- Qt 5.15
- Qt Network
- C++ Classes
- QAbstractSocket
- Qt 5.15.18 Reference Documentation

# QAbstractSocket Class

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

Header: | #include <QAbstractSocket>
qmake: | QT += network
Inherited By: | QTcpSocket and QUdpSocket

- List of all members, including inherited members
- Obsolete members

Note: All functions in this class are reentrant.

## Public Types

enum | BindFlag { ShareAddress, DontShareAddress, ReuseAddressHint, DefaultForPlatform }
flags | BindMode
enum | NetworkLayerProtocol { IPv4Protocol, IPv6Protocol, AnyIPProtocol, UnknownNetworkLayerProtocol }
enum | PauseMode { PauseNever, PauseOnSslErrors }
flags | PauseModes
enum | SocketError { ConnectionRefusedError, RemoteHostClosedError, HostNotFoundError, SocketAccessError, SocketResourceError, …, UnknownSocketError }
enum | SocketOption { LowDelayOption, KeepAliveOption, MulticastTtlOption, MulticastLoopbackOption, TypeOfServiceOption, …, PathMtuSocketOption }
enum | SocketState { UnconnectedState, HostLookupState, ConnectingState, ConnectedState, BoundState, …, ListeningState }
enum | SocketType { TcpSocket, UdpSocket, SctpSocket, UnknownSocketType }

## Public Functions

| QAbstractSocket(SocketType socketType, QObject *parent)
virtual | ~QAbstractSocket()
void | abort()
bool | bind(const QHostAddress &address, quint16 port = 0, BindMode mode = DefaultForPlatform)
bool | bind(quint16 port = 0, BindMode mode = DefaultForPlatform)
virtual void | connectToHost(const QString &hostName, quint16 port, OpenMode openMode = ReadWrite, NetworkLayerProtocol protocol = AnyIPProtocol)
virtual void | connectToHost(const QHostAddress &address, quint16 port, OpenMode openMode = ReadWrite)
virtual void | disconnectFromHost()
SocketError | error() const
bool | flush()
bool | isValid() const
QHostAddress | localAddress() const
quint16 | localPort() const
PauseModes | pauseMode() const
QHostAddress | peerAddress() const
QString | peerName() const
quint16 | peerPort() const
QString | protocolTag() const
QNetworkProxy | proxy() const
qint64 | readBufferSize() const
virtual void | resume()
void | setPauseMode(PauseModes pauseMode)
void | setProtocolTag(const QString &tag)
void | setProxy(const QNetworkProxy &networkProxy)
virtual void | setReadBufferSize(qint64 size)
virtual bool | setSocketDescriptor(qintptr socketDescriptor, SocketState socketState = ConnectedState, OpenMode openMode = ReadWrite)
virtual void | setSocketOption(QAbstractSocket::SocketOption option, const QVariant &value)
virtual qintptr | socketDescriptor() const
virtual QVariant | socketOption(QAbstractSocket::SocketOption option)
SocketType | socketType() const
SocketState | state() const
virtual bool | waitForConnected(int msecs = 30000)
virtual bool | waitForDisconnected(int msecs = 30000)

## Reimplemented Public Functions

virtual bool | atEnd() const override
virtual qint64 | bytesAvailable() const override
virtual qint64 | bytesToWrite() const override
virtual bool | canReadLine() const override
virtual void | close() override
virtual bool | isSequential() const override
virtual bool | waitForBytesWritten(int msecs = 30000) override
virtual bool | waitForReadyRead(int msecs = 30000) override

## Signals

void | connected()
void | disconnected()
void | errorOccurred(QAbstractSocket::SocketError socketError)
void | hostFound()
void | proxyAuthenticationRequired(const QNetworkProxy &proxy, QAuthenticator *authenticator)
void | stateChanged(QAbstractSocket::SocketState socketState)

## Protected Functions

void | setLocalAddress(const QHostAddress &address)
void | setLocalPort(quint16 port)
void | setPeerAddress(const QHostAddress &address)
void | setPeerName(const QString &name)
void | setPeerPort(quint16 port)
void | setSocketError(SocketError socketError)
void | setSocketState(SocketState state)

