QAbstractSocket Class Reference
^{[QtNetwork module]}

Detailed Description

The QAbstractSocket class provides the base functionality common to all socket types.

QAbstractSocket is the base class for QTcpSocket and QUdpSocket and contains all common functionality of these two classes. If you need a socket, you have two options:

• Instantiate QTcpSocket or QUdpSocket.
• Create a native socket descriptor, instantiate QAbstractSocket, and call setSocketDescriptor() to wrap the native socket.

TCP (Transmission Control Protocol) is a reliable, stream-oriented, connection-oriented transport protocol. UDP (User Datagram Protocol) is an unreliable, datagram-oriented, connectionless protocol. In practice, this means that TCP is better suited for continuous transmission of data, whereas the more lightweight UDP can be used when reliability isn't important.

QAbstractSocket's API unifies most of the differences between the two protocols. For example, although UDP is connectionless, connectToHost() establishes a virtual connection for UDP sockets, enabling you to use QAbstractSocket in more or less the same way regardless of the underlying protocol. Internally, QAbstractSocket remembers the address and port passed to connectToHost(), and functions like read() and write() use these values.

At any time, QAbstractSocket has a state (returned by state()). The initial state is QAbstractSocket::UnconnectedState. After calling connectToHost(), the socket first enters QAbstractSocket::HostLookupState. If the host is found, QAbstractSocket enters QAbstractSocket::ConnectingState and emits the hostFound() signal. When the connection has been established, it enters QAbstractSocket::ConnectedState and emits connected(). If an error occurs at any stage, error() is emitted. Whenever the state changes, stateChanged() is emitted. For convenience, isValid() returns true if the socket is ready for reading and writing.

Read or write data by calling read() or write(), or use the convenience functions readLine() and readAll(). QAbstractSocket also inherits getChar(), putChar(), and ungetChar() from QIODevice, which work on single bytes. For every chunk of data that has been written to the socket, the bytesWritten() signal is emitted.

The readyRead() signal is emitted every time a new chunk of data has arrived. bytesAvailable() then returns the number of bytes that are available for reading. Typically, you would connect the readyRead() signal to a slot and read all available data there. If you don't read all the data at once, the remaining data will still be available later, and any new incoming data will be appended to QAbstractSocket's internal read buffer. To limit the size of the read buffer, call setReadBufferSize().

To close the socket, call close(). QAbstractSocket enters QAbstractSocket::ClosingState, then emits closing(). After all pending data has been written to the socket, QAbstractSocket actually closes the socket, enters QAbstractSocket::ClosedState, and emits closed(). If you want to abort a connection immediately, discarding all pending data, call abort() instead.

The port and address of the connected peer is fetched by calling peerPort() and peerAddress(). peerName() returns the host name of the peer, as passed to connectToHost(). localPort() and localAddress() return the port and address of the local socket.

QAbstractSocket provides a set of functions that suspend the calling thread until certain signals are emitted. These functions can be used to implement blocking sockets:

• waitForConnected() blocks until a connection has been established.
• waitForReadyRead() blocks until new data is available for reading.
• waitForBytesWritten() blocks until one payload of data has been written to the socket.
• waitForClosed() blocks until the connection has closed.

Programming with a blocking socket is radically different from programming with a non-blocking socket. A blocking socket doesn't require an event loop and typically leads to simpler code. However, in a GUI application, blocking sockets should only be used in non-GUI threads, to avoid freezing the user interface. See the network/fortuneclient and network/blockingfortuneclient examples for an overview of both approaches.

QAbstractSocket can be used with QTextStream and QDataStream's stream operators (operator<<() and operator>>()). There is one issue to be aware of, though: You must make sure that enough data is available before attempting to read it using operator>>().

See also QFtp, QHttp, and QTcpServer.

Member Type Documentation

enum QAbstractSocket::NetworkLayerProtocol

This enum describes the network layer protocol values used in Qt.

See also QSocketLayer::protocol().

enum QAbstractSocket::SocketError

This enum describes the socket errors that can occur.

See also QAbstractSocket::error().

enum QAbstractSocket::SocketState

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

See also QAbstractSocket::state().

enum QAbstractSocket::SocketType

This enum describes the transport layer protocol.

See also QAbstractSocket::socketType().

Member Function Documentation

QAbstractSocket::QAbstractSocket ( SocketType socketType, QObject * parent )

Creates a new abstract socket of type socketType. The parent argument is passed to QObject's constructor.

See also socketType(), QTcpSocket, and QUdpSocket.

QAbstractSocket::~QAbstractSocket ()   [virtual]

Destroys the socket.

void QAbstractSocket::abort ()

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

See also close().

qint64 QAbstractSocket::bytesAvailable () const   [virtual]

Returns the number of incoming bytes that are waiting to be read.

Reimplemented from QIODevice.

