- Qt 6.9
- Qt Network
- C++ Classes
- QUdpSocket
- Qt 6.9.1 Reference Documentation

# QUdpSocket Class

The QUdpSocket class provides a UDP socket. More...

Header: | `#include <QUdpSocket>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Network)` `target_link_libraries(mytarget PRIVATE Qt6::Network)`
qmake: | `QT += network`
Inherits: | QAbstractSocket

- List of all members, including inherited members
- QUdpSocket is part of Network Programming API.

Note: All functions in this class are reentrant.

## Public Functions

| QUdpSocket(QObject *parent = nullptr)
virtual | ~QUdpSocket()
bool | hasPendingDatagrams() const
bool | joinMulticastGroup(const QHostAddress &groupAddress)
bool | joinMulticastGroup(const QHostAddress &groupAddress, const QNetworkInterface &iface)
bool | leaveMulticastGroup(const QHostAddress &groupAddress)
bool | leaveMulticastGroup(const QHostAddress &groupAddress, const QNetworkInterface &iface)
QNetworkInterface | multicastInterface() const
qint64 | pendingDatagramSize() const
qint64 | readDatagram(char *data, qint64 maxSize, QHostAddress *address = nullptr, quint16 *port = nullptr)
QNetworkDatagram | receiveDatagram(qint64 maxSize = -1)
void | setMulticastInterface(const QNetworkInterface &iface)
qint64 | writeDatagram(const char *data, qint64 size, const QHostAddress &address, quint16 port)
qint64 | writeDatagram(const QNetworkDatagram &datagram)
qint64 | writeDatagram(const QByteArray &datagram, const QHostAddress &host, quint16 port)

## Detailed Description

UDP (User Datagram Protocol) is a lightweight, unreliable, datagram-oriented, connectionless protocol. It can be used when reliability isn't important. QUdpSocket is a subclass of QAbstractSocket that allows you to send and receive UDP datagrams.

The most common way to use this class is to bind to an address and port using bind(), then call writeDatagram() and readDatagram() / receiveDatagram() to transfer data. If you want to use the standard QIODevice functions read(), readLine(), write(), etc., you must first connect the socket directly to a peer by calling connectToHost().

The socket emits the bytesWritten() signal every time a datagram is written to the network. If you just want to send datagrams, you don't need to call bind().

The readyRead() signal is emitted whenever datagrams arrive. In that case, hasPendingDatagrams() returns `true`. Call pendingDatagramSize() to obtain the size of the first pending datagram, and readDatagram() or receiveDatagram() to read it.

Note: An incoming datagram should be read when you receive the readyRead() signal, otherwise this signal will not be emitted for the next datagram.

Example:

     void Server::initSocket()
     {
         udpSocket = new QUdpSocket(this);
         udpSocket->bind(QHostAddress::LocalHost, 7755);

         connect(udpSocket, &QUdpSocket::readyRead,
                 this, &Server::readPendingDatagrams);
     }

     void Server::readPendingDatagrams()
     {
         while (udpSocket->hasPendingDatagrams()) {
             QNetworkDatagram datagram = udpSocket->receiveDatagram();
             processTheDatagram(datagram);
         }
     }

QUdpSocket also supports UDP multicast. Use joinMulticastGroup() and leaveMulticastGroup() to control group membership, and QAbstractSocket::MulticastTtlOption and QAbstractSocket::MulticastLoopbackOption to set the TTL and loopback socket options. Use setMulticastInterface() to control the outgoing interface for multicast datagrams, and multicastInterface() to query it.

With QUdpSocket, you can also establish a virtual connection to a UDP server using connectToHost() and then use read() and write() to exchange datagrams without specifying the receiver for each datagram.

The Broadcast Sender, Broadcast Receiver, Multicast Sender, and Multicast Receiver examples illustrate how to use QUdpSocket in applications.

See also QTcpSocket and QNetworkDatagram.

## Member Function Documentation

### `[explicit]` QUdpSocket::QUdpSocket(QObject *parent = nullptr)

Creates a QUdpSocket object.

parent is passed to the QObject constructor.

See also socketType().

### `[virtual noexcept]` QUdpSocket::~QUdpSocket()

Destroys the socket, closing the connection if necessary.

See also close().

### bool QUdpSocket::hasPendingDatagrams() const

Returns `true` if at least one datagram is waiting to be read; otherwise returns `false`.

See also pendingDatagramSize() and readDatagram().

### bool QUdpSocket::joinMulticastGroup(const QHostAddress &groupAddress)

Joins the multicast group specified by groupAddress on the default interface chosen by the operating system. The socket must be in BoundState, otherwise an error occurs.

Note that if you are attempting to join an IPv4 group, your socket must not be bound using IPv6 (or in dual mode, using QHostAddress::Any). You must use QHostAddress::AnyIPv4 instead.

This function returns `true` if successful; otherwise it returns `false` and sets the socket error accordingly.

Note: Joining IPv6 multicast groups without an interface selection is not supported in all operating systems. Consider using the overload where the interface is specified.

See also leaveMulticastGroup().

