Qt Reference Documentation

QFile Class Reference

The QFile class provides an interface for reading from and writing to files. More...

 #include <QFile>

Inherits QIODevice.

Inherited by QTemporaryFile.

Note: All functions in this class are reentrant, except for setEncodingFunction() and setDecodingFunction(), which are nonreentrant.

Public Types

typedef DecoderFn
typedef EncoderFn
enum FileError { NoError, ReadError, WriteError, FatalError, ..., CopyError }
enum MemoryMapFlags { NoOptions }
enum Permission { ReadOwner, WriteOwner, ExeOwner, ReadUser, ..., ExeOther }
typedef PermissionSpec
flags Permissions

Public Functions

QFile ( const QString & name )
QFile ( QObject * parent )
QFile ( const QString & name, QObject * parent )
~QFile ()
bool copy ( const QString & newName )
FileError error () const
bool exists () const
QString fileName () const
bool flush ()
int handle () const
bool link ( const QString & linkName )
uchar * map ( qint64 offset, qint64 size, MemoryMapFlags flags = NoOptions )
bool open ( FILE * fh, OpenMode mode )
bool open ( int fd, OpenMode mode )
Permissions permissions () const
bool remove ()
bool rename ( const QString & newName )
bool resize ( qint64 sz )
void setFileName ( const QString & name )
bool setPermissions ( Permissions permissions )
QString symLinkTarget () const
bool unmap ( uchar * address )
void unsetError ()

Reimplemented Public Functions

virtual bool atEnd () const
virtual void close ()
virtual bool isSequential () const
virtual bool open ( OpenMode mode )
virtual qint64 pos () const
virtual bool seek ( qint64 off )
virtual qint64 size () const
  • 33 public functions inherited from QIODevice
  • 29 public functions inherited from QObject

Static Public Members

bool copy ( const QString & fileName, const QString & newName )
QString decodeName ( const QByteArray & localFileName )
QString decodeName ( const char * localFileName )
QByteArray encodeName ( const QString & fileName )
bool exists ( const QString & fileName )
bool link ( const QString & fileName, const QString & linkName )
Permissions permissions ( const QString & fileName )
bool remove ( const QString & fileName )
bool rename ( const QString & oldName, const QString & newName )
bool resize ( const QString & fileName, qint64 sz )
void setDecodingFunction ( DecoderFn function )
void setEncodingFunction ( EncoderFn function )
bool setPermissions ( const QString & fileName, Permissions permissions )
QString symLinkTarget ( const QString & fileName )
  • 5 static public members inherited from QObject

Reimplemented Protected Functions

virtual qint64 readData ( char * data, qint64 len )
virtual qint64 readLineData ( char * data, qint64 maxlen )
virtual qint64 writeData ( const char * data, qint64 len )
  • 5 protected functions inherited from QIODevice
  • 7 protected functions inherited from QObject

Additional Inherited Members

  • 1 property inherited from QObject
  • 1 public slot inherited from QObject
  • 4 signals inherited from QIODevice
  • 1 signal inherited from QObject
  • 5 protected functions inherited from QIODevice
  • 7 protected functions inherited from QObject

Detailed Description

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.

Reading Files Directly

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.

Using Streams to Read Files

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();
     }

Signals

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.

Platform Specific Issues

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.

See also QTextStream, QDataStream, QFileInfo, QDir, and The Qt Resource System.

Member Type Documentation

typedef QFile::DecoderFn

This is a typedef for a pointer to a function with the following signature:

 QString myDecoderFunc(const QByteArray &localFileName);

See also setDecodingFunction().

typedef QFile::EncoderFn

This is a typedef for a pointer to a function with the following signature:

 QByteArray myEncoderFunc(const QString &fileName);

See also setEncodingFunction() and encodeName().