## Reimplemented Protected Functions

virtual qint64 | readData(char *data, qint64 maxSize) override
virtual qint64 | readLineData(char *data, qint64 maxlen) override
virtual qint64 | writeData(const char *data, qint64 size) override

## Detailed Description

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 UnconnectedState. After calling connectToHost(), the socket first enters HostLookupState. If the host is found, QAbstractSocket enters ConnectingState and emits the hostFound() signal. When the connection has been established, it enters ConnectedState and emits connected(). If an error occurs at any stage, errorOccurred() is emitted. Whenever the state changes, stateChanged() is emitted. For convenience, isValid() returns `true` if the socket is ready for reading and writing, but note that the socket's state must be ConnectedState before reading and writing can occur.

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. The bytesWritten() 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.

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 disconnectFromHost(). QAbstractSocket enters QAbstractSocket::ClosingState. After all pending data has been written to the socket, QAbstractSocket actually closes the socket, enters QAbstractSocket::UnconnectedState, and emits disconnected(). If you want to abort a connection immediately, discarding all pending data, call abort() instead. If the remote host closes the connection, QAbstractSocket will emit errorOccurred(QAbstractSocket::RemoteHostClosedError), during which the socket state will still be ConnectedState, and then the disconnected() signal will be emitted.

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.
- 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;
         }

If waitForReadyRead() returns `false`, 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 the fortuneclient and blockingfortuneclient examples for an overview of both approaches.

Note: We discourage the use of the blocking functions together with signals. One of the two possibilities should be used.

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 QNetworkAccessManager and QTcpServer.

## Member Type Documentation

### enum QAbstractSocket::BindFlag
flags QAbstractSocket::BindMode

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

Constant | Value | Description
--- | --- | ---

`QAbstractSocket::ShareAddress`

`0x1`

Allow other services to bind to the same address and port. This is useful when multiple processes share the load of a single service by listening to the same address and port (e.g., a web server with several pre-forked listeners can greatly improve response time). However, because any service is allowed to rebind, this option is subject to certain security considerations. Note that by combining this option with ReuseAddressHint, you will also allow your service to rebind an existing shared address. On Unix, this is equivalent to the SO_REUSEADDR socket option. On Windows, this is the default behavior, so this option is ignored.

`QAbstractSocket::DontShareAddress`

`0x2`

Bind the address and port exclusively, so that no other services are allowed to rebind. By passing this option to QAbstractSocket::bind(), you are guaranteed that on success, your service is the only one that listens to the address and port. No services are allowed to rebind, even if they pass ReuseAddressHint. This option provides more security than ShareAddress, but on certain operating systems, it requires you to run the server with administrator privileges. On Unix and macOS, not sharing is the default behavior for binding an address and port, so this option is ignored. On Windows, this option uses the SO_EXCLUSIVEADDRUSE socket option.

`QAbstractSocket::ReuseAddressHint` | `0x4` | Provides a hint to QAbstractSocket that it should try to rebind the service even if the address and port are already bound by another socket. On Windows and Unix, this is equivalent to the SO_REUSEADDR socket option.
`QAbstractSocket::DefaultForPlatform` | `0x0` | The default option for the current platform. On Unix and macOS, this is equivalent to (DontShareAddress + ReuseAddressHint), and on Windows, it is equivalent to ShareAddress.

This enum was introduced or modified in Qt 5.0.

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

### enum QAbstractSocket::NetworkLayerProtocol

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

Constant | Value | Description
--- | --- | ---
`QAbstractSocket::IPv4Protocol` | `0` | IPv4
`QAbstractSocket::IPv6Protocol` | `1` | IPv6
`QAbstractSocket::AnyIPProtocol` | `2` | Either IPv4 or IPv6
`QAbstractSocket::UnknownNetworkLayerProtocol` | `-1` | Other than IPv4 and IPv6

See also QHostAddress::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().

Constant | Value | Description
--- | --- | ---
`QAbstractSocket::PauseNever` | `0x0` | Do not pause data transfer on the socket. This is the default and matches the behavior of Qt 4.
`QAbstractSocket::PauseOnSslErrors` | `0x1` | Pause data transfer on the socket upon receiving an SSL error notification. I.E. QSslSocket::sslErrors().