See also bytesToWrite() and read().

qint64 QAbstractSocket::bytesToWrite () const   [virtual]

Returns the number of bytes that are waiting to be written. The bytes are written when control goes back to the event loop or when flush() is called.

Reimplemented from QIODevice.

See also bytesAvailable() and flush().

bool QAbstractSocket::canReadLine () const   [virtual]

Returns true if a line of data can be read from the socket; otherwise returns false.

Reimplemented from QIODevice.

See also readLine().

void QAbstractSocket::close ()   [virtual]

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

Reimplemented from QIODevice.

See also abort().

void QAbstractSocket::connectToHost ( const QString & hostName, quint16 port, OpenMode openMode = ReadWrite )

Attempts to make a connection to hostName on the given port.

The socket is opened in the given openMode and first enters HostLookupState, then performs a host name lookup of hostName. If the lookup succeeds, hostFound() is emitted and QAbstractSocket enters ConnectingState. It then attempts to connect to the address or addresses returned by the lookup. Finally, if a connection is established, QAbstractSocket enters ConnectedState and emits connected().

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

hostName may be an IP address in string form (e.g., "43.195.83.32"), or it may be a host name (e.g., "www.trolltech.com"). QAbstractSocket will do a lookup only if required. port is in native byte order.

See also state(), peerName(), peerAddress(), peerPort(), and waitForConnected().

void QAbstractSocket::connectToHost ( const QHostAddress & address, quint16 port, OpenMode openMode = ReadWrite )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Attempts to make a connection to address on port port.

void QAbstractSocket::connected ()   [signal]

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

See also connectToHost() and connectionClosed().

void QAbstractSocket::disconnectFromHost ()

Disconnects the socket's connection with the host.

See also connectToHost().

void QAbstractSocket::disconnected ()   [signal]

This signal is emitted when the socket has been disconnected.

See also connectToHost() and close().

SocketError QAbstractSocket::error () const

Returns the type of error that last occurred.

See also state() and errorString().

void QAbstractSocket::error ( QAbstractSocket::SocketError socketError )   [signal]

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

See also error() and errorString().

bool QAbstractSocket::flush ()

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

Call this function if you need QAbstractSocket 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 QAbstractSocket 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().

void QAbstractSocket::hostFound ()   [signal]

This signal is emitted after connectToHost() has been called and the host lookup has succeeded.

See also connected().

bool QAbstractSocket::isValid () const

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

See also state().

QHostAddress QAbstractSocket::localAddress () const

Returns the host address of the local socket if available; otherwise returns QHostAddress::Null.

This is normally the main IP address of the host, but can be QHostAddress::LocalHost (127.0.0.1) for connections to the local host.

See also localPort() and peerAddress().

quint16 QAbstractSocket::localPort () const

Returns the host port number (in native byte order) of the local socket if available; otherwise returns 0.

See also localAddress() and peerPort().

QHostAddress QAbstractSocket::peerAddress () const

Returns the address of the connected peer if the socket is in ConnectedState; otherwise returns QHostAddress::Null.

See also peerName(), peerPort(), and localAddress().

QString QAbstractSocket::peerName () const

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

See also peerAddress() and peerPort().

quint16 QAbstractSocket::peerPort () const

Returns the port of the connected peer if the socket is in ConnectedState; otherwise returns 0.

See also peerAddress() and localPort().

qint64 QAbstractSocket::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().

void QAbstractSocket::setReadBufferSize ( qint64 size )

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

If the buffer size is limited to a certain size, QAbstractSocket 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 QAbstractSocket::setSocketDescriptor ( int socketDescriptor, SocketState socketState = ConnectedState, OpenMode openMode = ReadWrite )

Initializes QAbstractSocket 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.

See also socketDescriptor().

void QAbstractSocket::setSocketError ( SocketError socketError )   [protected]

Sets the type of error that last occurred to socketError.

See also setSocketState() and setErrorString().

void QAbstractSocket::setSocketState ( SocketState state )   [protected]

Sets the state of the socket to state.

See also state().

int QAbstractSocket::socketDescriptor () const

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

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

See also setSocketDescriptor().

SocketType QAbstractSocket::socketType () const

Returns the socket type (TCP, UDP, or other).

See also QTcpSocket and QUdpSocket.

SocketState QAbstractSocket::state () const

Returns the state of the socket.

See also error().

void QAbstractSocket::stateChanged ( QAbstractSocket::SocketState socketState )   [signal]

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

See also state().

bool QAbstractSocket::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:

    socket->connectToHost("imap", 143);
    if (socket->waitForConnected(1000))
        qDebug("Connected!");

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

See also connectToHost() and connected().

bool QAbstractSocket::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:

    socket->disconnect();
    if (socket->waitForDisconnected(1000))
        qDebug("Disconnected!");

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

See also disconnect() and close().