Subtleties of the X Clipboard

A few months back, I started running the lean Archlinux build that I've been using through to today. I elected not to use a display manager, instead preferring booting Awesome (an aptly named tiling window manager) via the startx script. After getting used to Awesome's key bindings, and throwing Luakit, Urxvt and Tmux into the mix, I got about as close to an optimized Linux build as I was likely to get.

Everything was perfect ... except for one aspect: the clipboard. Its behavior was utterly perplexing: I could select text and middle-click (or Shift-Insert) it most places I wanted, but I could only copy out of Chromium; while pasting it seemed to only respect text that had been copied from itself. Vim was even worse, even with set clipboard=unnamed it didn't seem to play nice with anything else.

This was pretty frustrating—the clipboard's importance in the everyday workflow really can't be understated. So what was the problem? To understand, we have to know a little more about the X clipboard.

The X Clipboard

In X10, cut-buffers were introduced. The concept behind this now obsolete mechanism was that when text was selected in a window, the window owner would copy it to a property of the root window called CUT_BUFFER1, a universally owned bucket available to every application on the system. General consensus on cut-buffers was that they were the absolute salt of the Earth, so a new system was devised.

Thus selections came about. Rather than applications copying data to a global bucket, they request ownership of a selection. When paste is called from another application, it requests data from the client that currently owns the selection. Aside from being much more versatile and less volatile than cut-buffers, selections can also be faster because no data has to be sent on a copy (only on paste). This is especially advantageous when there's a slow connection to the X server, but this strength is also a weakness because data made available by an application disappears when it closes.

Three selections are defined in the ICCCM: CLIPBOARD, PRIMARY, and SECONDARY, each of which behaves like a clipboard in its own right:

  • CLIPBOARD: traditionally used when text is copied and pasted from the edit menu, or via the Ctrl+C and Ctrl+V shortcuts in applications that support them.
  • PRIMARY: traditionally used when a mouse selection is made, and pasted with middle-click or Shift-Insert.
  • SECONDARY: ill-defined secondary selection. Most applications don't use it.

The heart of the problem for me is that I expected the X clipboard to behave like the clipboard on Windows or Mac OS X, but in fact X's architecture is fundamentally different with two separate, yet equally important, clipboards in use.

Vim

Naturally, I had to know how Vim interacts with the X clipboard and was pleased to discover that it has some really great documentation on the subject (see for yourself with :help x11-selection). When running a GUI or X11-aware version of Vim, it has two registers that interact with X:

  • * (as in "*yy): is the PRIMARY selection. :set clipboard=unnamed aliases it to the unnamed register.
  • +: is the CLIPBOARD selection. :set clipboard=unnamedplus aliases it to the unnamed register.

Vim does not interact with the SECONDARY selection.

In Practice

I'm a Linux person at heart, but for me the two equal and separate selections remain an unfortunate usability problem. Luckily for anyone with the same disposition, Autocutsel can help make X's behavior more logical and intuitive. It's a great little program that synchronizes the cut-buffer with CLIPBOARD, or both the cut-buffer and CLIPBOARD with PRIMARY as well.

Install Autocutsel (pacman -S autocutsel on Arch) and put the following two lines into your .xinitrc (or just run them from a terminal to immediately observe the effects):

autocutsel -fork &
autocutsel -selection PRIMARY -fork &

Now, no matter where you copy and paste from, be it Ctrl+C in Chrome, p in Vim, or through text selection in X, your clipboard is consistent across the entire system.

Posted on August 3, 2011 from Calgary

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