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# Customization

Ediff has a rather self-explanatory interface, and in most cases you won't need to change anything. However, should the need arise, there are extensive facilities to change the default behavior.

Most of the customization can be done by setting various variables in the ``.emacs'` file. Some customization (mostly window-related customization and faces) can be done by putting appropriate lines in ``.Xdefaults'`, ``.xrdb'`, or whatever X resource file is in use.

With respect to the latter, please note that the X resource for Ediff customization is `Ediff', not `emacs'. See section Window and Frame Configuration, See section Highlighting Difference Regions, for further details. Please also refer to Emacs manual for the information on how to set Emacs X resources.

- Hooks: Customization via the hooks.
- Quick Help: How to customize Ediff's quick help feature.
- Window and Frame Configuration: Controlling the way Ediff displays things.
- Selective Browsing: Advanced browsing through difference regions.
- Highlighting Difference Regions: Controlling highlighting.
- Narrowing: Comparing regions, windows, etc.
- Refinement of Difference Regions: How to control the refinement process.
- Patch and Diff Programs: Changing the utilities that compute differences and apply patches.
- Merging and diff3: How to customize Ediff in its Merge Mode.
- Support for Version Control: Changing the version control package. You are not likely to do that.
- Customizing the Mode Line: Changing the look of the mode line in Ediff.
- Miscellaneous: Other customization.
- Notes on Heavy-duty Customization: Customization for the gurus.

## Hooks

The bulk of customization can be done via the following hooks:

`ediff-load-hook`
  This hook can be used to change defaults after Ediff is loaded.
`ediff-keymap-setup-hook`
  This hook can be used to alter bindings in Ediff's keymap. These hooks are run right after the default bindings are set but before `ediff-load-hook`. The regular user needs not be concerned with this hook--it is provided for implementors of other Emacs packages built on top of Ediff.
`ediff-before-setup-windows-hook`
`ediff-after-setup-windows-hook`
  These two hooks are called before and after Ediff sets up its window configuration. Can be used to save the configuration that existed before Ediff starts or for whatever other purposes.
`ediff-suspend-hook`
`ediff-quit-hook`
  These two hooks are run when you suspend or quit Ediff. They can be used to set desired window configurations, delete files Ediff didn't want to clean up after exiting, etc. By default, `ediff-quit-hook` holds one hook function, `ediff-cleanup-mess`, which cleans after Ediff, as appropriate in most cases. You probably won't want to change it, but you might want to add other hook functions. Keep in mind that hooks executing before `ediff-cleanup-mess` start in `ediff-control-buffer;` they should also leave `ediff-control-buffer` as the current buffer when they finish. Hooks that are executed after `ediff-cleanup-mess` should expect the current buffer be either buffer A or buffer B. `ediff-cleanup-mess` doesn't kill the buffers being compared or merged (see `ediff-cleanup-hook`, below).
`ediff-cleanup-hook`
  This hook is run just before `ediff-quit-hook`. This is a good place to do various cleanups, such as deleting the variant buffers. Ediff provides a function, `ediff-janitor`, as one such possible hook, which you can add to `ediff-cleanup-hook` with `add-hooks`. This function kills buffers A, B, and, possibly, C, if these buffers aren't modified. In merge jobs, buffer C is never deleted. However, the side effect of using this function is that you may not be able to compare the same buffer in two separate Ediff sessions: quitting one of them will delete this buffer in another session as well.
`ediff-before-setup-control-frame-hook`
`ediff-after-setup-control-frame-hook`
  These two hooks run before and after Ediff sets up the control frame. They can be used to relocate Ediff control frame when Ediff runs in a multiframe mode (i.e., when the control buffer is in its own dedicated frame). Be aware that many variables that drive Ediff are local to Ediff Control Panel (`ediff-control-buffer`), which requires special care in writing these hooks. Take a look at `ediff-default-suspend-hook` and `ediff-default-quit-hook` to see what's involved.
`ediff-startup-hook`
  This hook is run at the end of Ediff startup.
`ediff-select-hook`
  This hook is run after Ediff selects the next difference region.
`ediff-unselect-hook`
  This hook is run after Ediff unselects the current difference region.
`ediff-prepare-buffer-hook`
  This hook is run for each Ediff buffer (A, B, C) right after the buffer is arranged.
`ediff-display-help-hook`
  Ediff runs this hook each time after setting up the help message. It can be used to alter the help message for custom packages that run on top of Ediff.
`ediff-mode-hook`
  This hook is run just after Ediff mode is set up in the control buffer. This is done before any Ediff window is created. You can use it to set local variables that alter the look of the display.
`ediff-registry-setup-hook`
  Hooks run after setting up the registry for all active Ediff session. See section Session Groups, for details.
`ediff-session-group-setup-hook`
  Hooks run after setting up a control panel for a group of related Ediff sessions. See section Session Groups, for details.