This enum was introduced or modified in Qt 5.0.

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

### enum QAbstractSocket::SocketError

This enum describes the socket errors that can occur.

Constant | Value | Description
--- | --- | ---
`QAbstractSocket::ConnectionRefusedError` | `0` | The connection was refused by the peer (or timed out).
`QAbstractSocket::RemoteHostClosedError` | `1` | The remote host closed the connection. Note that the client socket (i.e., this socket) will be closed after the remote close notification has been sent.
`QAbstractSocket::HostNotFoundError` | `2` | The host address was not found.
`QAbstractSocket::SocketAccessError` | `3` | The socket operation failed because the application lacked the required privileges.
`QAbstractSocket::SocketResourceError` | `4` | The local system ran out of resources (e.g., too many sockets).
`QAbstractSocket::SocketTimeoutError` | `5` | The socket operation timed out.
`QAbstractSocket::DatagramTooLargeError` | `6` | The datagram was larger than the operating system's limit (which can be as low as 8192 bytes).
`QAbstractSocket::NetworkError` | `7` | An error occurred with the network (e.g., the network cable was accidentally plugged out).
`QAbstractSocket::AddressInUseError` | `8` | The address specified to QAbstractSocket::bind() is already in use and was set to be exclusive.
`QAbstractSocket::SocketAddressNotAvailableError` | `9` | The address specified to QAbstractSocket::bind() does not belong to the host.
`QAbstractSocket::UnsupportedSocketOperationError` | `10` | The requested socket operation is not supported by the local operating system (e.g., lack of IPv6 support).
`QAbstractSocket::ProxyAuthenticationRequiredError` | `12` | The socket is using a proxy, and the proxy requires authentication.
`QAbstractSocket::SslHandshakeFailedError` | `13` | The SSL/TLS handshake failed, so the connection was closed (only used in QSslSocket)
`QAbstractSocket::UnfinishedSocketOperationError` | `11` | Used by QAbstractSocketEngine only, The last operation attempted has not finished yet (still in progress in the background).
`QAbstractSocket::ProxyConnectionRefusedError` | `14` | Could not contact the proxy server because the connection to that server was denied
`QAbstractSocket::ProxyConnectionClosedError` | `15` | The connection to the proxy server was closed unexpectedly (before the connection to the final peer was established)
`QAbstractSocket::ProxyConnectionTimeoutError` | `16` | The connection to the proxy server timed out or the proxy server stopped responding in the authentication phase.
`QAbstractSocket::ProxyNotFoundError` | `17` | The proxy address set with setProxy() (or the application proxy) was not found.
`QAbstractSocket::ProxyProtocolError` | `18` | The connection negotiation with the proxy server failed, because the response from the proxy server could not be understood.
`QAbstractSocket::OperationError` | `19` | An operation was attempted while the socket was in a state that did not permit it.
`QAbstractSocket::SslInternalError` | `20` | The SSL library being used reported an internal error. This is probably the result of a bad installation or misconfiguration of the library.
`QAbstractSocket::SslInvalidUserDataError` | `21` | Invalid data (certificate, key, cypher, etc.) was provided and its use resulted in an error in the SSL library.
`QAbstractSocket::TemporaryError` | `22` | A temporary error occurred (e.g., operation would block and socket is non-blocking).
`QAbstractSocket::UnknownSocketError` | `-1` | An unidentified error occurred.

See also QAbstractSocket::error() and QAbstractSocket::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 the connected() signal from the socket or after having received a new socket from a QTcpServer.

