X Window selection

Selections, cut buffers, and drag-and-drop are the mechanisms used in the X Window System to allow a user to transfer data from one 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 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.

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:

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.

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