## Quick Help

Ediff provides quick help using its control panel window. Since this window takes a fair share of the screen real estate, you can toggle it off by typing `?`. The control window will then shrink to just one line and a mode line, displaying a short help message.

The variable `ediff-use-long-help-message` tells Ediff whether you use the short message or the long one. By default, it is set to `nil`, meaning that the short message is used. Set this to `t`, if you want Ediff to use the long message by default. This property can always be changed interactively, by typing `?` into Ediff Control Buffer.

If you want to change the appearance of the help message on a per-buffer basis, you must use `ediff-startup-hook` to change the value of the variable `ediff-help-message`, which is local to `ediff-control-buffer`.

## Window and Frame Configuration

On a non-windowing display, Ediff sets things up in one frame, splitting it between a small control window and the windows for buffers A, B, and C. The split between these windows can be horizontal or vertical, which can be changed interactively by typing `|` while the cursor is in the control window.

On a window display, Ediff sets up a dedicated frame for Ediff Control Panel and then it chooses windows as follows: If one of the buffers is invisible, it is displayed in the currently selected frame. If a buffer is visible, it is displayed in the frame where it is visible. If, according to the above criteria, the two buffers fall into the same frame, then so be it--the frame will be shared by the two. The same algorithm works when you type `C-l` (`ediff-recenter`), `p` (`ediff-previous-difference`), `n` (`ediff-next-difference`), etc.

The above behavior also depends on whether the current frame is splittable, dedicated, etc. Unfortunately, the margin is too small to present this remarkable algorithm.

The bottom line of all this is that you can compare buffers in one frame or in different frames. The former is done by default, while the latter can be achieved by arranging buffers A, B (and C, if applicable) to be seen in different frames. Ediff respects these arrangements, automatically adapting itself to the multi-frame mode.

Ediff uses the following variables to set up its control panel (a.k.a. control buffer, a.k.a. quick help window):

`ediff-control-frame-parameters`
  You can change or augment this variable including the font, color, etc. The X resource name of Ediff Control Panel frames is ``Ediff'`. Under X-windows, you can use this name to set up preferences in your ``~/.Xdefaults'`, ``~/.xrdb'`, or whatever X resource file is in use. Usually this is preferable to changing `ediff-control-frame-parameters` directly. For instance, you can specify in ``~/.Xdefaults'` the color of the control frame using the resource ``Ediff*background'`. In general, any X resource pertaining the control frame can be reached via the prefix `Ediff*`.
`ediff-control-frame-position-function`
  The preferred way of specifying the position of the control frame is by setting the variable `ediff-control-frame-position-function` to an appropriate function. The default value of this variable is `ediff-make-frame-position`. This function places the control frame in the vicinity of the North-East corner of the frame displaying buffer A.

The following variables can be used to adjust the location produced by `ediff-make-frame-position` and for related customization.

`ediff-narrow-control-frame-leftward-shift`
  Specifies the number of characters for shifting the control frame from the rightmost edge of frame A when the control frame is displayed as a small window.
`ediff-wide-control-frame-rightward-shift`
  Specifies the rightward shift of the control frame from the left edge of frame A when the control frame shows the full menu of options.
`ediff-control-frame-upward-shift`
  Specifies the number of pixels for the upward shift of the control frame.
`ediff-prefer-iconified-control-frame`
  If this variable is `t`, the control frame becomes iconified automatically when you toggle the quick help message off. This saves valuable real estate on the screen. Toggling help back will deiconify the control frame. To start Ediff with an iconified Control Panel, you should set this variable to `t` and `ediff-prefer-long-help-message` to `nil` (see section Quick Help). This behavior is useful only if the window manager is TWM or a derivative. Also, this doesn't work under XEmacs, since this editor ignores input from icons.