enum QFile::FileError

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

ConstantValueDescription
QFile::NoError0No error occurred.
QFile::ReadError1An error occurred when reading from the file.
QFile::WriteError2An error occurred when writing to the file.
QFile::FatalError3A fatal error occurred.
QFile::ResourceError4 
QFile::OpenError5The file could not be opened.
QFile::AbortError6The operation was aborted.
QFile::TimeOutError7A timeout occurred.
QFile::UnspecifiedError8An unspecified error occurred.
QFile::RemoveError9The file could not be removed.
QFile::RenameError10The file could not be renamed.
QFile::PositionError11The position in the file could not be changed.
QFile::ResizeError12The file could not be resized.
QFile::PermissionsError13The file could not be accessed.
QFile::CopyError14The file could not be copied.

enum QFile::MemoryMapFlags

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

ConstantValueDescription
QFile::NoOptions0No options.

This enum was introduced in Qt 4.4.

enum QFile::Permission
flags QFile::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.

ConstantValueDescription
QFile::ReadOwner0x4000The file is readable by the owner of the file.
QFile::WriteOwner0x2000The file is writable by the owner of the file.
QFile::ExeOwner0x1000The file is executable by the owner of the file.
QFile::ReadUser0x0400The file is readable by the user.
QFile::WriteUser0x0200The file is writable by the user.
QFile::ExeUser0x0100The file is executable by the user.
QFile::ReadGroup0x0040The file is readable by the group.
QFile::WriteGroup0x0020The file is writable by the group.
QFile::ExeGroup0x0010The file is executable by the group.
QFile::ReadOther0x0004The file is readable by anyone.
QFile::WriteOther0x0002The file is writable by anyone.
QFile::ExeOther0x0001The 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.

typedef QFile::PermissionSpec

Use QFile::Permission instead.

Member Function Documentation

QFile::QFile ( const QString & name )

Constructs a new file object to represent the file with the given name.

QFile::QFile ( QObject * parent )

Constructs a new file object with the given parent.

QFile::QFile ( const QString & name, QObject * parent )

Constructs a new file object with the given parent to represent the file with the specified name.

QFile::~QFile ()

Destroys the file object, closing it if necessary.

bool QFile::atEnd () const [virtual]

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.

void QFile::close () [virtual]

Reimplemented from QIODevice::close().

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

See also QIODevice::close().

bool QFile::copy ( const QString & newName )

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().

bool QFile::copy ( const QString & fileName, const QString & newName ) [static]

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().

QString QFile::decodeName ( const QByteArray & localFileName ) [static]

This does the reverse of QFile::encodeName() using localFileName.

See also setDecodingFunction() and encodeName().

QString QFile::decodeName ( const char * localFileName ) [static]

This is an overloaded function.

Returns the Unicode version of the given localFileName. See encodeName() for details.

QByteArray QFile::encodeName ( const QString & fileName ) [static]

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().

FileError QFile::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().

bool QFile::exists ( const QString & fileName ) [static]

Returns true if the file specified by fileName exists; otherwise returns false.

bool QFile::exists () const

This is an overloaded function.

Returns true if the file specified by fileName() exists; otherwise returns false.

See also fileName() and setFileName().

QString QFile::fileName () const

Returns the name set by setFileName() or to the QFile constructors.

See also setFileName() and QFileInfo::fileName().

bool QFile::flush ()

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

int QFile::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.

This function is not supported on Windows CE.

See also QSocketNotifier.

bool QFile::isSequential () const [virtual]

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().

bool QFile::link ( const QString & linkName )

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: On Symbian, no link is created and false is returned if fileName() currently specifies a directory.

See also setFileName().

bool QFile::link ( const QString & fileName, const QString & linkName ) [static]

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().

uchar * QFile::map ( qint64 offset, qint64 size, 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.

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().

bool QFile::open ( OpenMode mode ) [virtual]

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().

bool QFile::open ( FILE * fh, OpenMode mode )

This is an overloaded function.

Opens the existing file handle fh in the given mode. Returns true if successful; otherwise returns false.

