Detailed Description

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

• InstantiateQTcpSocket orQUdpSocket.
• Create a native socket descriptor, instantiate QAbstractSocket, and callsetSocketDescriptor() 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 toconnectToHost(), and functions likeread() andwrite() use these values.

At any time, QAbstractSocket has a state (returned bystate()). The initial state isUnconnectedState. After callingconnectToHost(), the socket first entersHostLookupState. If the host is found, QAbstractSocket entersConnectingState and emits thehostFound() signal. When the connection has been established, it entersConnectedState and emitsconnected(). If an error occurs at any stage,errorOccurred() is emitted. Whenever the state changes,stateChanged() is emitted. For convenience,isValid() returnstrue if the socket is ready for reading and writing, but note that the socket's state must beConnectedState before reading and writing can occur.

Read or write data by callingread() orwrite(), or use the convenience functionsreadLine() andreadAll(). QAbstractSocket also inheritsgetChar(),putChar(), andungetChar() fromQIODevice, which work on single bytes. ThebytesWritten() signal is emitted when data has been written to the socket. Note that Qt does not limit the write buffer size. You can monitor its size by listening to this signal.

ThereadyRead() 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 thereadyRead() 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, callsetReadBufferSize().

To close the socket, calldisconnectFromHost(). QAbstractSocket entersQAbstractSocket::ClosingState. After all pending data has been written to the socket, QAbstractSocket actually closes the socket, entersQAbstractSocket::UnconnectedState, and emitsdisconnected(). If you want to abort a connection immediately, discarding all pending data, callabort() instead. If the remote host closes the connection, QAbstractSocket will emiterrorOccurred(QAbstractSocket::RemoteHostClosedError), during which the socket state will still beConnectedState, and then thedisconnected() signal will be emitted.

The port and address of the connected peer is fetched by callingpeerPort() andpeerAddress().peerName() returns the host name of the peer, as passed toconnectToHost().localPort() andlocalAddress() 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.
• waitForDisconnected() blocks until the connection has closed.

We show an example:

int numRead=0, numReadTotal=0;char buffer[50];    forever {        numRead= socket.read(buffer,50);// do whatever with array        numReadTotal+= numRead;if (numRead==0&&!socket.waitForReadyRead())break;    }

IfwaitForReadyRead() returnsfalse, the connection has been closed or an error has occurred.

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 thefortuneclient andblockingfortuneclient examples for an overview of both approaches.

QAbstractSocket can be used withQTextStream andQDataStream'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>>().

Member Type Documentation

enum QAbstractSocket::BindFlag
flags QAbstractSocket::BindMode

This enum describes the different flags you can pass to modify the behavior ofQAbstractSocket::bind().

This enum was introduced or modified in Qt 5.0.

The BindMode type is a typedef forQFlags<BindFlag>. It stores an OR combination of BindFlag values.

enum QAbstractSocket::NetworkLayerProtocol

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

See alsoQHostAddress::protocol().

enum QAbstractSocket::PauseMode
flags QAbstractSocket::PauseModes

This enum describes the behavior of when the socket should hold back with continuing data transfer. The only notification currently supported is QSslSocket::sslErrors().

This enum was introduced or modified in Qt 5.0.

The PauseModes type is a typedef forQFlags<PauseMode>. It stores an OR combination of PauseMode values.

enum QAbstractSocket::SocketError

This enum describes the socket errors that can occur.

See alsoQAbstractSocket::error() andQAbstractSocket::errorOccurred().

enum QAbstractSocket::SocketOption

This enum represents the options that can be set on a socket. If desired, they can be set after having received theconnected() signal from the socket or after having received a new socket from aQTcpServer.

Possible values forTypeOfServiceOption are:

This enum was introduced or modified in Qt 4.6.

See alsoQAbstractSocket::setSocketOption() andQAbstractSocket::socketOption().

enum QAbstractSocket::SocketState

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

See alsoQAbstractSocket::state().

enum QAbstractSocket::SocketType

This enum describes the transport layer protocol.

See alsoQAbstractSocket::socketType().

