- Qt 6.12
- Qt Bluetooth
- C++ Classes
- QBluetoothSocket
- Qt 6.12.0 Reference Documentation

# QBluetoothSocket Class

The QBluetoothSocket class enables connection to a Bluetooth device running a bluetooth server. More...

Header: | `#include <QBluetoothSocket>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Bluetooth)` `target_link_libraries(mytarget PRIVATE Qt6::Bluetooth)`
qmake: | `QT += bluetooth`
Inherits: | QIODevice

- List of all members, including inherited members

## Public Types

enum class | SocketError { UnknownSocketError, NoSocketError, HostNotFoundError, ServiceNotFoundError, NetworkError, …, MissingPermissionsError }
enum class | SocketState { UnconnectedState, ServiceLookupState, ConnectingState, ConnectedState, BoundState, …, ListeningState }

## Public Functions

| QBluetoothSocket(QObject *parent = nullptr)
| QBluetoothSocket(QBluetoothServiceInfo::Protocol socketType, QObject *parent = nullptr)
virtual | ~QBluetoothSocket()
void | abort()
void | connectToService(const QBluetoothServiceInfo &service, QIODeviceBase::OpenMode openMode = ReadWrite)
void | connectToService(const QBluetoothAddress &address, const QBluetoothUuid &uuid, QIODeviceBase::OpenMode openMode = ReadWrite)
void | connectToService(const QBluetoothAddress &address, quint16 port, QIODeviceBase::OpenMode openMode = ReadWrite)
void | disconnectFromService()
QBluetoothSocket::SocketError | error() const
QString | errorString() const
QBluetoothAddress | localAddress() const
QString | localName() const
quint16 | localPort() const
QBluetoothAddress | peerAddress() const
QString | peerName() const
quint16 | peerPort() const
QBluetooth::SecurityFlags | preferredSecurityFlags() const
void | setPreferredSecurityFlags(QBluetooth::SecurityFlags flags)
bool | setSocketDescriptor(int socketDescriptor, QBluetoothServiceInfo::Protocol socketType, QBluetoothSocket::SocketState socketState = SocketState::ConnectedState, QIODeviceBase::OpenMode openMode = ReadWrite)
int | socketDescriptor() const
QBluetoothServiceInfo::Protocol | socketType() const
QBluetoothSocket::SocketState | state() const

## Reimplemented Public Functions

virtual qint64 | bytesAvailable() const override
virtual qint64 | bytesToWrite() const override
virtual bool | canReadLine() const override
virtual void | close() override
virtual bool | isSequential() const override

## Signals

void | connected()
void | disconnected()
`(since 6.2)` void | errorOccurred(QBluetoothSocket::SocketError error)
void | stateChanged(QBluetoothSocket::SocketState state)

## Protected Functions

void | doDeviceDiscovery(const QBluetoothServiceInfo &service, QIODeviceBase::OpenMode openMode)
void | setSocketError(QBluetoothSocket::SocketError error_)
void | setSocketState(QBluetoothSocket::SocketState state)

## Reimplemented Protected Functions

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

## Detailed Description

QBluetoothSocket supports two socket types, L2CAP and RFCOMM.

L2CAP is a low level datagram-oriented Bluetooth socket. Android does not support L2CAP for socket connections.

RFCOMM is a reliable, stream-oriented socket. RFCOMM sockets emulate an RS-232 serial port.

To create a connection to a Bluetooth service, create a socket of the appropriate type and call connectToService() passing the Bluetooth address and port number. QBluetoothSocket will emit the connected() signal when the connection is established.

If the Protocol is not supported on a platform, calling connectToService() will emit a UnsupportedProtocolError error.

Note: QBluetoothSocket does not support synchronous read and write operations. Functions such as waitForReadyRead() and waitForBytesWritten() are not implemented. I/O operations should be performed using readyRead(), read() and write().

On iOS, this class cannot be used because the platform does not expose an API which may permit access to QBluetoothSocket related features.

Note: On macOS Monterey (12) the socket data flow is paused when a modal dialogue is executing, or an event tracking mode is entered (for example by long-pressing a Window close button). This issue has been fixed in macOS Sequoia (15).

## Member Type Documentation

### enum class QBluetoothSocket::SocketError

This enum describes Bluetooth socket error types.

Constant | Value | Description
--- | --- | ---
`QBluetoothSocket::SocketError::UnknownSocketError` | `1` | An unknown error has occurred.
`QBluetoothSocket::SocketError::NoSocketError` | `0` | No error. Used for testing.
`QBluetoothSocket::SocketError::HostNotFoundError` | `3` | Could not find the remote host.
`QBluetoothSocket::SocketError::ServiceNotFoundError` | `4` | Could not find the service UUID on remote host.
`QBluetoothSocket::SocketError::NetworkError` | `5` | Attempt to read or write from socket returned an error
`QBluetoothSocket::SocketError::UnsupportedProtocolError` | `6` | The Protocol is not supported on this platform.
`QBluetoothSocket::SocketError::OperationError` | `7` | An operation was attempted while the socket was in a state that did not permit it.
`QBluetoothSocket::SocketError::RemoteHostClosedError (since Qt 5.10)` | `2` | The remote host closed the connection.
`QBluetoothSocket::SocketError::MissingPermissionsError (since Qt 6.4)` | `8` | The operating system requests permissions which were not granted by the user.