Example:

 #include <stdio.h>

 void printError(const char* msg)
 {
     QFile file;
     file.open(stderr, QIODevice::WriteOnly);
     file.write(msg, qstrlen(msg));        // write to stderr
     file.close();
 }

When a QFile is opened using this function, close() does not actually close the file, but only flushes it.

Warning:

  1. If fh does not refer to a regular file, e.g., it is stdin, stdout, or stderr, you may not be able to seek(). size() returns 0 in those cases. See QIODevice::isSequential() for more information.
  2. Since this function opens the file without specifying the file name, you cannot use this QFile with a QFileInfo.

Note: For Windows CE you may not be able to call resize().

Note for the Windows Platform

fh must be opened in binary mode (i.e., the mode string must contain 'b', as in "rb" or "wb") when accessing files and other random-access devices. Qt will translate the end-of-line characters if you pass QIODevice::Text to mode. Sequential devices, such as stdin and stdout, are unaffected by this limitation.

You need to enable support for console applications in order to use the stdin, stdout and stderr streams at the console. To do this, add the following declaration to your application's project file:

 CONFIG += console

See also close() and qmake Variable Reference.

bool QFile::open ( int fd, OpenMode mode )

This is an overloaded function.

Opens the existing file descriptor fd in the given mode. Returns true if successful; otherwise returns false.

When a QFile is opened using this function, close() does not actually close the file.

The QFile that is opened using this function is automatically set to be in raw mode; this means that the file input/output functions are slow. If you run into performance issues, you should try to use one of the other open functions.

Warning: If fd is not a regular file, e.g, it is 0 (stdin), 1 (stdout), or 2 (stderr), you may not be able to seek(). In those cases, size() returns 0. See QIODevice::isSequential() for more information.

Warning: For Windows CE you may not be able to call seek(), setSize(), fileTime(). size() returns 0.

Warning: Since this function opens the file without specifying the file name, you cannot use this QFile with a QFileInfo.

See also close().

Permissions QFile::permissions () const

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

See also setPermissions() and setFileName().

Permissions QFile::permissions ( const QString & fileName ) [static]

This is an overloaded function.

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

qint64 QFile::pos () const [virtual]

Reimplemented from QIODevice::pos().

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

Reimplemented from QIODevice::readData().

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

Reimplemented from QIODevice::readLineData().

bool QFile::remove ()

Removes the file specified by fileName(). Returns true if successful; otherwise returns false.

The file is closed before it is removed.

See also setFileName().

bool QFile::remove ( const QString & fileName ) [static]

This is an overloaded function.

Removes the file specified by the fileName given.

Returns true if successful; otherwise returns false.

See also remove().

bool QFile::rename ( const QString & newName )

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().

bool QFile::rename ( const QString & oldName, const QString & newName ) [static]

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().

bool QFile::resize ( qint64 sz )

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().

bool QFile::resize ( const QString & fileName, qint64 sz ) [static]

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().

bool QFile::seek ( qint64 off ) [virtual]

Reimplemented from QIODevice::seek().

void QFile::setDecodingFunction ( DecoderFn function ) [static]

Sets the function for decoding 8-bit file names. The default uses the locale-specific 8-bit encoding.

Warning: This function is not reentrant.

See also setEncodingFunction() and decodeName().

void QFile::setEncodingFunction ( EncoderFn function ) [static]

Sets the function for encoding Unicode file names. The default encodes in the locale-specific 8-bit encoding.

Warning: This function is not reentrant.

See also encodeName() and setDecodingFunction().

void QFile::setFileName ( const QString & name )

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.

bool QFile::setPermissions ( Permissions permissions )

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().

bool QFile::setPermissions ( const QString & fileName, Permissions permissions ) [static]

This is an overloaded function.

Sets the permissions for fileName file to permissions.

qint64 QFile::size () const [virtual]

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().

QString QFile::symLinkTarget ( const QString & fileName ) [static]

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.

QString QFile::symLinkTarget () const

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().

bool QFile::unmap ( uchar * address )

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().

void QFile::unsetError ()

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

See also error().

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

Reimplemented from QIODevice::writeData().

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