Member Function Documentation

QAbstractSocket::QAbstractSocket(QAbstractSocket::SocketTypesocketType,QObject *parent)

Creates a new abstract socket of typesocketType. Theparent argument is passed toQObject's constructor.

See alsosocketType(),QTcpSocket, andQUdpSocket.

[signal]void QAbstractSocket::connected()

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

See alsoconnectToHost() anddisconnected().

[signal]void QAbstractSocket::disconnected()

This signal is emitted when the socket has been disconnected.

See alsoconnectToHost(),disconnectFromHost(), andabort().

[signal]void QAbstractSocket::errorOccurred(QAbstractSocket::SocketErrorsocketError)

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

When this signal is emitted, the socket may not be ready for a reconnect attempt. In that case, attempts to reconnect should be done from the event loop. For example, use aQTimer::singleShot() with 0 as the timeout.

QAbstractSocket::SocketError is not a registered metatype, so for queued connections, you will have to register it withQ_DECLARE_METATYPE() andqRegisterMetaType().

This function was introduced in Qt 5.15.

See alsoerror(),errorString(), andCreating Custom Qt Types.

[signal]void QAbstractSocket::hostFound()

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

See alsoconnected().

[signal]void QAbstractSocket::proxyAuthenticationRequired(constQNetworkProxy &proxy,QAuthenticator *authenticator)

This signal can be emitted when aproxy that requires authentication is used. Theauthenticator object can then be filled in with the required details to allow authentication and continue the connection.

This function was introduced in Qt 4.3.

See alsoQAuthenticator andQNetworkProxy.

[signal]void QAbstractSocket::stateChanged(QAbstractSocket::SocketStatesocketState)

This signal is emitted wheneverQAbstractSocket's state changes. ThesocketState parameter is the new state.

QAbstractSocket::SocketState is not a registered metatype, so for queued connections, you will have to register it withQ_DECLARE_METATYPE() andqRegisterMetaType().

See alsostate() andCreating Custom Qt Types.

[virtual]QAbstractSocket::~QAbstractSocket()

Destroys the socket.

void QAbstractSocket::abort()

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

See alsodisconnectFromHost() andclose().

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

Reimplements:QIODevice::atEnd() const.

Returnstrue if no more data is currently available for reading; otherwise returnsfalse.

This function is most commonly used when reading data from the socket in a loop. For example:

// This slot is connected to QAbstractSocket::readyRead()void SocketClass::readyReadSlot() {while (!socket.atEnd()) {QByteArray data= socket.read(100);....     } }

See alsobytesAvailable() andreadyRead().

bool QAbstractSocket::bind(constQHostAddress &address,quint16port = 0,QAbstractSocket::BindModemode = ...)

Binds toaddress on portport, using theBindModemode.

For UDP sockets, after binding, the signalQUdpSocket::readyRead() is emitted whenever a UDP datagram arrives on the specified address and port. Thus, this function is useful to write UDP servers.

For TCP sockets, this function may be used to specify which interface to use for an outgoing connection, which is useful in case of multiple network interfaces.

By default, the socket is bound using theDefaultForPlatformBindMode. If a port is not specified, a random port is chosen.

On success, the function returnstrue and the socket entersBoundState; otherwise it returnsfalse.

This function was introduced in Qt 5.0.

bool QAbstractSocket::bind(quint16port = 0,QAbstractSocket::BindModemode = DefaultForPlatform)

This is an overloaded function.

Binds toQHostAddress:Any on portport, using theBindModemode.

By default, the socket is bound using theDefaultForPlatformBindMode. If a port is not specified, a random port is chosen.

This function was introduced in Qt 5.0.

[override virtual]qint64 QAbstractSocket::bytesAvailable() const

Reimplements:QIODevice::bytesAvailable() const.

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

See alsobytesToWrite() andread().

[override virtual]qint64 QAbstractSocket::bytesToWrite() const

Reimplements:QIODevice::bytesToWrite() const.

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

See alsobytesAvailable() andflush().

[override virtual]bool QAbstractSocket::canReadLine() const

Reimplements:QIODevice::canReadLine() const.

Returnstrue if a line of data can be read from the socket; otherwise returnsfalse.

See alsoreadLine().

