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DCOP, which stands for Desktop COmmunication Protocol, is a light-weight interprocess and software componentry communication system. The main point of this system is to allow applications to interoperate, and to share complex tasks. Essentially, DCOP is a ‘remote control’ system, which allows an application or a script to enlist the help of other applications. It is built on top of the X Window System’s Inter-Client Exchange protocol.

The use of DCOP provides extensive new capabilities, without requiring entirely new applications to be written, as might otherwise be the case. KDE applications and the KDE libraries make heavy use of DCOP and most of the KDE applications can be controlled by scripts via the DCOP mechanism. DCOP was replaced by D-Bus in KDE 4.

In modern KDE systems, every KDE application supports a basic set of DCOP interfaces, even if the programmer of the application did not explicitly code in such support. For instance, every application automatically supports the “quit” command to close the application.

There is a command-line tool called ‘dcop’ (note the lower-case letters) that can be used for communication with the applications from the shell. ‘kdcop’ is a GUI tool to explore the interfaces of an application.

For example, the KDE desktop provides a way to display different wallpapers at timed intervals. However, it does not directly provide an interface for changing to the next wallpaper, if the current one does not fit your mood. Neither does it provide a way of permanently removing desktop wallpapers that you decide you do not like, after seeing them as actual wallpaper on your screen.

These features can be added in a few moments, however, using dcop. The command:

 $ dcop kdesktop KBackgroundIface changeWallpaper

will switch to the next wallpaper from a shell, and the command:

 $ dcop kdesktop KBackgroundIface currentWallpaper 1

will get the filename of the wallpaper on desktop 1. (KDE, and most X environments, support multiple ‘virtual’ desktops for organizing work.) By combining the two in a short shell script, you can switch to the next wallpaper, then delete the previous wallpaper, like so:

 OLDWALLPAPER=`dcop kdesktop KBackgroundIface currentWallpaper 1`
 dcop kdesktop KBackgroundIface changeWallpaper

As you can see, DCOP makes it possible to add new features that were never envisaged when an application was first created.

DCOP Model

The model is simple. Each application using DCOP is a client. They communicate to each other through a DCOP server, which functions like a traffic director, dispatching messages/calls to the proper destinations. All clients are peers of each other.

Two types of actions are possible with DCOP: "send and forget" messages, which do not block, and "calls," which block waiting for some data to be returned.

Any data that will be sent is serialized (also referred to as marshalling in CORBA speak) using the built-in QDataStream operators available in all of the Qt classes. There is also a simple IDL-like compiler available (dcopidl and dcopidl2cpp) that generates stubs and skeletons. Using the dcopidl compiler has the additional benefit of type safety.

D-Bus, a message bus system standardized by, was heavily influenced by the DCOP system and replaces DCOP in KDE 4.

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