To make more creative changes in the way Ediff sets up windows, you can rewrite the function `ediff-setup-windows`. However, we believe that detaching Ediff Control Panel from the rest and making it into a separate frame offers an important opportunity by allowing you to iconify that frame. Under Emacs, the icon will usually accept all of the Ediff commands, but will free up valuable real estate on your screen (this may depend on the window manager, though). Iconifying won't do any good under XEmacs since XEmacs icons are not sensitive to keyboard input. The saving grace is that, even if not iconified, the control frame is very small, smaller than some icons, so it does not take much space in any case.

The following variable controls how windows are set up:

`ediff-window-setup-function`
  The multiframe setup is done by the `ediff-setup-windows-multiframe` function, which is the default on windowing displays. The plain setup, one where all windows are always in one frame, is done by `ediff-setup-windows-plain`, which is the default on a non-windowing display (or in an xterm window). In fact, under Emacs, you can switch freely between these two setups by executing the command `ediff-toggle-multiframe` using the Minibuffer. If you don't like any of these setups, write your own function. See the documentation for `ediff-window-setup-function` for the basic guidelines. However, writing window setups is not easy, so you should first take a close look at `ediff-setup-windows-plain` and `ediff-setup-windows-multiframe`.

You can run multiple Ediff sessions at once, by invoking Ediff several times without exiting previous Ediff sessions. Different sessions may even operate on the same pair of files.

Each session has its own Ediff Control Panel and all the regarding a particular session is local to the associated control panel buffer. You can switch between sessions by suspending one session and then switching to another control panel. (Different control panel buffers are distinguished by a numerical suffix, e.g., ``Ediff Control Panel<3>'`.)

## Selective Browsing

Sometimes it is convenient to be able to step through only some difference regions, those that satisfy certain regular expressions, and to ignore all others. On other occasions, you may want to ignore difference regions that satisfy some regular expressions, and to look only at the rest.

The commands `#f` and `#h` let you do precisely this.

Typing `#f` lets you specify regular expressions that match difference regions you want to focus on. We shall call these regular expressions regexp-A, regexp-B and regexp-C. Ediff will then start stepping through only those difference regions where the region in buffer A matches regexp-A and/or the region in buffer B matches regexp-B, etc. Whether `and' or `or' will be used depends on how you respond to a question.

When scanning difference regions for the aforesaid regular expressions, Ediff narrows the buffers to those regions. This means that you can use the expressions `\`` and `\'` to tie search to the beginning or end of the difference regions.

On the other hand, typing `#h` lets you specify (hide) uninteresting regions. That is, if a difference region in buffer A matches regexp-A, the corresponding region in buffer B matches regexp-B and (if applicable) buffer-C's region matches regexp-C, then the region will be ignored by the commands `n`/`SPC` (`ediff-next-difference`) and `p`/`DEL` (`ediff-previous-difference`) commands.

Typing `#f` and `#h` toggles selective browsing on and off.

Note that selective browsing affects only `ediff-next-difference` and `ediff-previous-difference`, i.e., the commands `n`/`SPC` and `p`/`DEL`. `#f` and `#h` do not change the position of the point in the buffers. And you can still jump directly (using `j`) to any numbered difference.

Users can supply their own functions to specify how Ediff should do selective browsing. To change the default Ediff function, add a function to `ediff-load-hook` which will do the following assignments:

    (setq ediff-hide-regexp-matches-function 'your-hide-function)
    (setq ediff-focus-on-regexp-matches-function 'your-focus-function)

Useful hint: To specify a regexp that matches everything, don't simply type `RET` in response to a prompt. Typing `RET` tells Ediff to accept the default value, which may not be what you want. Instead, you should enter something like `^` or `$`. These match every line.

You can use the status command, `i`, to find out whether selective browsing is currently in use.

The regular expressions you specified are kept in the local variables `ediff-regexp-focus-A`, `ediff-regexp-focus-B`, `ediff-regexp-focus-C`, `ediff-regexp-hide-A`, `ediff-regexp-hide-B`, `ediff-regexp-hide-C`. Their default value is the empty string (i.e., nothing is hidden or focused on). To change the default, set these variables in ``.emacs'` using `setq-default`.

In addition to the ability to ignore regions that match regular expressions, Ediff can be ordered to start skipping over certain "inessential" regions. This is controlled by the following variable:

`ediff-ignore-similar-regions`
  If `t`, causes Ediff to skip over difference regions that deemed inessential, i.e., where the only differences are those in the white space and newlines.

