Many modern editors and IDEs can graphically indicate the location of the fill column by drawing a thin line (in design parlance, a "rule") down the length of the editing window. Fill-column-indicator implements this facility in Emacs:

Installation and Usage

Put the package file in your load path and put

(require 'fill-column-indicator)

in your .emacs.

To toggle graphical indication of the fill column in a buffer, use the command fci-mode.

Configuration

• On graphical displays the fill-column rule is drawn using a bitmap image. Its color is controlled by the variable fci-rule-color, whose value can be any valid color name. The rule's width in pixels is determined by the variable fci-rule-width; the default value is 2.

• The image formats fci-mode can use are XPM, PBM, and XBM. If Emacs has been compiled with the appropriate library it uses XPM images by default; if not it uses PBM images, which are natively supported. You can specify a particular format by setting fci-rule-image-format to either xpm, xpm, or xbm.

• On character terminals the rule is drawn using the character specified by fci-rule-character; the default is `|' (ascii 124). If fci-rule-character-color is nil, then it is drawn using fci-rule-color (or the closest approximation thereto that the terminal is capable of); if it is a color name, then that color is used instead.

• If you'd like the rule to be drawn using fci-rule-character even on graphical displays, set fci-always-use-textual-rule to a non-nil value.

• If you'd like the rule to be drawn at a column other than the fill-column, set fci-fill-column to a non-nil value. This is useful if you want a fill-column of, say, 70 or 72 characters for wrapping flowing blocks of text such as code comments, but want a vertical rule at, say, 80 or 100 characters to indicate the maximum line length for lines of code.

These variables (as well as those in the next section) can be given buffer-local bindings.

Other Options

When truncate-lines is nil, the effect of drawing a fill-column rule is very odd looking. Indeed, it makes little sense to use a rule to indicate the position of the fill column in that case (the positions at which the fill column falls in the visual display space won't in general be collinear). For this reason, fci-mode sets truncate-lines to t in buffers in which it is enabled and restores it to its previous value when disabled. You can turn this feature off by setting fci-handle-truncate-lines to nil.

If line-move-visual is t, then vertical navigation can behave oddly in several edge cases while fci-mode is enabled (this is due to a bug in C code). Accordingly, fci-mode sets line-move-visual to nil in buffers in which it is enabled and restores it to its previous value when disabled. This can be suppressed by setting fci-handle-line-move-visual to nil. (But you shouldn't want to do this. There's no reason to use line-move-visual if truncate-lines is t, and it doesn't make sense to use something like fci-mode when truncate-lines is nil.)

Fci-mode needs free use of two characters (specifically, it needs the use of two characters whose display table entries it can change arbitrarily). By default, it uses the first two characters of the Private Use Area of the Unicode BMP, viz. U+E000 and U+E001. If you need to use those characters for some other purpose, set fci-eol-char and fci-blank-char to different values.

Troubleshooting

• Fci-mode is intended to be used with monospaced fonts. If you're using a monospaced font and the fill-column rule is missing or misaligned on a few lines but otherwise appears normal, then most likely (a) there are non-ascii characters on those lines that are being displayed using a non-monospaced font, or (b) your font-lock settings use bold or italics and those font variants aren't monospaced.

• Although the XBM and PBM formats are natively supported by Emacs, the implementations are different in different ports and sometimes incomplete; for example, on some ports XBM images are always drawn in black. Explicitly setting fci-rule-image-format to a different value will usually resolve such issues.

Known Issues

• The indicator extends only to end of the buffer contents (as opposed to running the full length of the editing window).

• When portions of a buffer are invisible, such as when outline mode is used to hide certain lines, the fill-column rule is hidden as well.

• Fci-mode should work smoothly when simultaneously displaying the same buffer on both a graphical display and on a character terminal. It does not currently support simultaneous display of the same buffer on window frames with different default font sizes. (It would be feasible to support this use case, but thus far there seems to be no demand for it.)

• An issue specific to the Mac OS X (NextStep) port, versions 23.0-23.2: Emacs won't, in these particular versions, draw a cursor on top of an image. Thus on graphical displays the cursor will disappear when positioned directly on top of the fill-column rule. The best way to deal with this is to upgrade to v23.3 or v24 (or downgrade to v22). If that isn't practical, a fix is available via the mini-package fci-osx-23-fix.el, which can be downloaded from this page. Directions for its use are given in the file header.

Todo

• Accommodate non-nil values of hl-line-sticky-flag and similar cases.

• Accommodate linum-mode more robustly.

• Compatibility with non-nil show-trailing-whitespace.