QLocalServer Class

The QLocalServer class provides a local socket based server. More...

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

Public Types

enum SocketOption { NoOptions, UserAccessOption, GroupAccessOption, OtherAccessOption, WorldAccessOption, AbstractNamespaceOption }
flags SocketOptions


Public Functions

QLocalServer(QObject *parent = nullptr)
virtual ~QLocalServer()
QBindable<QLocalServer::SocketOptions> bindableSocketOptions()
void close()
QString errorString() const
QString fullServerName() const
virtual bool hasPendingConnections() const
bool isListening() const
bool listen(const QString &name)
bool listen(qintptr socketDescriptor)
int listenBacklogSize() const
int maxPendingConnections() const
virtual QLocalSocket *nextPendingConnection()
QAbstractSocket::SocketError serverError() const
QString serverName() const
void setListenBacklogSize(int size)
void setMaxPendingConnections(int numConnections)
void setSocketOptions(QLocalServer::SocketOptions options)
qintptr socketDescriptor() const
QLocalServer::SocketOptions socketOptions() const
bool waitForNewConnection(int msec = 0, bool *timedOut = nullptr)


void newConnection()

Static Public Members

bool removeServer(const QString &name)

Protected Functions

virtual void incomingConnection(quintptr socketDescriptor)

Detailed Description

This class makes it possible to accept incoming local socket connections.

Call listen() to have the server start listening for incoming connections on a specified key. The newConnection() signal is then emitted each time a client connects to the server.

Call nextPendingConnection() to accept the pending connection as a connected QLocalSocket. The function returns a pointer to a QLocalSocket that can be used 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 name which the server is listening on is available through serverName().

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

Although QLocalServer is 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 QLocalSocket and QTcpServer.

Member Type Documentation

enum QLocalServer::SocketOption
flags QLocalServer::SocketOptions

This enum describes the possible options that can be used to create the socket. This changes the access permissions on platforms (Linux, Windows) that support access permissions on the socket. Both GroupAccess and OtherAccess may vary slightly in meanings depending on the platform. On Linux and Android it is possible to use sockets with abstract addresses; socket permissions have no meaning for such sockets.

QLocalServer::NoOptions0x0No access restrictions have been set.
QLocalServer::UserAccessOption0x01Access is restricted to the same user as the process that created the socket.
QLocalServer::GroupAccessOption0x2Access is restricted to the same group but not the user that created the socket on Linux. Access is restricted to the primary group of the process on Windows
QLocalServer::OtherAccessOption0x4Access is available to everyone but the user and group that created the socket on Linux. Access is available to everyone on Windows.
QLocalServer::WorldAccessOption0x7No access restrictions.
QLocalServer::AbstractNamespaceOption0x8The listening socket will be created in the abstract namespace. This flag is specific to Linux. In case of other platforms, for the sake of code portability, this flag is equivalent to WorldAccessOption.

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

See also socketOptions.

Property Documentation

[bindable] socketOptions : SocketOptions

Note: This property supports QProperty bindings.

This property holds the socket options that control how the socket operates.

For example, the socket may restrict access to what user ids can connect to the socket.

These options must be set before listen() is called.

In some cases, such as with Unix domain sockets on Linux, the access to the socket will be determined by file system permissions, and are created based on the umask. Setting the access flags will override this and will restrict or permit access as specified.

Other Unix-based operating systems, such as macOS, do not honor file permissions for Unix domain sockets and by default have WorldAccess and these permission flags will have no effect.

On Windows, UserAccessOption is sufficient to allow a non elevated process to connect to a local server created by an elevated process run by the same user. GroupAccessOption refers to the primary group of the process (see TokenPrimaryGroup in the Windows documentation). OtherAccessOption refers to the well known "Everyone" group.

On Linux platforms it is possible to create a socket in the abstract namespace, which is independent of the filesystem. Using this kind of socket implies ignoring permission options. On other platforms AbstractNamespaceOption is equivalent to WorldAccessOption.

By default none of the flags are set, access permissions are the platform default.

See also listen().

Member Function Documentation

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

Create a new local socket server with the given parent.

See also listen().

[virtual] QLocalServer::~QLocalServer()

Destroys the QLocalServer object. If the server is listening for connections, it is automatically closed.

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

See also close().

void QLocalServer::close()

Stop listening for incoming connections. Existing connections are not affected, but any new connections will be refused.

