QTcpServer Class

The QTcpServer class provides a TCP-based server. More...

Header: #include <QTcpServer>
CMake: find_package(Qt6 REQUIRED COMPONENTS Network)
target_link_libraries(mytarget PRIVATE Qt6::Network)
qmake: QT += network
Inherits: QObject
Inherited By:

QSctpServer and QSslServer

Note: All functions in this class are reentrant.

Public Functions

QTcpServer(QObject *parent = nullptr)
virtual ~QTcpServer()
void close()
QString errorString() const
virtual bool hasPendingConnections() const
bool isListening() const
bool listen(const QHostAddress &address = QHostAddress::Any, quint16 port = 0)
(since 6.3) int listenBacklogSize() const
int maxPendingConnections() const
virtual QTcpSocket *nextPendingConnection()
void pauseAccepting()
QNetworkProxy proxy() const
void resumeAccepting()
QHostAddress serverAddress() const
QAbstractSocket::SocketError serverError() const
quint16 serverPort() const
(since 6.3) void setListenBacklogSize(int size)
void setMaxPendingConnections(int numConnections)
void setProxy(const QNetworkProxy &networkProxy)
bool setSocketDescriptor(qintptr socketDescriptor)
qintptr socketDescriptor() const
bool waitForNewConnection(int msec = 0, bool *timedOut = nullptr)


void acceptError(QAbstractSocket::SocketError socketError)
void newConnection()
(since 6.4) void pendingConnectionAvailable()

Protected Functions

void addPendingConnection(QTcpSocket *socket)
virtual void incomingConnection(qintptr socketDescriptor)

Detailed Description

This class makes it possible to accept incoming TCP connections. You can specify the port or have QTcpServer pick one automatically. You can listen on a specific address or on all the machine's addresses.

Call listen() to have the server listen for incoming connections. The newConnection() signal is then emitted each time a client connects to the server. When the client connection has been added to the pending connection queue using the addPendingConnection() function, the pendingConnectionAvailable() signal is emitted.

Call nextPendingConnection() to accept the pending connection as a connected QTcpSocket. The function returns a pointer to a QTcpSocket in QAbstractSocket::ConnectedState that you can use for communicating with the client.

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

When listening for connections, the address and port on which the server is listening are available as serverAddress() and serverPort().

Calling close() makes QTcpServer stop listening for incoming connections.

Although QTcpServer is mostly designed for use with an event loop, it's possible to use it without one. In that case, you must use waitForNewConnection(), which blocks until either a connection is available or a timeout expires.

See also QTcpSocket, Fortune Server, Threaded Fortune Server, and Torrent Example.

Member Function Documentation

[explicit] QTcpServer::QTcpServer(QObject *parent = nullptr)

Constructs a QTcpServer object.

parent is passed to the QObject constructor.

See also listen() and setSocketDescriptor().

[virtual noexcept] QTcpServer::~QTcpServer()

Destroys the QTcpServer object. If the server is listening for connections, the socket is automatically closed.

Any client QTcpSockets that are still connected must either disconnect or be reparented before the server is deleted.

See also close().

[signal] void QTcpServer::acceptError(QAbstractSocket::SocketError socketError)

This signal is emitted when accepting a new connection results in an error. The socketError parameter describes the type of error that occurred.

See also pauseAccepting() and resumeAccepting().

[protected] void QTcpServer::addPendingConnection(QTcpSocket *socket)

This function is called by QTcpServer::incomingConnection() to add the socket to the list of pending incoming connections.

Note: Don't forget to call this member from reimplemented incomingConnection() if you do not want to break the Pending Connections mechanism. This function emits the pendingConnectionAvailable() signal after the socket has been added.

See also incomingConnection() and pendingConnectionAvailable().

void QTcpServer::close()

Closes the server. The server will no longer listen for incoming connections.

See also listen().

QString QTcpServer::errorString() const

Returns a human readable description of the last error that occurred.

See also serverError().

[virtual] bool QTcpServer::hasPendingConnections() const

Returns true if the server has a pending connection; otherwise returns false.

See also nextPendingConnection() and setMaxPendingConnections().

[virtual protected] void QTcpServer::incomingConnection(qintptr socketDescriptor)

This virtual function is called by QTcpServer when a new connection is available. The socketDescriptor argument is the native socket descriptor for the accepted connection.

The base implementation creates a QTcpSocket, sets the socket descriptor and then stores the QTcpSocket in an internal list of pending connections. Finally newConnection() is emitted.

Reimplement this function to alter the server's behavior when a connection is available.

If this server is using QNetworkProxy then the socketDescriptor may not be usable with native socket functions, and should only be used with QTcpSocket::setSocketDescriptor().

Note: If another socket is created in the reimplementation of this method, it needs to be added to the Pending Connections mechanism by calling addPendingConnection().

