Qt 4.3: QSocketNotifier Class Reference

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# QSocketNotifier Class Reference
[QtCore module]

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

     #include <QSocketNotifier>

Inherits QObject.

- List of all members, including inherited members
- Qt 3 support members

### Public Types

- enum Type { Read, Write, Exception }

### Public Functions

- QSocketNotifier ( int socket, Type type, QObject * parent = 0 )
- ~QSocketNotifier ()
- bool isEnabled () const
- int socket () const
- Type type () const

- 29 public functions inherited from QObject

### Public Slots

- void setEnabled ( bool enable )

- 1 public slot inherited from QObject

### Signals

- void activated ( int socket )

- 1 signal inherited from QObject

### Additional Inherited Members

- 1 property inherited from QObject
- 5 static public members inherited from QObject
- 7 protected functions inherited from QObject

## Detailed Description

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

The QSocketNotifier makes it possible to integrate Qt's event loop with other event loops based on file descriptors. For example, the CORBA Framework uses it to process CORBA events. 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.

### Notes for Windows Users

The socket passed to QSocketNotifier will become non-blocking, even if it was created as a blocking socket. The activated() signal is sometimes triggered by high general activity on the host, even if there is nothing to read. A subsequent read from the socket can then fail, the error indicating that there is no data available (e.g., `WSAEWOULDBLOCK`). This is an operating system limitation, and not a bug in QSocketNotifier.

To ensure that the socket notifier handles read notifications correctly, follow these steps when you receive a notification:

1. Disable the notifier.
2. Read data from the socket.
3. Re-enable the notifier if you are interested in more data (such as after having written a new command to a remote server).

To ensure that the socket notifier handles write notifications correctly, follow these steps when you receive a notification:

1. Disable the notifier.
2. Write as much data as you can (before `EWOULDBLOCK` is returned).
3. Re-enable notifier if you have more data to write.

Further information: On Windows, Qt always disables the notifier after getting a notification, and only re-enables it if more data is expected. For example, if data is read from the socket and it can be used to read more, or if reading or writing is not possible because the socket would block, in which case it is necessary to wait before attempting to read or write again.

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 ( int socket, Type type, QObject * parent = 0 )

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().

### QSocketNotifier::~QSocketNotifier ()

Destroys this socket notifier.

### void QSocketNotifier::activated ( int socket ) `[signal]`

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.

See also type() and socket().

### bool QSocketNotifier::isEnabled () const

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

See also setEnabled().

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

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().

### int QSocketNotifier::socket () const

Returns the socket identifier specified to the constructor.

See also type().

### Type QSocketNotifier::type () const

Returns the socket event type specified to the constructor.

See also socket().