Note: In order for this feature to work, auto-refining of difference regions must be on, since otherwise Ediff won't know if there are fine differences between regions. On devices where Emacs can display faces, auto-refining is a default, but it is not turned on by default on text-only terminals. In that case, you must explicitly turn auto-refining on (such as, by typing `@`).

Reassurance: If many inessential regions appear in a row, Ediff may take a long time to jump to the next region because it has to compute fine differences of all intermediate regions. This delay does not indicate any problem.

## Highlighting Difference Regions

The following variables control the way Ediff highlights difference regions:

`ediff-before-flag-bol`
`ediff-after-flag-eol`
`ediff-before-flag-mol`
`ediff-after-flag-mol`
  These variables hold strings that Ediff uses to mark the beginning and the end of the differences found in files A, B, and C on devices where Emacs cannot display faces. Ediff uses different flags to highlight regions that begin/end at the beginning/end of a line or in a middle of a line.
`ediff-current-diff-face-A`
`ediff-current-diff-face-B`
`ediff-current-diff-face-C`
  Ediff uses these faces to highlight current differences on devices where Emacs can display faces. These and subsequently described faces can be set either in ``.emacs'` or in ``.Xdefaults'`. The X resource for Ediff is ``Ediff'`, not ``emacs'`. Please refer to Emacs manual for the information on how to set X resources.
`ediff-fine-diff-face-A`
`ediff-fine-diff-face-B`
`ediff-fine-diff-face-C`
  Ediff uses these faces to show the fine differences between the current differences regions in buffers A, B, and C, respectively.
`ediff-even-diff-face-A`
`ediff-even-diff-face-B`
`ediff-even-diff-face-C`
`ediff-odd-diff-face-A`
`ediff-odd-diff-face-B`
`ediff-odd-diff-face-C`
  Non-current difference regions are displayed using these alternating faces. The odd and the even faces are actually identical on monochrome displays, because without colors options are limited. So, Ediff uses italics to highlight non-current differences.
`ediff-force-faces`
  Ediff generally can detect when Emacs is running on a device where it can use highlighting with faces. However, if it fails to determine that faces can be used, the user can set this variable to `t` to make sure that Ediff uses faces to highlight differences.
`ediff-highlight-all-diffs`
  Indicates whether--on a window system--Ediff should highlight differences using inserted strings (as on a text-only terminal) or using colors and highlighting. Normally, Ediff highlights all differences, but the selected difference is highlighted more visibly. One can cycle through various modes of highlighting by typing `h`. By default, Ediff starts in the mode where all difference regions are highlighted. If you prefer to start in the mode where unselected differences are not highlighted, you should set `ediff-highlight-all-diffs` to `nil`. Type `h` to restore highlighting of all differences. Ediff lets you switch between the two modes of highlighting. That is, you can switch interactively from highlighting using faces to highlighting using string flags, and back. Of course, switching has effect only under a windowing system. On a text-only terminal or in an xterm window, the only available option is highlighting with strings.

If you want to change the above variables, you must do it before Ediff is loaded.

There are two ways to change the default setting for highlighting faces: either change the variables, as shown here,

    (setq ediff-current-diff-face-A 'bold-italic)

or here,

    (setq ediff-current-diff-face-A
    (copy-face 'bold-italic 'ediff-current-diff-face-A))

or modify the defaults selectively:

    (add-hook 'ediff-load-hook
              (function (lambda ()
                          (set-face-foreground
                            ediff-current-diff-face-B "blue")
                          (set-face-background
                            ediff-current-diff-face-B "red")
                          (make-face-italic
                            ediff-current-diff-face-B))))

You may also want to take a look at how the above faces are defined in the source code of Ediff.

Note: it is not recommended to use `internal-get-face` (or `get-face` in XEmacs) when defining Ediff's faces, since this may cause problems when there are several frames with different font sizes. Instead, use `copy-face` or `set/make-face-...` as shown above.

## Narrowing

If buffers being compared are narrowed at the time of invocation of Ediff, `ediff-buffers` will preserve the narrowing range. However, if `ediff-files` is invoked on the files visited by these buffers, that widens the buffers, since this command is defined to compare the entire files.

Calling `ediff-regions-linewise` or `ediff-windows-linewise`, or the corresponding ``-wordwise'` commands, narrows the buffers being compared (corresponding to the regions being compared). The original accessible ranges are restored when you quit Ediff. During the command, you can toggle this narrowing on and off with the `%` command.