[override virtual]void QAbstractSocket::close()

Reimplements:QIODevice::close().

Closes the I/O device for the socket and callsdisconnectFromHost() to close the socket's connection.

SeeQIODevice::close() for a description of the actions that occur when an I/O device is closed.

See alsoabort().

[virtual]void QAbstractSocket::connectToHost(constQString &hostName,quint16port,QIODevice::OpenModeopenMode = ReadWrite,QAbstractSocket::NetworkLayerProtocolprotocol = AnyIPProtocol)

Attempts to make a connection tohostName on the givenport. Theprotocol parameter can be used to specify which network protocol to use (eg. IPv4 or IPv6).

The socket is opened in the givenopenMode and first entersHostLookupState, then performs a host name lookup ofhostName. If the lookup succeeds,hostFound() is emitted andQAbstractSocket entersConnectingState. It then attempts to connect to the address or addresses returned by the lookup. Finally, if a connection is established,QAbstractSocket entersConnectedState and emitsconnected().

At any point, the socket can emiterrorOccurred() 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., "example.com").QAbstractSocket will do a lookup only if required.port is in native byte order.

See alsostate(),peerName(),peerAddress(),peerPort(), andwaitForConnected().

[virtual]void QAbstractSocket::connectToHost(constQHostAddress &address,quint16port,QIODevice::OpenModeopenMode = ReadWrite)

This is an overloaded function.

Attempts to make a connection toaddress on portport.

[virtual]void QAbstractSocket::disconnectFromHost()

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

See alsoconnectToHost().

QAbstractSocket::SocketError QAbstractSocket::error() const

Returns the type of error that last occurred.

See alsostate() anderrorString().

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 returnstrue; otherwise false is returned.

Call this function if you needQAbstractSocket 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, becauseQAbstractSocket will start sending data automatically once control goes back to the event loop. In the absence of an event loop, callwaitForBytesWritten() instead.

See alsowrite() andwaitForBytesWritten().

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

Reimplements:QIODevice::isSequential() const.

bool QAbstractSocket::isValid() const

Returnstrue if the socket is valid and ready for use; otherwise returnsfalse.

See alsostate().

QHostAddress QAbstractSocket::localAddress() const

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

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

See alsolocalPort(),peerAddress(), andsetLocalAddress().

quint16 QAbstractSocket::localPort() const

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

See alsolocalAddress(),peerPort(), andsetLocalPort().

QAbstractSocket::PauseModes QAbstractSocket::pauseMode() const

Returns the pause mode of this socket.

This function was introduced in Qt 5.0.

See alsosetPauseMode() andresume().

QHostAddress QAbstractSocket::peerAddress() const

Returns the address of the connected peer if the socket is inConnectedState; otherwise returnsQHostAddress::Null.

See alsopeerName(),peerPort(),localAddress(), andsetPeerAddress().

QString QAbstractSocket::peerName() const

Returns the name of the peer as specified byconnectToHost(), or an emptyQString ifconnectToHost() has not been called.

See alsopeerAddress(),peerPort(), andsetPeerName().

quint16 QAbstractSocket::peerPort() const

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

See alsopeerAddress(),localPort(), andsetPeerPort().

QString QAbstractSocket::protocolTag() const

Returns the protocol tag for this socket. If the protocol tag is set then this is passed toQNetworkProxyQuery when this is created internally to indicate the protocol tag to be used.

This function was introduced in Qt 5.13.

See alsosetProtocolTag() andQNetworkProxyQuery.

QNetworkProxy QAbstractSocket::proxy() const

Returns the network proxy for this socket. By defaultQNetworkProxy::DefaultProxy is used, which means this socket will query the default proxy settings for the application.

This function was introduced in Qt 4.1.

See alsosetProxy(),QNetworkProxy, andQNetworkProxyFactory.

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 callread() orreadAll().

A read buffer size of 0 (the default) means that the buffer has no size limit, ensuring that no data is lost.

See alsosetReadBufferSize() andread().

