shall not immediately extend the file. If a write is performed at this position, then the file shall 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.
Sets the name of the file. The name can have no path, a relative path, or an absolute path.
Do not call this function if the file has already been opened.
If the file name has no path or a relative path, the path used will be the application's current directory path at the time of the open() call.
Example:
QFile file; QDir.setCurrent("/tmp"); file.setFileName("readme.txt"); QDir.setCurrent("/home"); file.open(QIODevice.ReadOnly); // opens "/home/readme.txt" under Unix
Note that the directory separator "/" works for all operating systems supported by Qt.
See also fileName(), QFileInfo, and QDir.
Sets the permissions for the file to the permissions specified. Returns true if successful, or false if the permissions cannot be modified.
See also permissions() and setFileName().
This is an overloaded function.
Sets the permissions for fileName file to permissions.
Reimplemented from QIODevice.size().
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().
Returns the absolute path of the file or directory referred to by the symlink (or shortcut on Windows) specified by fileName, or returns an empty string if the fileName does not correspond to a symbolic link.
This name may not represent an existing file; it is only a string. QFile.exists() returns true if the symlink points to an existing file.
This function was introduced in Qt 4.2.
This is an overloaded function.
Returns the absolute path of the file or directory a symlink (or shortcut on Windows) points to, or a an empty string if the object isn't a symbolic link.
This name may not represent an existing file; it is only a string. QFile.exists() returns true if the symlink points to an existing file.
This function was introduced in Qt 4.2.
See also fileName() and setFileName().
Unmaps the memory address.
Returns true if the unmap succeeds; false otherwise.
This function was introduced in Qt 4.4.
See also map() and QAbstractFileEngine.supportsExtension().
Sets the file's error to QFile.NoError.
See also error().
Reimplemented from QIODevice.writeData().
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