These two variables control this narrowing behavior:

`ediff-start-narrowed`
  If `t`, Ediff narrows the display to the appropriate range when it is invoked with an ``ediff-regions...'` or ``ediff-windows...'` command. If `nil`, these commands do not automatically narrow, but you can still toggle narrowing on and off by typing `%`.
`ediff-quit-widened`
  Controls whether on quitting Ediff should restore the accessible range that existed before the current invocation.

## Refinement of Difference Regions

Ediff has variables to control the way fine differences are highlighted. This feature gives you control over the process of refinement. Note that refinement ignores spaces, tabs, and newlines.

`ediff-auto-refine`
  This variable controls whether fine differences within regions are highlighted automatically ("auto-refining"). The default is yes (``on'`). On a slow machine, automatic refinement may be painful. In that case, you can turn auto-refining on or off interactively by typing `@`. You can also turn off display of refining that has already been done. When auto-refining is off, fine differences are shown only for regions for which these differences have been computed and saved before. If auto-refining and display of refining are both turned off, fine differences are not shown at all. Typing `*` computes and displays fine differences for the current difference region, regardless of whether auto-refining is turned on.
`ediff-auto-refine-limit`
  If auto-refining is on, this variable limits the size of the regions to be auto-refined. This guards against the possible slowdown that may be caused by extraordinary large difference regions. You can always refine the current region by typing `*`.
`ediff-forward-word-function`
  This variable gives control over how fine differences are computed. The value must be a Lisp function that determines how the current difference region should be split into words. Fine differences are computed by first splitting the current difference region into words and then passing this along to `ediff-diff-program`. For the default `ediff-forward-word-function` (which is `ediff-forward-word`), a word is a string consisting of letters, ``-'`, or ``_'`; a string of punctuation symbols; a string of digits, or a string consisting of symbols that are neither space, nor a letter. This default behavior is controlled by four variables: `ediff-word-1`, ..., `ediff-word-4`. See the on-line documentation for these variables and for the function `ediff-forward-word` for an explanation of how to modify these variables.

Sometimes, when a region has too many differences between the variants, highlighting of fine differences is inconvenient, especially on color displays. If that is the case, type `*` with a negative prefix argument. This unhighlights fine differences for the current region.

To unhighlight fine differences in all difference regions, use the command `@`. Repeated typing of this key cycles through three different states: auto-refining, no-auto-refining, and no-highlighting of fine differences.

## Patch and Diff Programs

This section describes variables that specify the programs to be used for applying patches and for computing the main difference regions (not the fine difference regions):

`ediff-patch-program`
`ediff-diff-program`
`ediff-diff3-program`
  These variables specify the programs to use to produce differences and do patching.
`ediff-patch-options`
`ediff-diff-options`
`ediff-diff3-options`
  These variables specify the options to pass to the above utilities. In `ediff-diff-options`, it may be useful to specify options such as ``-w'` that ignore certain kinds of changes. However, Ediff does not let you use the option ``-c'`, as it doesn't recognize this format yet.
`ediff-custom-diff-program`
`ediff-custom-diff-options`
  Because Ediff limits the options you may want to pass to the `diff` program, it partially makes up for this drawback by letting you save the output from `diff` in your preferred format, which is specified via the above two variables. The output generated by `ediff-custom-diff-program` (which doesn't even have to be a standard-style ``diff'`!) is not used by Ediff. It is provided exclusively so that you can refer to it later, send it over email, etc. For instance, after reviewing the differences, you may want to send context differences to a colleague. Since Ediff ignores the ``-c'` option in `ediff-diff-program`, you would have to run `diff -c` separately just to produce the list of differences. Fortunately, `ediff-custom-diff-program` and `ediff-custom-diff-options` eliminate this nuisance by keeping a copy of a difference list in the desired format in a buffer that can be displayed via the command `D`.
`ediff-patch-default-directory`
  Specifies the default directory to look for patches.

Warning: Ediff does not support the output format of VMS `diff`. Instead, make sure to use some implementation of POSIX `diff`, such as `gnudiff`.

## Merging and diff3

Ediff supports three-way comparison via the functions `ediff-files3` and `ediff-buffers3`. The interface is the same as for two-way comparison. In three-way comparison and merging, Ediff reports if any two difference regions are identical. For instance, if the current region in buffer A is the same as the region in buffer C, then the mode line of buffer A will display ``[=diff(C)]'` and the mode line of buffer C will display ``[=diff(A)]'`.