Constant | Value | Description
--- | --- | ---
`QAbstractSocket::LowDelayOption` | `0` | Try to optimize the socket for low latency. For a QTcpSocket this would set the TCP_NODELAY option and disable Nagle's algorithm. Set this to 1 to enable.
`QAbstractSocket::KeepAliveOption` | `1` | Set this to 1 to enable the SO_KEEPALIVE socket option
`QAbstractSocket::MulticastTtlOption` | `2` | Set this to an integer value to set IP_MULTICAST_TTL (TTL for multicast datagrams) socket option.
`QAbstractSocket::MulticastLoopbackOption` | `3` | Set this to 1 to enable the IP_MULTICAST_LOOP (multicast loopback) socket option.
`QAbstractSocket::TypeOfServiceOption` | `4` | This option is not supported on Windows. This maps to the IP_TOS socket option. For possible values, see table below.
`QAbstractSocket::SendBufferSizeSocketOption` | `5` | Sets the socket send buffer size in bytes at the OS level. This maps to the SO_SNDBUF socket option. This option does not affect the QIODevice or QAbstractSocket buffers. This enum value has been introduced in Qt 5.3.
`QAbstractSocket::ReceiveBufferSizeSocketOption` | `6` | Sets the socket receive buffer size in bytes at the OS level. This maps to the SO_RCVBUF socket option. This option does not affect the QIODevice or QAbstractSocket buffers (see setReadBufferSize()). This enum value has been introduced in Qt 5.3.
`QAbstractSocket::PathMtuSocketOption` | `7` | Retrieves the Path Maximum Transmission Unit (PMTU) value currently known by the IP stack, if any. Some IP stacks also allow setting the MTU for transmission. This enum value was introduced in Qt 5.11.

Possible values for TypeOfServiceOption are:

Value | Description
--- | ---
224 | Network control
192 | Internetwork control
160 | CRITIC/ECP
128 | Flash override
96 | Flash
64 | Immediate
32 | Priority
0 | Routine

This enum was introduced or modified in Qt 4.6.

See also QAbstractSocket::setSocketOption() and QAbstractSocket::socketOption().

### enum QAbstractSocket::SocketState

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

Constant | Value | Description
--- | --- | ---
`QAbstractSocket::UnconnectedState` | `0` | The socket is not connected.
`QAbstractSocket::HostLookupState` | `1` | The socket is performing a host name lookup.
`QAbstractSocket::ConnectingState` | `2` | The socket has started establishing a connection.
`QAbstractSocket::ConnectedState` | `3` | A connection is established.
`QAbstractSocket::BoundState` | `4` | The socket is bound to an address and port.
`QAbstractSocket::ClosingState` | `6` | The socket is about to close (data may still be waiting to be written).
`QAbstractSocket::ListeningState` | `5` | For internal use only.

See also QAbstractSocket::state().

### enum QAbstractSocket::SocketType

This enum describes the transport layer protocol.

Constant | Value | Description
--- | --- | ---
`QAbstractSocket::TcpSocket` | `0` | TCP
`QAbstractSocket::UdpSocket` | `1` | UDP
`QAbstractSocket::SctpSocket` | `2` | SCTP
`QAbstractSocket::UnknownSocketType` | `-1` | Other than TCP, UDP and SCTP

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.

### `[signal]` void QAbstractSocket::connected()

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

Note: On some operating systems the connected() signal may be directly emitted from the connectToHost() call for connections to the localhost.

See also connectToHost() and disconnected().

### `[signal]` void QAbstractSocket::disconnected()

This signal is emitted when the socket has been disconnected.

Warning: If you need to delete the sender() of this signal in a slot connected to it, use the deleteLater() function.

See also connectToHost(), disconnectFromHost(), and abort().

### `[signal]` void QAbstractSocket::errorOccurred(QAbstractSocket::SocketError socketError)

This signal is emitted after an error occurred. The socketError 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 a QTimer::singleShot() with 0 as the timeout.

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

This function was introduced in Qt 5.15.

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

### `[signal]` void QAbstractSocket::hostFound()

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

Note: Since Qt 4.6.3 QAbstractSocket may emit hostFound() directly from the connectToHost() call since a DNS result could have been cached.

See also connected().

### `[signal]` void QAbstractSocket::proxyAuthenticationRequired(const QNetworkProxy &proxy, QAuthenticator *authenticator)

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

Note: It is not possible to use a QueuedConnection to connect to this signal, as the connection will fail if the authenticator has not been filled in with new information when the signal returns.

This function was introduced in Qt 4.3.

See also QAuthenticator and QNetworkProxy.

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

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

QAbstractSocket::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]` QAbstractSocket::~QAbstractSocket()

Destroys the socket.

### void QAbstractSocket::abort()

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

See also disconnectFromHost() and close().

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

Returns `true` if no more data is currently available for reading; otherwise returns `false`.