### bool QUdpSocket::joinMulticastGroup(const QHostAddress &groupAddress, const QNetworkInterface &iface)

This is an overloaded function.

Joins the multicast group address groupAddress on the interface iface.

See also leaveMulticastGroup().

### bool QUdpSocket::leaveMulticastGroup(const QHostAddress &groupAddress)

Leaves the multicast group specified by groupAddress on the default interface chosen by the operating system. The socket must be in BoundState, otherwise an error occurs.

This function returns `true` if successful; otherwise it returns `false` and sets the socket error accordingly.

Note: This function should be called with the same arguments as were passed to joinMulticastGroup().

See also joinMulticastGroup().

### bool QUdpSocket::leaveMulticastGroup(const QHostAddress &groupAddress, const QNetworkInterface &iface)

This is an overloaded function.

Leaves the multicast group specified by groupAddress on the interface iface.

Note: This function should be called with the same arguments as were passed to joinMulticastGroup().

See also joinMulticastGroup().

### QNetworkInterface QUdpSocket::multicastInterface() const

Returns the interface for the outgoing interface for multicast datagrams. This corresponds to the IP_MULTICAST_IF socket option for IPv4 sockets and the IPV6_MULTICAST_IF socket option for IPv6 sockets. If no interface has been previously set, this function returns an invalid QNetworkInterface. The socket must be in BoundState, otherwise an invalid QNetworkInterface is returned.

See also setMulticastInterface().

### qint64 QUdpSocket::pendingDatagramSize() const

Returns the size of the first pending UDP datagram. If there is no datagram available, this function returns -1.

See also hasPendingDatagrams() and readDatagram().

### qint64 QUdpSocket::readDatagram(char *data, qint64 maxSize, QHostAddress *address = nullptr, quint16 *port = nullptr)

Receives a datagram no larger than maxSize bytes and stores it in data. The sender's host address and port is stored in *address and *port (unless the pointers are `nullptr`).

Returns the size of the datagram on success; otherwise returns -1.

If maxSize is too small, the rest of the datagram will be lost. To avoid loss of data, call pendingDatagramSize() to determine the size of the pending datagram before attempting to read it. If maxSize is 0, the datagram will be discarded.

See also writeDatagram(), hasPendingDatagrams(), and pendingDatagramSize().

### QNetworkDatagram QUdpSocket::receiveDatagram(qint64 maxSize = -1)

Receives a datagram no larger than maxSize bytes and returns it in the QNetworkDatagram object, along with the sender's host address and port. If possible, this function will also try to determine the datagram's destination address, port, and the number of hop counts at reception time.

On failure, returns a QNetworkDatagram that reports not valid.

If maxSize is too small, the rest of the datagram will be lost. If maxSize is 0, the datagram will be discarded. If maxSize is -1 (the default), this function will attempt to read the entire datagram.

See also writeDatagram(), hasPendingDatagrams(), and pendingDatagramSize().

### void QUdpSocket::setMulticastInterface(const QNetworkInterface &iface)

Sets the outgoing interface for multicast datagrams to the interface iface. This corresponds to the IP_MULTICAST_IF socket option for IPv4 sockets and the IPV6_MULTICAST_IF socket option for IPv6 sockets. The socket must be in BoundState, otherwise this function does nothing.

See also multicastInterface(), joinMulticastGroup(), and leaveMulticastGroup().

### qint64 QUdpSocket::writeDatagram(const char *data, qint64 size, const QHostAddress &address, quint16 port)

Sends the datagram at data of size size to the host address address at port port. Returns the number of bytes sent on success; otherwise returns -1.

Datagrams are always written as one block. The maximum size of a datagram is highly platform-dependent, but can be as low as 8192 bytes. If the datagram is too large, this function will return -1 and error() will return DatagramTooLargeError.

Sending datagrams larger than 512 bytes is in general disadvised, as even if they are sent successfully, they are likely to be fragmented by the IP layer before arriving at their final destination.

Warning: Calling this function on a connected UDP socket may result in an error and no packet being sent. If you are using a connected socket, use write() to send datagrams.

See also readDatagram() and write().

### qint64 QUdpSocket::writeDatagram(const QNetworkDatagram &datagram)

This is an overloaded function.

Sends the datagram datagram to the host address and port numbers contained in datagram, using the network interface and hop count limits also set there. If the destination address and port numbers are unset, this function will send to the address that was passed to connectToHost().

If the destination address is IPv6 with a non-empty scope id but differs from the interface index in datagram, it is undefined which interface the operating system will choose to send on.

The function returns the number of bytes sent if it succeeded or -1 if it encountered an error.

Warning: Calling this function on a connected UDP socket may result in an error and no packet being sent. If you are using a connected socket, use write() to send datagrams.

See also QNetworkDatagram::setDestination(), QNetworkDatagram::setHopLimit(), and QNetworkDatagram::setInterfaceIndex().

### qint64 QUdpSocket::writeDatagram(const QByteArray &datagram, const QHostAddress &host, quint16 port)

This is an overloaded function.

Sends the datagram datagram to the host address host and at port port.

The function returns the number of bytes sent if it succeeded or -1 if it encountered an error.