Merging is done according to the following algorithm.

If a difference region in one of the buffers, say B, differs from the ancestor file while the region in the other buffer, A, doesn't, then the merge buffer, C, gets B's region. Similarly when buffer A's region differs from the ancestor and B's doesn't, A's region is used.

If both regions in buffers A and B differ from the ancestor file, Ediff chooses the region according to the value of the variable `ediff-default-variant`. If its value is `default-A` then A's region is chosen. If it is `default-B` then B's region is chosen. If it is `combined` then the region in buffer C will look like this:

    #ifdef NEW  /* variant A */
    difference region from buffer A
    #else  /* variant B */
    difference region from buffer B
    #endif  /* NEW */

The actual strings that separate the regions copied from buffer A and B are controlled by the variable `ediff-combination-pattern`. Its value should be a list of three strings. The first is inserted before the difference region of buffer A; the second string goes between the regions; the third goes after region B, as shown in the above example.

In addition to the state of the difference, during merging Ediff displays the state of the merge for each region. If a difference came from buffer A by default (because both regions A and B were different from the ancestor and `ediff-default-variant` was set to `default-A`) then ``[=diff(A) default-A]'` is displayed in the mode line. If the difference in buffer C came, say, from buffer B because the difference region in that buffer differs from the ancestor, but the region in buffer A does not (if merging with an ancestor) then ``[=diff(B) prefer-B]'` is displayed. The indicators default-A/B and prefer-A/B are inspired by Emerge and have the same meaning.

Another indicator of the state of merge is ``combined'`. It appears with any difference region in buffer C that was obtained by combining the difference regions in buffers A and B as explained above.

In addition to state of merge and difference indicator, while merging with an ancestor file or buffer, Ediff informs the user when the current difference region in the (normally invisible) ancestor buffer is empty via the AncestorEmpty indicator. This helps determine if the changes made to the original in variants A and B represent pure insertion or deletion of text: if the mode line shows AncestorEmpty and the corresponding region in buffers A or B is not empty, this means that new text was inserted. If this indicator is not present and the difference regions in buffers A or B are non-empty, this means that text was modified. Otherwise, the original text was deleted.

Although the ancestor buffer is normally invisible, Ediff maintains difference regions there and advances the current difference region accordingly. All highlighting of difference regions is provided in the ancestor buffer, except for the fine differences. Therefore, if desired, the user can put the ancestor buffer in a separate frame and watch it there. However, on a TTY, only one frame can be visible at any given time, and Ediff doesn't support any single-frame window configuration where all buffers, including the ancestor buffer, would be visible. However, the ancestor buffer can be displayed by typing `/` to the control window. (Type `C-l` to hide it again.)

Note that the state-of-difference indicators ``=diff(A)'` and ``=diff(B)'` above are not redundant, even in the presence of a state-of-merge indicator. In fact, the two serve different purposes.

For instance, if the mode line displays ``=diff(B) prefer(B)'` and you copy a difference region from buffer A to buffer C then ``=diff(B)'` will change to ``diff-A'` and the mode line will display ``=diff(A) prefer-B'`. This indicates that the difference region in buffer C is identical to that in buffer A, but originally buffer C's region came from buffer B. This is useful to know because you can recover the original difference region in buffer C by typing `r`.

Ediff never changes the state-of-merge indicator, except in response to the `!` command (see below), in which case the indicator is lost. On the other hand, the state-of-difference indicator is changed automatically by the copying/recovery commands, `a`, `b`, `r`, `+`.

The `!` command loses the information about origins of the regions in the merge buffer (default-A, prefer-B, or combined). This is because recomputing differences in this case means running `diff3` on buffers A, B, and the merge buffer, not on the ancestor buffer. (It makes no sense to recompute differences using the ancestor file, since in the merging mode Ediff assumes that you have not edited buffers A and B, but that you may have edited buffer C, and these changes are to be preserved.) Since some difference regions may disappear as a result of editing buffer C and others may arise, there is generally no simple way to tell where the various regions in the merge buffer came from.

In three-way comparison, Ediff tries to disregard regions that consist entirely of white space. For instance, if, say, the current region in buffer A consists of the white space only (or if it is empty), Ediff will not take it into account for the purpose of computing fine differences. The result is that Ediff can provide a better visual information regarding the actual fine differences in the non-white regions in buffers B and C. Moreover, if the regions in buffers B and C differ in the white space only, then a message to this effect will be displayed.

