QLocalSocket Class

The QLocalSocket class provides a local socket. More...

Header: #include <QLocalSocket>
Since: Qt 4.4
Inherits: QIODevice.

Public Types

enum LocalSocketError { ConnectionRefusedError, PeerClosedError, ServerNotFoundError, SocketAccessError, ..., UnknownSocketError }
enum LocalSocketState { UnconnectedState, ConnectingState, ConnectedState, ClosingState }

Public Functions

QLocalSocket(QObject * parent = 0)
void abort()
void connectToServer(const QString & name, OpenMode openMode = ReadWrite)
void disconnectFromServer()
LocalSocketError error() const
bool flush()
QString fullServerName() const
bool isValid() const
qint64 readBufferSize() const
QString serverName() const
void setReadBufferSize(qint64 size)
bool setSocketDescriptor(quintptr socketDescriptor, LocalSocketState socketState = ConnectedState, OpenMode openMode = ReadWrite)
quintptr socketDescriptor() const
LocalSocketState state() const
bool waitForConnected(int msecs = 30000)
bool waitForDisconnected(int msecs = 30000)

Reimplemented Public Functions

virtual qint64 bytesAvailable() const
virtual qint64 bytesToWrite() const
virtual bool canReadLine() const
virtual void close()
virtual bool isSequential() const
virtual bool waitForBytesWritten(int msecs = 30000)
virtual bool waitForReadyRead(int msecs = 30000)
  • 33 public functions inherited from QIODevice
  • 29 public functions inherited from QObject


void connected()
void disconnected()
void error(QLocalSocket::LocalSocketError socketError)
void stateChanged(QLocalSocket::LocalSocketState socketState)

Reimplemented Protected Functions

virtual qint64 readData(char * data, qint64 c)
virtual qint64 writeData(const char * data, qint64 c)
  • 5 protected functions inherited from QIODevice
  • 8 protected functions inherited from QObject

Additional Inherited Members

  • 1 property inherited from QObject
  • 1 public slot inherited from QObject
  • 7 static public members inherited from QObject
  • 5 protected functions inherited from QIODevice
  • 8 protected functions inherited from QObject

Detailed Description

The QLocalSocket class provides a local socket.

On Windows this is a named pipe and on Unix this is a local domain socket.

If an error occurs, socketError() returns the type of error, and errorString() can be called to get a human readable description of what happened.

Although QLocalSocket is designed for use with an event loop, it's possible to use it without one. In that case, you must use waitForConnected(), waitForReadyRead(), waitForBytesWritten(), and waitForDisconnected() which blocks until the operation is complete or the timeout expires.

Note that this feature is not supported on versions of Windows earlier than Windows XP.

See also QLocalServer.

Member Type Documentation

enum QLocalSocket::LocalSocketError

The LocalServerError enumeration represents the errors that can occur. The most recent error can be retrieved through a call to QLocalSocket::error().

QLocalSocket::ConnectionRefusedErrorQAbstractSocket::ConnectionRefusedErrorThe connection was refused by the peer (or timed out).
QLocalSocket::PeerClosedErrorQAbstractSocket::RemoteHostClosedErrorThe remote socket closed the connection. Note that the client socket (i.e., this socket) will be closed after the remote close notification has been sent.
QLocalSocket::ServerNotFoundErrorQAbstractSocket::HostNotFoundErrorThe local socket name was not found.
QLocalSocket::SocketAccessErrorQAbstractSocket::SocketAccessErrorThe socket operation failed because the application lacked the required privileges.
QLocalSocket::SocketResourceErrorQAbstractSocket::SocketResourceErrorThe local system ran out of resources (e.g., too many sockets).
QLocalSocket::SocketTimeoutErrorQAbstractSocket::SocketTimeoutErrorThe socket operation timed out.
QLocalSocket::DatagramTooLargeErrorQAbstractSocket::DatagramTooLargeErrorThe datagram was larger than the operating system's limit (which can be as low as 8192 bytes).
QLocalSocket::ConnectionErrorQAbstractSocket::NetworkErrorAn error occurred with the connection.
QLocalSocket::UnsupportedSocketOperationErrorQAbstractSocket::UnsupportedSocketOperationErrorThe requested socket operation is not supported by the local operating system.
QLocalSocket::UnknownSocketErrorQAbstractSocket::UnknownSocketErrorAn unidentified error occurred.

enum QLocalSocket::LocalSocketState

This enum describes the different states in which a socket can be.