[override virtual protected]qint64 QAbstractSocket::readData(char *data,qint64maxSize)

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

[override virtual protected]qint64 QAbstractSocket::readLineData(char *data,qint64maxlen)

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

[virtual]void QAbstractSocket::resume()

Continues data transfer on the socket. This method should only be used after the socket has been set to pause upon notifications and a notification has been received. The only notification currently supported is QSslSocket::sslErrors(). Calling this method if the socket is not paused results in undefined behavior.

This function was introduced in Qt 5.0.

See alsopauseMode() andsetPauseMode().

[protected]void QAbstractSocket::setLocalAddress(constQHostAddress &address)

Sets the address on the local side of a connection toaddress.

You can call this function in a subclass ofQAbstractSocket to change the return value of thelocalAddress() function after a connection has been established. This feature is commonly used by proxy connections for virtual connection settings.

Note that this function does not bind the local address of the socket prior to a connection (e.g.,QAbstractSocket::bind()).

This function was introduced in Qt 4.1.

See alsolocalAddress(),setLocalPort(), andsetPeerAddress().

[protected]void QAbstractSocket::setLocalPort(quint16port)

Sets the port on the local side of a connection toport.

You can call this function in a subclass ofQAbstractSocket to change the return value of thelocalPort() function after a connection has been established. This feature is commonly used by proxy connections for virtual connection settings.

Note that this function does not bind the local port of the socket prior to a connection (e.g.,QAbstractSocket::bind()).

This function was introduced in Qt 4.1.

See alsolocalPort(),localAddress(),setLocalAddress(), andsetPeerPort().

void QAbstractSocket::setPauseMode(QAbstractSocket::PauseModespauseMode)

Controls whether to pause upon receiving a notification. ThepauseMode parameter specifies the conditions in which the socket should be paused. The only notification currently supported is QSslSocket::sslErrors(). If set toPauseOnSslErrors, data transfer on the socket will be paused and needs to be enabled explicitly again by callingresume(). By default this option is set toPauseNever. This option must be called before connecting to the server, otherwise it will result in undefined behavior.

This function was introduced in Qt 5.0.

See alsopauseMode() andresume().

[protected]void QAbstractSocket::setPeerAddress(constQHostAddress &address)

Sets the address of the remote side of the connection toaddress.

You can call this function in a subclass ofQAbstractSocket to change the return value of thepeerAddress() function after a connection has been established. This feature is commonly used by proxy connections for virtual connection settings.

This function was introduced in Qt 4.1.

See alsopeerAddress(),setPeerPort(), andsetLocalAddress().

[protected]void QAbstractSocket::setPeerName(constQString &name)

Sets the host name of the remote peer toname.

You can call this function in a subclass ofQAbstractSocket to change the return value of thepeerName() function after a connection has been established. This feature is commonly used by proxy connections for virtual connection settings.

This function was introduced in Qt 4.1.

See alsopeerName().

[protected]void QAbstractSocket::setPeerPort(quint16port)

Sets the port of the remote side of the connection toport.

You can call this function in a subclass ofQAbstractSocket to change the return value of thepeerPort() function after a connection has been established. This feature is commonly used by proxy connections for virtual connection settings.

This function was introduced in Qt 4.1.

See alsopeerPort(),setPeerAddress(), andsetLocalPort().

void QAbstractSocket::setProtocolTag(constQString &tag)

Sets the protocol tag for this socket totag.

This function was introduced in Qt 5.13.

See alsoprotocolTag().

void QAbstractSocket::setProxy(constQNetworkProxy &networkProxy)

Sets the explicit network proxy for this socket tonetworkProxy.

To disable the use of a proxy for this socket, use theQNetworkProxy::NoProxy proxy type:

socket->setProxy(QNetworkProxy::NoProxy);

The default value for the proxy isQNetworkProxy::DefaultProxy, which means the socket will use the application settings: if a proxy is set withQNetworkProxy::setApplicationProxy, it will use that; otherwise, if a factory is set withQNetworkProxyFactory::setApplicationProxyFactory, it will query that factory with typeQNetworkProxyQuery::TcpSocket.

This function was introduced in Qt 4.1.

See alsoproxy(),QNetworkProxy, andQNetworkProxyFactory::queryProxy().