Note: If you want to handle an incoming connection as a new QTcpSocket object in another thread you have to pass the socketDescriptor to the other thread and create the QTcpSocket object there and use its setSocketDescriptor() method.

See also newConnection(), nextPendingConnection(), and addPendingConnection().

bool QTcpServer::isListening() const

Returns true if the server is currently listening for incoming connections; otherwise returns false.

See also listen().

bool QTcpServer::listen(const QHostAddress &address = QHostAddress::Any, quint16 port = 0)

Tells the server to listen for incoming connections on address address and port port. If port is 0, a port is chosen automatically. If address is QHostAddress::Any, the server will listen on all network interfaces.

Returns true on success; otherwise returns false.

See also isListening().

[since 6.3] int QTcpServer::listenBacklogSize() const

Returns the backlog queue size of to be accepted connections.

This function was introduced in Qt 6.3.

See also setListenBacklogSize().

int QTcpServer::maxPendingConnections() const

Returns the maximum number of pending accepted connections. The default is 30.

See also setMaxPendingConnections() and hasPendingConnections().

[signal] void QTcpServer::newConnection()

This signal is emitted every time a new connection is available, regardless of whether it has been added to the pending connections queue or not.

See also hasPendingConnections() and nextPendingConnection().

[virtual] QTcpSocket *QTcpServer::nextPendingConnection()

Returns the next pending connection as a connected QTcpSocket object.

The socket is created as a child of the server, which means that it is automatically deleted when the QTcpServer object is destroyed. It is still a good idea to delete the object explicitly when you are done with it, to avoid wasting memory.

nullptr is returned if this function is called when there are no pending connections.

Note: The returned QTcpSocket object cannot be used from another thread. If you want to use an incoming connection from another thread, you need to override incomingConnection().

See also hasPendingConnections().

void QTcpServer::pauseAccepting()

Pauses accepting new connections. Queued connections will remain in queue.

See also resumeAccepting().

[private signal, since 6.4] void QTcpServer::pendingConnectionAvailable()

This signal is emitted every time a new connection has been added to the pending connections queue.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

This function was introduced in Qt 6.4.

See also hasPendingConnections() and nextPendingConnection().

QNetworkProxy QTcpServer::proxy() const

Returns the network proxy for this socket. By default QNetworkProxy::DefaultProxy is used.

See also setProxy() and QNetworkProxy.

void QTcpServer::resumeAccepting()

Resumes accepting new connections.

See also pauseAccepting().

QHostAddress QTcpServer::serverAddress() const

Returns the server's address if the server is listening for connections; otherwise returns QHostAddress::Null.

See also serverPort() and listen().

QAbstractSocket::SocketError QTcpServer::serverError() const

Returns an error code for the last error that occurred.

See also errorString().

quint16 QTcpServer::serverPort() const

Returns the server's port if the server is listening for connections; otherwise returns 0.

See also serverAddress() and listen().

[since 6.3] void QTcpServer::setListenBacklogSize(int size)

Sets the backlog queue size of to be accepted connections to size. The operating system might reduce or ignore this value. By default, the queue size is 50.

Note: This property must be set prior to calling listen().

This function was introduced in Qt 6.3.

See also listenBacklogSize().

void QTcpServer::setMaxPendingConnections(int numConnections)

Sets the maximum number of pending accepted connections to numConnections. QTcpServer will accept no more than numConnections incoming connections before nextPendingConnection() is called. By default, the limit is 30 pending connections.

Clients may still able to connect after the server has reached its maximum number of pending connections (i.e., QTcpSocket can still emit the connected() signal). QTcpServer will stop accepting the new connections, but the operating system may still keep them in queue.

See also maxPendingConnections() and hasPendingConnections().

void QTcpServer::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:


See also proxy() and QNetworkProxy.

bool QTcpServer::setSocketDescriptor(qintptr socketDescriptor)

Sets the socket descriptor this server should use when listening for incoming connections to socketDescriptor. Returns true if the socket is set successfully; otherwise returns false.

The socket is assumed to be in listening state.

See also socketDescriptor() and isListening().

qintptr QTcpServer::socketDescriptor() const

Returns the native socket descriptor the server uses to listen for incoming instructions, or -1 if the server is not listening.

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

See also setSocketDescriptor() and isListening().

bool QTcpServer::waitForNewConnection(int msec = 0, bool *timedOut = nullptr)

Waits for at most msec milliseconds or until an incoming connection is available. Returns true if a connection is available; otherwise returns false. If the operation timed out and timedOut is not nullptr, *timedOut will be set to true.

This is a blocking function call. Its use is disadvised in a single-threaded GUI application, since the whole application will stop responding until the function returns. waitForNewConnection() is mostly useful when there is no event loop available.

The non-blocking alternative is to connect to the newConnection() signal.

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

See also hasPendingConnections() and nextPendingConnection().

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