See also isListening() and listen().

QString QLocalServer::errorString() const

Returns the human-readable message appropriate to the current error reported by serverError(). If no suitable string is available, an empty string is returned.

See also serverError().

QString QLocalServer::fullServerName() const

Returns the full path that the server is listening on.

Note: This is platform specific

See also listen() and serverName().

[virtual] bool QLocalServer::hasPendingConnections() const

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

See also nextPendingConnection() and setMaxPendingConnections().

[virtual protected] void QLocalServer::incomingConnection(quintptr socketDescriptor)

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

The base implementation creates a QLocalSocket, sets the socket descriptor and then stores the QLocalSocket 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.

See also newConnection(), nextPendingConnection(), and QLocalSocket::setSocketDescriptor().

bool QLocalServer::isListening() const

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

See also listen() and close().

bool QLocalServer::listen(const QString &name)

Tells the server to listen for incoming connections on name. If the server is currently listening then it will return false. Return true on success otherwise false.

name can be a single name and QLocalServer will determine the correct platform specific path. serverName() will return the name that is passed into listen.

Usually you would just pass in a name like "foo", but on Unix this could also be a path such as "/tmp/foo" and on Windows this could be a pipe path such as "\\.\pipe\foo"

Note: On Unix if the server crashes without closing listen will fail with AddressInUseError. To create a new server the file should be removed. On Windows two local servers can listen to the same pipe at the same time, but any connections will go to one of the server.

See also serverName(), isListening(), and close().

bool QLocalServer::listen(qintptr socketDescriptor)

Instructs the server to listen for incoming connections on socketDescriptor. The property returns false if the server is currently listening. It returns true on success; otherwise, it returns false. The socket must be ready to accept new connections with no extra platform-specific functions called. The socket is set into non-blocking mode.

serverName(), fullServerName() may return a string with a name if this option is supported by the platform; otherwise, they return an empty QString. In particular, the addresses of sockets in the abstract namespace supported by Linux will not yield useful names if they contain unprintable characters.

See also isListening() and close().

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

Returns the backlog queue size of to be accepted connections.

This function was introduced in Qt 6.3.

See also setListenBacklogSize().

int QLocalServer::maxPendingConnections() const

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

See also setMaxPendingConnections() and hasPendingConnections().

[signal] void QLocalServer::newConnection()

This signal is emitted every time a new connection is available.

See also hasPendingConnections() and nextPendingConnection().

[virtual] QLocalSocket *QLocalServer::nextPendingConnection()

Returns the next pending connection as a connected QLocalSocket object.

The socket is created as a child of the server, which means that it is automatically deleted when the QLocalServer 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.

See also hasPendingConnections(), newConnection(), and incomingConnection().

[static] bool QLocalServer::removeServer(const QString &name)

Removes any server instance that might cause a call to listen() to fail and returns true if successful; otherwise returns false. This function is meant to recover from a crash, when the previous server instance has not been cleaned up.

On Windows, this function does nothing; on Unix, it removes the socket file given by name.

Warning: Be careful to avoid removing sockets of running instances.

QAbstractSocket::SocketError QLocalServer::serverError() const

Returns the type of error that occurred last or NoError.

See also errorString().

QString QLocalServer::serverName() const

Returns the server name if the server is listening for connections; otherwise returns QString()

See also listen() and fullServerName().

[since 6.3] void QLocalServer::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 QLocalServer::setMaxPendingConnections(int numConnections)

Sets the maximum number of pending accepted connections to numConnections. QLocalServer will accept no more than numConnections incoming connections before nextPendingConnection() is called.

Note: Even though QLocalServer will stop accepting new connections after it has reached its maximum number of pending connections, the operating system may still keep them in queue which will result in clients signaling that it is connected.

See also maxPendingConnections() and hasPendingConnections().

qintptr QLocalServer::socketDescriptor() const

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

The type of the descriptor depends on the platform:

  • On Windows, the returned value is a Winsock 2 Socket Handle.
  • On INTEGRITY, the returned value is the QTcpServer socket descriptor and the type is defined by socketDescriptor.
  • On all other UNIX-like operating systems, the type is a file descriptor representing a listening socket.

See also listen().

QLocalServer::SocketOptions QLocalServer::socketOptions() const

Returns the socket options set on the socket.

Note: Getter function for property socketOptions.

See also setSocketOptions().

bool QLocalServer::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 ill-advised 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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