QSocketNotifier Class

The QSocketNotifier class provides support for monitoring activity on a file descriptor. More...

Header:	#include <QSocketNotifier>
qmake:	QT += core
Inherits:	QObject

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

Public Types

enum	Type { Read, Write, Exception }

Public Functions

	QSocketNotifier(qintptr socket, QSocketNotifier::Type type, QObject *parent = nullptr)
virtual	~QSocketNotifier()
bool	isEnabled() const
qintptr	socket() const
QSocketNotifier::Type	type() const

Public Slots

void	setEnabled(bool enable)

Signals

void	activated(QSocketDescriptor socket, QSocketNotifier::Type type)

Reimplemented Protected Functions

virtual bool

event(QEvent *e) override

Detailed Description

The QSocketNotifier makes it possible to integrate Qt's event loop with other event loops based on file descriptors. File descriptor action is detected in Qt's main event loop (QCoreApplication::exec()).

Once you have opened a device using a low-level (usually platform-specific) API, you can create a socket notifier to monitor the file descriptor. The socket notifier is enabled by default, i.e. it emits the activated() signal whenever a socket event corresponding to its type occurs. Connect the activated() signal to the slot you want to be called when an event corresponding to your socket notifier's type occurs.

There are three types of socket notifiers: read, write, and exception. The type is described by the Type enum, and must be specified when constructing the socket notifier. After construction it can be determined using the type() function. Note that if you need to monitor both reads and writes for the same file descriptor, you must create two socket notifiers. Note also that it is not possible to install two socket notifiers of the same type (Read, Write, Exception) on the same socket.

The setEnabled() function allows you to disable as well as enable the socket notifier. It is generally advisable to explicitly enable or disable the socket notifier, especially for write notifiers. A disabled notifier ignores socket events (the same effect as not creating the socket notifier). Use the isEnabled() function to determine the notifier's current status.

Finally, you can use the socket() function to retrieve the socket identifier. Although the class is called QSocketNotifier, it is normally used for other types of devices than sockets. QTcpSocket and QUdpSocket provide notification through signals, so there is normally no need to use a QSocketNotifier on them.

See also QFile, QProcess, QTcpSocket, and QUdpSocket.

Member Type Documentation

enum QSocketNotifier::Type

This enum describes the various types of events that a socket notifier can recognize. The type must be specified when constructing the socket notifier.

Note that if you need to monitor both reads and writes for the same file descriptor, you must create two socket notifiers. Note also that it is not possible to install two socket notifiers of the same type (Read, Write, Exception) on the same socket.

Constant	Value	Description
QSocketNotifier::Read	0	There is data to be read.
QSocketNotifier::Write	1	Data can be written.
QSocketNotifier::Exception	2	An exception has occurred. We recommend against using this.

See also QSocketNotifier() and type().

Member Function Documentation

QSocketNotifier::QSocketNotifier(qintptr socket, QSocketNotifier::Type type, QObject *parent = nullptr)

Constructs a socket notifier with the given parent. It enables the socket, and watches for events of the given type.

It is generally advisable to explicitly enable or disable the socket notifier, especially for write notifiers.

Note for Windows users: The socket passed to QSocketNotifier will become non-blocking, even if it was created as a blocking socket.

See also setEnabled() and isEnabled().

[signal] void QSocketNotifier::activated(QSocketDescriptor socket, QSocketNotifier::Type type)

This signal is emitted whenever the socket notifier is enabled and a socket event corresponding to its type occurs.

The socket identifier is passed in the socket parameter.

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

Note: Signal activated is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

connect(socketNotifier, QOverload<QSocketDescriptor, QSocketNotifier::Type>::of(&QSocketNotifier::activated),
    [=](QSocketDescriptor socket, QSocketNotifier::Type type){ /* ... */ });

This function was introduced in Qt 5.15.

See also type() and socket().

[slot] void QSocketNotifier::setEnabled(bool enable)

If enable is true, the notifier is enabled; otherwise the notifier is disabled.

The notifier is enabled by default, i.e. it emits the activated() signal whenever a socket event corresponding to its type occurs. If it is disabled, it ignores socket events (the same effect as not creating the socket notifier).

Write notifiers should normally be disabled immediately after the activated() signal has been emitted

See also isEnabled() and activated().

[virtual] QSocketNotifier::~QSocketNotifier()

Destroys this socket notifier.

[override virtual protected] bool QSocketNotifier::event(QEvent *e)

Reimplements: QObject::event(QEvent *e).

bool QSocketNotifier::isEnabled() const

Returns true if the notifier is enabled; otherwise returns false.

See also setEnabled().

qintptr QSocketNotifier::socket() const

Returns the socket identifier specified to the constructor.

See also type().

QSocketNotifier::Type QSocketNotifier::type() const

Returns the socket event type specified to the constructor.

See also socket().