### enum class QBluetoothSocket::SocketState

This enum describes the state of the Bluetooth socket.

Constant | Value | Description
--- | --- | ---
`QBluetoothSocket::SocketState::UnconnectedState` | `0` | Socket is not connected.
`QBluetoothSocket::SocketState::ServiceLookupState` | `1` | Socket is querying connection parameters.
`QBluetoothSocket::SocketState::ConnectingState` | `2` | Socket is attempting to connect to a device.
`QBluetoothSocket::SocketState::ConnectedState` | `3` | Socket is connected to a device.
`QBluetoothSocket::SocketState::BoundState` | `4` | Socket is bound to a local address and port.
`QBluetoothSocket::SocketState::ClosingState` | `5` | Socket is connected and will be closed once all pending data is written to the socket.
`QBluetoothSocket::SocketState::ListeningState` | `6` | Socket is listening for incoming connections.

## Member Function Documentation

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

Constructs a Bluetooth socket with parent.

### `[explicit]` QBluetoothSocket::QBluetoothSocket(QBluetoothServiceInfo::Protocol socketType, QObject *parent = nullptr)

Constructs a Bluetooth socket of socketType type, with parent.

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

Destroys the Bluetooth socket.

### void QBluetoothSocket::abort()

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

Note: On Android, aborting the socket requires asynchronous interaction with Android threads. Therefore the associated disconnected() and stateChanged() signals are delayed until the threads have finished the closure.

See also disconnectFromService() and close().

### `[override virtual]` qint64 QBluetoothSocket::bytesAvailable() const

Reimplements: QIODevice::bytesAvailable() const.

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

See also bytesToWrite() and read().

### `[override virtual]` qint64 QBluetoothSocket::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.

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

Reimplements: QIODevice::canReadLine() const.

Returns true if you can read at least one line from the device

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

Reimplements: QIODevice::close().

Disconnects the socket's connection with the device.

Note: On Android, closing the socket requires asynchronous interaction with Android threads. Therefore the associated disconnected() and stateChanged() signals are delayed until the threads have finished the closure.

### void QBluetoothSocket::connectToService(const QBluetoothServiceInfo &service, QIODeviceBase::OpenMode openMode = ReadWrite)

Attempts to connect to the service described by service.

The socket is opened in the given openMode. The socketType() is ignored if service specifies a differing QBluetoothServiceInfo::socketProtocol().

The socket first enters ConnectingState and attempts to connect to the device providing service. If a connection is established, QBluetoothSocket enters ConnectedState and emits connected().

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

Note that most platforms require a pairing prior to connecting to the remote device. Otherwise the connection process may fail.

On Android, only RFCOMM connections are possible. This function ignores any socket protocol indicator and assumes RFCOMM.

See also state() and disconnectFromService().

### void QBluetoothSocket::connectToService(const QBluetoothAddress &address, const QBluetoothUuid &uuid, QIODeviceBase::OpenMode openMode = ReadWrite)

Attempts to make a connection to the service identified by uuid on the device with address address.

The socket is opened in the given openMode.

For BlueZ, the socket first enters the ServiceLookupState and queries the connection parameters for uuid. If the service parameters are successfully retrieved the socket enters ConnectingState, and attempts to connect to address. If a connection is established, QBluetoothSocket enters ConnectedState and emits connected().

On Android, the service connection can directly be established using the UUID of the remote service. Therefore the platform does not require the ServiceLookupState and socketType() is always set to QBluetoothServiceInfo::RfcommProtocol.

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

Note that most platforms require a pairing prior to connecting to the remote device. Otherwise the connection process may fail.

See also state() and disconnectFromService().

### void QBluetoothSocket::connectToService(const QBluetoothAddress &address, quint16 port, QIODeviceBase::OpenMode openMode = ReadWrite)

Attempts to make a connection with address on the given port.

The socket is opened in the given openMode.

The socket first enters ConnectingState, and attempts to connect to address. If a connection is established, QBluetoothSocket enters ConnectedState and emits connected().

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

On Android and BlueZ (version 5.46 or above), a connection to a service can not be established using a port. Calling this function will emit a ServiceNotFoundError.

Note that most platforms require a pairing prior to connecting to the remote device. Otherwise the connection process may fail.

See also state() and disconnectFromService().

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

This signal is emitted when a connection is established.

See also QBluetoothSocket::SocketState::ConnectedState and stateChanged().

### void QBluetoothSocket::disconnectFromService()

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

See also connectToService().

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

This signal is emitted when the socket is disconnected.

See also QBluetoothSocket::SocketState::UnconnectedState and stateChanged().