[virtual]void QAbstractSocket::setReadBufferSize(qint64size)

Sets the size ofQAbstractSocket's internal read buffer to besize 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.

OnlyQTcpSocket usesQAbstractSocket's internal buffer;QUdpSocket does not use any buffering at all, but rather relies on the implicit buffering provided by the operating system. Because of this, calling this function onQUdpSocket has no effect.

See alsoreadBufferSize() andread().

[virtual]bool QAbstractSocket::setSocketDescriptor(qintptrsocketDescriptor,QAbstractSocket::SocketStatesocketState = ConnectedState,QIODevice::OpenModeopenMode = ReadWrite)

InitializesQAbstractSocket with the native socket descriptorsocketDescriptor. Returnstrue ifsocketDescriptor is accepted as a valid socket descriptor; otherwise returnsfalse. The socket is opened in the mode specified byopenMode, and enters the socket state specified bysocketState. Read and write buffers are cleared, discarding any pending data.

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

See alsosocketDescriptor().

[protected]void QAbstractSocket::setSocketError(QAbstractSocket::SocketErrorsocketError)

Sets the type of error that last occurred tosocketError.

See alsosetSocketState() andsetErrorString().

[virtual]void QAbstractSocket::setSocketOption(QAbstractSocket::SocketOptionoption, constQVariant &value)

Sets the givenoption to the value described byvalue.

This function was introduced in Qt 4.6.

See alsosocketOption().

[protected]void QAbstractSocket::setSocketState(QAbstractSocket::SocketStatestate)

Sets the state of the socket tostate.

See alsostate().

[virtual]qintptr QAbstractSocket::socketDescriptor() const

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

If the socket is usingQNetworkProxy, the returned descriptor may not be usable with native socket functions.

The socket descriptor is not available whenQAbstractSocket is inUnconnectedState.

See alsosetSocketDescriptor().

[virtual]QVariant QAbstractSocket::socketOption(QAbstractSocket::SocketOptionoption)

Returns the value of theoption option.

This function was introduced in Qt 4.6.

See alsosetSocketOption().

QAbstractSocket::SocketType QAbstractSocket::socketType() const

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

See alsoQTcpSocket andQUdpSocket.

QAbstractSocket::SocketState QAbstractSocket::state() const

Returns the state of the socket.

See alsoerror().

[override virtual]bool QAbstractSocket::waitForBytesWritten(intmsecs = 30000)

Reimplements:QIODevice::waitForBytesWritten(int msecs).

This function blocks until at least one byte has been written on the socket and thebytesWritten() signal has been emitted. The function will timeout aftermsecs milliseconds; the default timeout is 30000 milliseconds.

The function returnstrue if thebytesWritten() signal is emitted; otherwise it returnsfalse (if an error occurred or the operation timed out).

See alsowaitForReadyRead().

[virtual]bool QAbstractSocket::waitForConnected(intmsecs = 30000)

Waits until the socket is connected, up tomsecs milliseconds. If the connection has been established, this function returnstrue; otherwise it returnsfalse. In the case where it returnsfalse, you can callerror() 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 alsoconnectToHost() andconnected().

[virtual]bool QAbstractSocket::waitForDisconnected(intmsecs = 30000)

Waits until the socket has disconnected, up tomsecs milliseconds. If the connection was successfully disconnected, this function returnstrue; otherwise it returnsfalse (if the operation timed out, if an error occurred, or if thisQAbstractSocket is already disconnected). In the case where it returnsfalse, you can callerror() to determine the cause of the error.

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

socket->disconnectFromHost();if (socket->state()==QAbstractSocket::UnconnectedState|| socket->waitForDisconnected(1000)) {qDebug("Disconnected!");}

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

See alsodisconnectFromHost() andclose().

[override virtual]bool QAbstractSocket::waitForReadyRead(intmsecs = 30000)

Reimplements:QIODevice::waitForReadyRead(int msecs).

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

The function returnstrue if thereadyRead() signal is emitted and there is new data available for reading; otherwise it returnsfalse (if an error occurred or the operation timed out).

See alsowaitForBytesWritten().

[override virtual protected]qint64 QAbstractSocket::writeData(constchar *data,qint64size)

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