QLocalSocket::UnconnectedStateQAbstractSocket::UnconnectedStateThe socket is not connected.
QLocalSocket::ConnectingStateQAbstractSocket::ConnectingStateThe socket has started establishing a connection.
QLocalSocket::ConnectedStateQAbstractSocket::ConnectedStateA connection is established.
QLocalSocket::ClosingStateQAbstractSocket::ClosingStateThe socket is about to close (data may still be waiting to be written).

See also QLocalSocket::state().

Member Function Documentation

QLocalSocket::QLocalSocket(QObject * parent = 0)

Creates a new local socket. The parent argument is passed to QObject's constructor.


Destroys the socket, closing the connection if necessary.

void QLocalSocket::abort()

Aborts the current connection and resets the socket. Unlike disconnectFromServer(), this function immediately closes the socket, clearing any pending data in the write buffer.

See also disconnectFromServer() and close().

[virtual] qint64 QLocalSocket::bytesAvailable() const

Reimplemented from QIODevice::bytesAvailable().

[virtual] qint64 QLocalSocket::bytesToWrite() const

Reimplemented from QIODevice::bytesToWrite().

[virtual] bool QLocalSocket::canReadLine() const

Reimplemented from QIODevice::canReadLine().

[virtual] void QLocalSocket::close()

Reimplemented from QIODevice::close().

void QLocalSocket::connectToServer(const QString & name, OpenMode openMode = ReadWrite)

Attempts to make a connection to name.

The socket is opened in the given openMode and first enters ConnectingState. It then attempts to connect to the address or addresses returned by the lookup. Finally, if a connection is established, QLocalSocket enters ConnectedState and emits connected().

At any point, the socket can emit error() to signal that an error occurred.

See also state(), serverName(), and waitForConnected().

[signal] void QLocalSocket::connected()

This signal is emitted after connectToServer() has been called and a connection has been successfully established.

See also connectToServer() and disconnected().

void QLocalSocket::disconnectFromServer()

Attempts to close the socket. If there is pending data waiting to be written, QLocalSocket will enter ClosingState and wait until all data has been written. Eventually, it will enter UnconnectedState and emit the disconnectedFromServer() signal.

See also connectToServer().

[signal] void QLocalSocket::disconnected()

This signal is emitted when the socket has been disconnected.

See also connectToServer(), disconnectFromServer(), abort(), and connected().

LocalSocketError QLocalSocket::error() const

Returns the type of error that last occurred.

See also state() and errorString().

[signal] void QLocalSocket::error(QLocalSocket::LocalSocketError socketError)

This signal is emitted after an error occurred. The socketError parameter describes the type of error that occurred.

QLocalSocket::LocalSocketError is not a registered metatype, so for queued connections, you will have to register it with Q_DECLARE_METATYPE() and qRegisterMetaType().

See also error(), errorString(), and Creating Custom Qt Types.

bool QLocalSocket::flush()

This function writes as much as possible from the internal write buffer to the socket, without blocking. If any data was written, this function returns true; otherwise false is returned.

Call this function if you need QLocalSocket to start sending buffered data immediately. The number of bytes successfully written depends on the operating system. In most cases, you do not need to call this function, because QLocalSocket will start sending data automatically once control goes back to the event loop. In the absence of an event loop, call waitForBytesWritten() instead.

See also write() and waitForBytesWritten().

QString QLocalSocket::fullServerName() const

Returns the server path that the socket is connected to.

Note: The return value of this function is platform specific.

See also connectToServer() and serverName().

[virtual] bool QLocalSocket::isSequential() const

Reimplemented from QIODevice::isSequential().

bool QLocalSocket::isValid() const

Returns true if the socket is valid and ready for use; otherwise returns false.

