Xclipboard

Selections, cut buffers, and drag-and-drop are the mechanisms used in the X Window System to allow a user to transfer data from a window to another. Selections and cut buffer are typically used when a user selects text or some other data in a window and pastes in another one. Drag-and-drop is used when a user selects something in a window, then clicks on the selection and drags it into another window.

Since the two windows may be handled by two different applications, these mechanisms require two different clients connected with the same X server to exchange data. The X Window core protocol includes some requests and events that are specific to selection exchange, but the transfer is mainly done using event sending and window properties, which are not specific to selection transfer.

Different types of data can be transferred: it is usually text, but can also be an image, a number, a list of objects, etc. In the following, only the case of text is considered.

Active and passive selections

The methods for transferring data can be classified into active and passive, depending on whether the client handling the selected data has to actively participate in the transfer to a client requesting it:

passive 

when some data is selected, the client handling the window where this selection is done transfers it somewhere, and needs not to care about it any longer;

active 

transfer of data to a client request the client "holding" the selection to actively participate to the exchange;

Selections and drag-and-drop are active mechanisms: after some text is selected in a window, the client handling the window must actively support a protocol for transferring the data to the application requesting it. By contrast, cut buffers are a passive mechanism: after some text is selected, its is transferred to a cut buffer and remains there even if the application handling the window terminates and the window is destroyed. The X clipboard is a passive mechanism as perceived by the client holding the selection, but require the xclipboard client to actively support any subsequent data transfer.

An advantage of active mechanisms is that the data can be converted to a different format before the transfer. In particular, the client receiving the data can request the selection data to be converted into a suitable form. If the sending client refuses to do so, first client can request a different format. For example, a piece of text rendering HTML code can be transferred as text to a requestor that can only handle text, but can also be transferred as HTML code if the requestor can handle it. Such negotiation of format cannot be done by passive mechanisms, in which the client holding the selection (and giving it a semantics) transfers the selection is not involved in the further transfer into a client requesting it.

Another advantage of the active mechanisms is that large pieces of data can be transferred in a sequence of transfers rather than a single one. Passive mechanisms instead require all data to be transferred somewhere from the selection owner, then transferred again to the client requesting it.

The advantage of the passive mechanisms is that the transfer can be done even after the client holding the data terminates. This is not possible in the active mechanisms, which require the client holding the data to actively participate in the transfer.

Selections

The X Window System supports an arbitrary number of selections; every selection is identified by a string (more precisely, an atom). The most used selection is the PRIMARY selection.

The following requests are specific to selection transfer, although transfer also involves other requests:

1. request to know which window owns the selection
2. request to set the window that owns the selection
3. request to convert the selection

The owner of the selection is typically the window in which the selected text is located, if any. When the user selects some text in a window, the client handling the window must tell the server that the window is the owner of the selection.

When the user tries to paste the selection into another window, that window's handler initiates a protocol for getting the selected text from the other client. This protocol involves the second and third requests of the list above, and is not specified by the X protocol but as a convention in the Inter-Client Communication Convention Manual (ICCCM).

In particular, the destination client begins by asking the server which window owns the selection. Then the two clients transfer the selection via the server. This exchange involves a property of a window, an arbitrary piece of data attached to the window. If the content of the selection is considered small enough to be transferred all at once, the steps that take place are:

1. the recipient of the selection requests the selection to be converted, specifying a property of a window (this may be the window where the text has to be pasted)
2. in response, the server sends to the current owner of the selection a SelectionRequest event;
3. the owner places the selected text in the property of the window that the requestor has specified by sending a ChangeProperty; request to the server
4. the owner sends a request to the server to send the requestor a SelectionNotify to notify that the selection has been transferred
5. the requestor can now read the selection in the property of the window by sending one or more GetProperty requests to the server;
6. the requestor destroys the property; if the owner has requested to be informed of this, it is sent a PropertyNotify event.

If the content is large, it should be transferred in chunks. In this case, both clients express interest in PropertyNotify events: this way, the selection owner knows when the selection has been read, and the requestor knows when another chunk has been placed in the property.

Clipboard

The most used selection is the PRIMARY selection, and is used when the user selects some data. The CLIPBOARD selection is used when the user selects some data and explicitly requests it to be "copied" to the clipboard, such as by invoking "Copy" under the "Edit" menu of an application. An associated request of "Paste" results in the data of the CLIPBOARD selection being used.