### `[protected]` void QBluetoothSocket::doDeviceDiscovery(const QBluetoothServiceInfo &service, QIODeviceBase::OpenMode openMode)

Start device discovery for service and open the socket with openMode. If the socket is created with a service uuid device address, use service discovery to find the port number to connect to.

### QBluetoothSocket::SocketError QBluetoothSocket::error() const

Returns the last error.

### `[signal, since 6.2]` void QBluetoothSocket::errorOccurred(QBluetoothSocket::SocketError error)

This signal is emitted when an error occurs.

This function was introduced in Qt 6.2.

See also error().

### QString QBluetoothSocket::errorString() const

Returns a user displayable text string for the error.

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

Reimplements: QIODevice::isSequential() const.

### QBluetoothAddress QBluetoothSocket::localAddress() const

Returns the address of the local device.

Although some platforms may differ the socket must generally be connected to guarantee the return of a valid address. In particular, this is true when dealing with platforms that support multiple local Bluetooth adapters.

### QString QBluetoothSocket::localName() const

Returns the name of the local device.

Although some platforms may differ the socket must generally be connected to guarantee the return of a valid name. In particular, this is true when dealing with platforms that support multiple local Bluetooth adapters.

### quint16 QBluetoothSocket::localPort() const

Returns the port number of the local socket if available, otherwise returns 0. Although some platforms may differ the socket must generally be connected to guarantee the return of a valid port number.

On Android and macOS, this feature is not supported and returns 0.

### QBluetoothAddress QBluetoothSocket::peerAddress() const

Returns the address of the peer device.

### QString QBluetoothSocket::peerName() const

Returns the name of the peer device.

### quint16 QBluetoothSocket::peerPort() const

Return the port number of the peer socket if available, otherwise returns 0. On Android, this feature is not supported.

### QBluetooth::SecurityFlags QBluetoothSocket::preferredSecurityFlags() const

Returns the security parameters used for the initial connection attempt.

The security parameters may be renegotiated between the two parties during or after the connection has been established. If such a change happens it is not reflected in the value of this flag.

On macOS, this flag is always set to QBluetooth::Security::Secure.

See also setPreferredSecurityFlags().

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

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

### void QBluetoothSocket::setPreferredSecurityFlags(QBluetooth::SecurityFlags flags)

Sets the preferred security parameter for the connection attempt to flags. This value is incorporated when calling connectToService(). Therefore it is required to reconnect to change this parameter for an existing connection.

On the Linux BlueZ backend, the flags are mapped to kernel security levels:

- QBluetooth::Security::Authorization maps to `BT_SECURITY_LOW`;
- QBluetooth::Security::Encryption maps to `BT_SECURITY_MEDIUM`;
- QBluetooth::Security::Secure maps to `BT_SECURITY_HIGH`;
- QBluetooth::Security::Authentication is not used by the kernel and is ignored.

The default value is QBluetooth::Security::Authorization.

On Linux with the BlueZ D-Bus backend and on Windows, this flag is not supported and ignored.

On macOS, this value is ignored as the platform does not permit access to the security parameter of the socket. By default the platform prefers secure/encrypted connections though and therefore this function always returns QBluetooth::Security::Secure.

Android only supports two levels of security (secure and non-secure). If this flag is set to QBluetooth::Security::NoSecurity the socket object will not employ any authentication or encryption. Any other security flag combination will trigger a secure Bluetooth connection. This flag is set to QBluetooth::Security::Secure by default.

Note: A secure connection requires a pairing between the two devices. On some platforms, the pairing is automatically initiated during the establishment of the connection. Other platforms require the application to manually trigger the pairing before attempting to connect.

See also preferredSecurityFlags().

### bool QBluetoothSocket::setSocketDescriptor(int socketDescriptor, QBluetoothServiceInfo::Protocol socketType, QBluetoothSocket::SocketState socketState = SocketState::ConnectedState, QIODeviceBase::OpenMode openMode = ReadWrite)

Sets the socket to use socketDescriptor with a type of socketType, which is in state socketState, and mode openMode.

The socket descriptor is owned by the QBluetoothSocket instance and may be closed once finished.

Returns `true` on success.

See also socketDescriptor().

### `[protected]` void QBluetoothSocket::setSocketError(QBluetoothSocket::SocketError error_)

Sets the type of error that last occurred to error_.

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

Sets the socket state to state.

### int QBluetoothSocket::socketDescriptor() const

Returns the platform-specific socket descriptor, if available. This function returns -1 if the descriptor is not available or an error has occurred.

See also setSocketDescriptor().

### QBluetoothServiceInfo::Protocol QBluetoothSocket::socketType() const

Returns the socket type. The socket automatically adjusts to the protocol offered by the remote service.

Android only support RFCOMM based sockets.

### QBluetoothSocket::SocketState QBluetoothSocket::state() const

Returns the current state of the socket.

### `[signal]` void QBluetoothSocket::stateChanged(QBluetoothSocket::SocketState state)

This signal is emitted when the socket state changes to state.

See also connected(), disconnected(), state(), and QBluetoothSocket::SocketState.

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

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