X Window System selection

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 no longer needs to care about it; ; Active: Transfer of data to a client requires the client "holding" the selection to actively participate in 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, it 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 requires 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, the receiver can request a different format. For example, a piece of text rendering HTML code can be transferred as text to a requester that can only handle text, but can also be transferred as HTML code if the requester can handle it. Such negotiation of format cannot be done by passive mechanisms, in which the client holding the selection (and giving it semantics) transfers the selection and 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: • request to know which window owns the selection • request to set the window that owns the selection • 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, and 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: • 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) • in response, the server sends to the current owner of the selection a SelectionRequest event; • 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 • the owner sends a request to the server to send the requester a SelectionNotify to notify that the selection has been transferred • the requester can now read the selection in the property of the window by sending one or more GetProperty requests to the server; • the requester 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 requester knows when another chunk has been placed in the property. The XFixes extension allow clients to listen for selection changes. == 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. == Programs ==

The following programs specifically operate on data transfer mechanisms: • xcutsel transfers data from selections to cut buffers or vice versa • xclipboard, glipper (Gnome), parcellite (LXDE), and klipper (KDE) are clipboard managers, maybe wmcliphist as well • xcb shows the content of the cut buffers and allows the user to manipulate them • xselection, xclip, xsel and xcopy are command line programs that copy data to or from the X selection. xcopy has a verbosity option that helps debug X selection issues. parcellite also has the ability to read from and write to specific X selections from the command line. • 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" and also a clipboard manager • xtranslate looks up words in the Xselection in a multi-lingual dictionary • autocutsel syncs cut buffer and selection buffer == See also ==