At the level of the core protocol, the PRIMARY and CLIPBOARD selections do not differ. But the xclipboard client makes them behave differently. In particular, when another client asserts the ownership of the CLIPBOARD selection, this program requests and displays it in a window. Any further request for this selection are handled by xclipboard. This way, the content of the selection survives the client having copied it.

Cut buffers

Cut buffers are another mechanism to transfer data, in particular selected text. They are window properties of the root window, named CUT_BUFFER1, etc. Unlike selections, cut buffers do not involve a direct interaction between clients. Rather, when text is selected in a window, the window owner copies this text into the property of the root window called CUT_BUFFER1. When the user pastes the text in another window, the window owner reads this property of the root window.

The xcutsel program transfers data between selections and cut buffers, and the xcb program allows various kinds of access to the cut buffers.

Cut buffers are considered obsolete.

Xdnd

Drag-and-drop in the X Window System is regulated by the Xdnd convention. When the user drags the selected text into a window and releases the mouse button, the exchange of data is done as for the primary selection. Drag-and-drop is complicated by what happens during the drag. Namely, when the user drags the selection in different parts of the screen or of a window, the user expects to be able to tell whether text can be dropped or not. In particular, the image of the cursor (which the user metaphorically identifies with the dragged text) should change depending on whether the text can be dropped in the current position or not.

In the Xdnd protocol, the window where the text is selected and the drag begins is called the source; the window where the cursor is currently in is called the target. The communication between the source and the target is driven by the source client, which also decides the shape of the cursor. Since this shape is supposed to change depending on whether the text can be dropped in the current position of the target window, an exchange between source and target is necessary.

A client can state that a window can be the target of a drop by creating a property of name XdndAware. This way, all applications that were written without taking drag-and-drop into consideration at all will be recognized by the lack of this property and ignored by all drag-and-drop sources.

When the cursor enters the target window, the source checks the presence of the XdndAware property on that window. If this property is present, an exchange begins:

• the source tells the target that the cursor has entered the target while dragging some data by sending an event XdndEnter
• the target can find out which kind of data is dragged (text, image, etc.) by looking at this event and possibly by further interaction with the source

When the cursor is inside the target window:

• the source sends XdndPosition events to tell the target where the cursor currently is
• the target replies with XdndStatus events to tell the source whether the data can be dropped in the current position
• the source sends a message XdndLeave or XdndDrop when the cursor has left the window or the button has been released

In case of drop the target requests the selection to the source as usual. When the transfer of the selection is concluded, the target sends an XdndFinish event to tell the source that the transfer has been successful, so that the source can give a visible feedback to the user.

Summarizing, the protocol is driven by the source, which keeps the target informed of what is happening with the cursor. In reply, the target tells the source whether a drop would be accepted or not. The target must also be informed when the user releases the mouse button, as this event starts a regular request for a selection, which is a protocol driven by the target.

The above is the description of the Xdnd convention for drag-and-drop. Different conventions for drag-and-drop are used in Motif, OffiX, and Amulet.

Programs

The following programs specifically operate on data transfer mechanisms:

• xcutsel transfers data from selections to cut buffers or vice versa
• xclipboard, glipper (Gnome), and klipper (KDE) are clipboard managers, maybe wmcliphist as well
• xselection transfers data to/from a selection
• xcb shows the content of the cut buffers and allows the user to manipulate them
• xclip, xsel and uclip are command line programs which are used to copy data to/or from a selection from scripts
• synergy is a cross platform tool that allows you to share a clipboard across multiple computers running multiple operating systems
• xfce4-clipman-plugin is a "clipboard history plugin for the Xfce4 panel"
• xtranslate looks up words in the Xselection in a multi-lingual dictionary

See also

• X Window System protocols and architecture
• X Window System core protocol
• ICCCM

External links

• ICCCM: Peer-to-Peer Communication by Means of Selections
• ICCCM: Peer-to-Peer Communication by Means of Cut Buffers
• Xdnd specification
• A paper by Keith Packard
• Selections in general and in Emacs
• autocutsel - syncs cut buffer and selection buffer