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 also bytesAvailable() and readyRead().

### bool QAbstractSocket::bind(const QHostAddress &address, quint16 port = 0, BindMode mode = DefaultForPlatform)

Binds to address on port port, using the BindMode mode.

For UDP sockets, after binding, the signal QUdpSocket::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 the DefaultForPlatform BindMode. If a port is not specified, a random port is chosen.

On success, the function returns `true` and the socket enters BoundState; otherwise it returns `false`.

This function was introduced in Qt 5.0.

### bool QAbstractSocket::bind(quint16 port = 0, BindMode mode = DefaultForPlatform)

This is an overloaded function.

Binds to QHostAddress:Any on port port, using the BindMode mode.

By default, the socket is bound using the DefaultForPlatform BindMode. 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

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

See also bytesToWrite() and read().

### `[override virtual]` qint64 QAbstractSocket::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 when flush() is called.

See also bytesAvailable() and flush().

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

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

See also readLine().

### `[override virtual]` void QAbstractSocket::close()

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

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

See also abort().

### `[virtual]` void QAbstractSocket::connectToHost(const QString &hostName, quint16 port, OpenMode openMode = ReadWrite, NetworkLayerProtocol protocol = AnyIPProtocol)

Attempts to make a connection to hostName on the given port. The protocol parameter can be used to specify which network protocol to use (eg. IPv4 or IPv6).

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

### `[virtual]` void QAbstractSocket::connectToHost(const QHostAddress &address, quint16 port, OpenMode openMode = ReadWrite)

This is an overloaded function.

Attempts to make a connection to address on port port.

### `[virtual]` void QAbstractSocket::disconnectFromHost()

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.

See also connectToHost().

### SocketError QAbstractSocket::error() const

Returns the type of error that last occurred.

See also state() 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().

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

### bool QAbstractSocket::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().

### 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(), peerAddress(), and setLocalAddress().

### 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(), peerPort(), and setLocalPort().

### PauseModes QAbstractSocket::pauseMode() const

Returns the pause mode of this socket.

This function was introduced in Qt 5.0.

See also setPauseMode() and resume().

### 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(), localAddress(), and setPeerAddress().

### 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(), peerPort(), and setPeerName().

### quint16 QAbstractSocket::peerPort() const

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

See also peerAddress(), localPort(), and setPeerPort().

### QString QAbstractSocket::protocolTag() const

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

This function was introduced in Qt 5.13.

See also setProtocolTag() and QNetworkProxyQuery.

### QNetworkProxy QAbstractSocket::proxy() const

Returns the network proxy for this socket. By default QNetworkProxy::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 also setProxy(), QNetworkProxy, and QNetworkProxyFactory.

### 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().

### `[override virtual protected]` qint64 QAbstractSocket::readData(char *data, qint64 maxSize)

### `[override virtual protected]` qint64 QAbstractSocket::readLineData(char *data, qint64 maxlen)

### `[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 also pauseMode() and setPauseMode().

### `[protected]` void QAbstractSocket::setLocalAddress(const QHostAddress &address)

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

You can call this function in a subclass of QAbstractSocket to change the return value of the localAddress() 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 also localAddress(), setLocalPort(), and setPeerAddress().

### `[protected]` void QAbstractSocket::setLocalPort(quint16 port)

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

You can call this function in a subclass of QAbstractSocket to change the return value of the localPort() 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 also localPort(), localAddress(), setLocalAddress(), and setPeerPort().

### void QAbstractSocket::setPauseMode(PauseModes pauseMode)

Controls whether to pause upon receiving a notification. The pauseMode parameter specifies the conditions in which the socket should be paused. The only notification currently supported is QSslSocket::sslErrors(). If set to PauseOnSslErrors, data transfer on the socket will be paused and needs to be enabled explicitly again by calling resume(). By default this option is set to PauseNever. 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 also pauseMode() and resume().

### `[protected]` void QAbstractSocket::setPeerAddress(const QHostAddress &address)

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

You can call this function in a subclass of QAbstractSocket to change the return value of the peerAddress() 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 also peerAddress(), setPeerPort(), and setLocalAddress().

### `[protected]` void QAbstractSocket::setPeerName(const QString &name)

Sets the host name of the remote peer to name.