Note: The socket's state must be ConnectedState before reading and writing can occur.

See also state() and connectToServer().

qint64 QLocalSocket::readBufferSize() const

Returns the size of the internal read buffer. This limits the amount of data that the client can receive before you call read() or readAll(). A read buffer size of 0 (the default) means that the buffer has no size limit, ensuring that no data is lost.

See also setReadBufferSize() and read().

[virtual protected] qint64 QLocalSocket::readData(char * data, qint64 c)

Reimplemented from QIODevice::readData().

QString QLocalSocket::serverName() const

Returns the name of the peer as specified by connectToServer(), or an empty QString if connectToServer() has not been called or it failed.

See also connectToServer() and fullServerName().

void QLocalSocket::setReadBufferSize(qint64 size)

Sets the size of QLocalSocket's internal read buffer to be size bytes.

If the buffer size is limited to a certain size, QLocalSocket won't buffer more than this size of data. Exceptionally, a buffer size of 0 means that the read buffer is unlimited and all incoming data is buffered. This is the default.

This option is useful if you only read the data at certain points in time (e.g., in a real-time streaming application) or if you want to protect your socket against receiving too much data, which may eventually cause your application to run out of memory.

See also readBufferSize() and read().

bool QLocalSocket::setSocketDescriptor(quintptr socketDescriptor, LocalSocketState socketState = ConnectedState, OpenMode openMode = ReadWrite)

Initializes QLocalSocket with the native socket descriptor socketDescriptor. Returns true if socketDescriptor is accepted as a valid socket descriptor; otherwise returns false. The socket is opened in the mode specified by openMode, and enters the socket state specified by socketState.

Note: It is not possible to initialize two local sockets with the same native socket descriptor.

See also socketDescriptor(), state(), and openMode().

quintptr QLocalSocket::socketDescriptor() const

Returns the native socket descriptor of the QLocalSocket object if this is available; otherwise returns -1.

The socket descriptor is not available when QLocalSocket is in UnconnectedState.

See also setSocketDescriptor().

LocalSocketState QLocalSocket::state() const

Returns the state of the socket.

See also error().

[signal] void QLocalSocket::stateChanged(QLocalSocket::LocalSocketState socketState)

This signal is emitted whenever QLocalSocket's state changes. The socketState parameter is the new state.

QLocalSocket::SocketState is not a registered metatype, so for queued connections, you will have to register it with Q_DECLARE_METATYPE() and qRegisterMetaType().

See also state() and Creating Custom Qt Types.

[virtual] bool QLocalSocket::waitForBytesWritten(int msecs = 30000)

Reimplemented from QIODevice::waitForBytesWritten().

bool QLocalSocket::waitForConnected(int msecs = 30000)

Waits until the socket is connected, up to msecs milliseconds. If the connection has been established, this function returns true; otherwise it returns false. In the case where it returns false, you can call error() to determine the cause of the error.

The following example waits up to one second for a connection to be established:

if (socket->waitForConnected(1000))

If msecs is -1, this function will not time out.

See also connectToServer() and connected().

bool QLocalSocket::waitForDisconnected(int msecs = 30000)

Waits until the socket has disconnected, up to msecs milliseconds. If the connection has been disconnected, this function returns true; otherwise it returns false. In the case where it returns false, you can call error() to determine the cause of the error.

The following example waits up to one second for a connection to be closed:

if (socket->waitForDisconnected(1000))

If msecs is -1, this function will not time out.

See also disconnectFromServer() and close().

[virtual] bool QLocalSocket::waitForReadyRead(int msecs = 30000)

Reimplemented from QIODevice::waitForReadyRead().

This function blocks until data is available for reading and the readyRead() signal has been emitted. The function will timeout after msecs milliseconds; the default timeout is 30000 milliseconds.

The function returns true if data is available for reading; otherwise it returns false (if an error occurred or the operation timed out).

See also waitForBytesWritten().

[virtual protected] qint64 QLocalSocket::writeData(const char * data, qint64 c)

Reimplemented from QIODevice::writeData().

© 2015 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.