You can call this function in a subclass of QAbstractSocket to change the return value of the peerName() 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 also peerName().

### `[protected]` void QAbstractSocket::setPeerPort(quint16 port)

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

You can call this function in a subclass of QAbstractSocket to change the return value of the peerPort() 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 also peerPort(), setPeerAddress(), and setLocalPort().

### void QAbstractSocket::setProtocolTag(const QString &tag)

Sets the protocol tag for this socket to tag.

This function was introduced in Qt 5.13.

See also protocolTag().

### void QAbstractSocket::setProxy(const QNetworkProxy &networkProxy)

Sets the explicit network proxy for this socket to networkProxy.

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

     socket->setProxy(QNetworkProxy::NoProxy);

The default value for the proxy is QNetworkProxy::DefaultProxy, which means the socket will use the application settings: if a proxy is set with QNetworkProxy::setApplicationProxy, it will use that; otherwise, if a factory is set with QNetworkProxyFactory::setApplicationProxyFactory, it will query that factory with type QNetworkProxyQuery::TcpSocket.

This function was introduced in Qt 4.1.

See also proxy(), QNetworkProxy, and QNetworkProxyFactory::queryProxy().

### `[virtual]` 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.

Only QTcpSocket uses QAbstractSocket'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 on QUdpSocket has no effect.

See also readBufferSize() and read().

### `[virtual]` bool QAbstractSocket::setSocketDescriptor(qintptr 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. 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 also socketDescriptor().

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

Sets the type of error that last occurred to socketError.

See also setSocketState() and setErrorString().

### `[virtual]` void QAbstractSocket::setSocketOption(QAbstractSocket::SocketOption option, const QVariant &value)

Sets the given option to the value described by value.

Note: On Windows Runtime, QAbstractSocket::KeepAliveOption must be set before the socket is connected.

This function was introduced in Qt 4.6.

See also socketOption().

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

Sets the state of the socket to state.

See also state().

### `[virtual]` qintptr QAbstractSocket::socketDescriptor() const

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

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

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

See also setSocketDescriptor().

### `[virtual]` QVariant QAbstractSocket::socketOption(QAbstractSocket::SocketOption option)

Returns the value of the option option.

This function was introduced in Qt 4.6.

See also setSocketOption().

### 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().

### `[override virtual]` bool QAbstractSocket::waitForBytesWritten(int msecs = 30000)

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

The function returns `true` if the bytesWritten() signal is emitted; otherwise it returns `false` (if an error occurred or the operation timed out).

Note: This function may fail randomly on Windows. Consider using the event loop and the bytesWritten() signal if your software will run on Windows.

See also waitForReadyRead().

### `[virtual]` 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.

Note: This function may wait slightly longer than msecs, depending on the time it takes to complete the host lookup.

Note: Multiple calls to this functions do not accumulate the time. If the function times out, the connecting process will be aborted.

Note: This function may fail randomly on Windows. Consider using the event loop and the connected() signal if your software will run on Windows.

See also connectToHost() and connected().

### `[virtual]` bool QAbstractSocket::waitForDisconnected(int msecs = 30000)

Waits until the socket has disconnected, up to msecs milliseconds. If the connection was successfully disconnected, this function returns `true`; otherwise it returns `false` (if the operation timed out, if an error occurred, or if this QAbstractSocket is already disconnected). 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->disconnectFromHost();
     if (socket->state() == QAbstractSocket::UnconnectedState
         || socket->waitForDisconnected(1000)) {
             qDebug("Disconnected!");
     }

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

Note: This function may fail randomly on Windows. Consider using the event loop and the disconnected() signal if your software will run on Windows.

See also disconnectFromHost() and close().

### `[override virtual]` bool QAbstractSocket::waitForReadyRead(int msecs = 30000)

This function blocks until new 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 the readyRead() signal is emitted and there is new data available for reading; otherwise it returns `false` (if an error occurred or the operation timed out).

Note: This function may fail randomly on Windows. Consider using the event loop and the readyRead() signal if your software will run on Windows.

See also waitForBytesWritten().

### `[override virtual protected]` qint64 QAbstractSocket::writeData(const char *data, qint64 size)