In the merge mode, the share of the split between window C (the window displaying the merge-buffer) and the windows displaying buffers A and B is controlled by the variable `ediff-merge-window-share`. Its default value is 0.5. To make the merge-buffer window smaller, reduce this amount.

We don't recommend increasing the size of the merge-window to more than half the frame (i.e., to increase the value of `ediff-merge-window-share`) to more than 0.5, since it would be hard to see the contents of buffers A and B.

You can temporarily shrink the merge window to just one line by typing `s`. This change is temporary, until Ediff finds a reason to redraw the screen. Typing `s` again restores the original window size.

With a positive prefix argument, the `s` command will make the merge window slightly taller. This change is persistent. With ``-`' or with a negative prefix argument, the command `s` makes the merge window slightly shorter. This change also persistent.

Ediff lets you automatically ignore the regions where only one of the buffers A and B disagrees with the ancestor. To do this, set the variable `ediff-show-clashes-only` to non-`nil`.

You can toggle this feature interactively by typing `$`.

Note that this variable controls only how Ediff chooses the next/previous difference to show. You can still jump directly to any difference using the command `j` (with a prefix argument specifying the difference number).

## Support for Version Control

Ediff supports version control and lets you compare versions of files visited by Emacs buffers via the function `ediff-revision`. This feature is controlled by the following variables:

`ediff-version-control-package`
  A symbol. The default is ``vc'`. If you are like most Emacs users, Ediff will use VC as the version control package. This is the standard Emacs interface to RCS, CVS, and SCCS. However, if your needs are better served by other interfaces, you will have to tell Ediff which version control package you are using, e.g.,

      (setq ediff-version-control-package 'rcs)

  Apart from the standard ``vc.el'`, Ediff supports three other interfaces to version control: ``rcs.el'`, ``pcl-cvs.el'`, and ``generic-sc.el'`. The package ``rcs.el'` is written by Sebastian Kremer <sk@thp.Uni-Koeln.DE> and is available as

      `ftp.cs.buffalo.edu:pub/Emacs/rcs.tar.Z'
      `ftp.uni-koeln.de:/pub/gnu/emacs/rcs.tar.Z'

  The packages ``pcl-cvs.el'` and ``generic-sc.el'` are found in XEmacs distribution.

Ediff's interface to the above packages allows the user to compare the versions of the current buffer or to merge them (with or without an ancestor-version). These operations can also be performed on directories containing files under version control.

In case of ``pcl-cvs.el'`, Ediff can also be invoked via the function `run-ediff-from-cvs-buffer`---see the documentation string for this function.

## Customizing the Mode Line

When Ediff is running, the mode line of ``Ediff Control Panel'` buffer shows the current difference number and the total number of difference regions in the two files.

The mode line of the buffers being compared displays the type of the buffer (``A:'`, ``B:'`, or ``C:'`) and (usually) the file name. Ediff tries to be intelligent in choosing the mode line buffer identification. In particular, it works well with the ``uniquify.el'` and ``mode-line.el'` packages (which improve on the default way in which Emacs displays buffer identification). If you don't like the way Ediff changes the mode line, you can use `ediff-prepare-buffer-hook` to modify the mode line.

## Miscellaneous

Here are a few other variables for customizing Ediff:

`ediff-split-window-function`
  Controls the way you want the window be split between file-A and file-B (and file-C, if applicable). It defaults to the vertical split (`split-window-vertically`, but you can set it to `split-window-horizontally`, if you so wish. Ediff also lets you switch from vertical to horizontal split and back interactively. Note that if Ediff detects that all the buffers it compares are displayed in separate frames, it assumes that the user wants them to be so displayed and stops splitting windows. Instead, it arranges for each buffer to be displayed in a separate frame. You can switch to the one-frame mode by hiding one of the buffers A/B/C. You can also swap the windows where buffers are displayed by typing `~`.
`ediff-merge-split-window-function`
  Controls how windows are split between buffers A and B in the merge mode. This variable is like `ediff-split-window-function`, but it defaults to `split-window-horizontally` instead of `split-window-vertically`.
`ediff-make-wide-display-function`
  The value is a function to be called to widen the frame for displaying the Ediff buffers. See the on-line documentation for `ediff-make-wide-display-function` for details. It is also recommended to look into the source of the default function `ediff-make-wide-display`. You can toggle wide/regular display by typing `m`. In the wide display mode, buffers A, B (and C, when applicable) are displayed in a single frame that is as wide as the entire workstation screen. This is useful when files are compared side-by-side. By default, the display is widened without changing its height.
`ediff-use-last-dir`
  Controls the way Ediff presents the default directory when it prompts the user for files to compare. If `nil`, Ediff uses the default directory of the current buffer when it prompts the user for file names. Otherwise, it will use the directories it had previously used for files A, B, or C, respectively.
`ediff-no-emacs-help-in-control-buffer`
  If `t`, makes `C-h` behave like the `DEL` key, i.e., it will move you back to the previous difference rather than invoking help. This is useful when, in an xterm window or a text-only terminal, the Backspace key is bound to `C-h` and is positioned more conveniently than the `DEL` key.
`ediff-toggle-read-only-function`
  This variable's value is a function that Ediff uses to toggle the read-only property in its buffers. The default function that Ediff uses simply toggles the read-only property, unless the file is under version control. For a checked-in file under version control, Ediff first tries to check the file out.
`ediff-keep-variants`
  The default is `t`, meaning that the buffers being compared or merged will be preserved when Ediff quits. Setting this to `nil` causes Ediff to offer the user a chance to delete these buffers (if they are not modified). Supplying a prefix argument to the quit command (`q`) temporarily reverses the meaning of this variable. This is convenient when the user prefers one of the behaviors most of the time, but occasionally needs the other behavior. However, Ediff temporarily resets this variable to `t` if it is invoked via one of the "buffer" jobs, such as `ediff-buffers`. This is because it is all too easy to loose day's work otherwise. Besides, in a "buffer" job, the variant buffers have already been loaded prior to starting Ediff, so Ediff just preserves status quo here. Using `ediff-cleanup-hook`, one can make Ediff delete the variants unconditionally (e.g., by making `ediff-janitor` into one of these hooks).
`ediff-grab-mouse`
  Default is `t`. Normally, Ediff grabs mouse and puts it in its control frame. This is useful since the user can be sure that when he needs to type an Ediff command the focus will be in an appropriate Ediff's frame. However, some users prefer to move the mouse by themselves. The above variable, if set to `maybe`, will prevent Ediff from grabbing the mouse in many situations, usually after commands that may take more time than usual. In other situation, Ediff will continue grabbing the mouse and putting it where it believes is appropriate. If the value is `nil`, then mouse is entirely user's responsibility. Try different settings and see which one is for you.

## Notes on Heavy-duty Customization

Some users need to customize Ediff in rather sophisticated ways, which requires different defaults for different kinds of files (e.g., SGML, etc.). Ediff supports this kind of customization in several ways. First, most customization variables are buffer-local. Those that aren't are usually accessible from within Ediff Control Panel, so one can make them local to the panel by calling make-local-variable from within `ediff-startup-hook`.

Second, the function `ediff-setup` accepts an optional sixth argument which has the form `((var-name-1 . val-1) (var-name-2 . val-2) ...)`. The function `ediff-setup` sets the variables in the list to the respective values, locally in the Ediff control buffer. This is an easy way to throw in custom variables (which usually should be buffer-local) that can then be tested in various hooks.

Make sure the variable `ediff-job-name` and `ediff-word-mode` are set properly in this case, as some things in Ediff depend on this.

Finally, if you want custom-tailored help messages, you can set the variables `ediff-brief-help-message-function` and `ediff-long-help-message-function` to functions that return help strings.

When customizing Ediff, some other variables are useful, although they are not user-definable. They are local to the Ediff control buffer, so this buffer must be current when you access these variables. The control buffer is accessible via the variable `ediff-control-buffer`, which is also local to that buffer. It is usually used for checking if the current buffer is also the control buffer.

Other variables of interest are:

`ediff-buffer-A`
  The first of the data buffers being compared.
`ediff-buffer-B`
  The second of the data buffers being compared.
`ediff-buffer-C`
  In three-way comparisons, this is the third buffer being compared. In merging, this is the merge buffer. In two-way comparison, this variable is nil.
`ediff-window-A`
  The window displaying buffer A. If buffer A is not visible, this variable is nil or it may be a dead window.
`ediff-window-B`
  The window displaying buffer B.
`ediff-window-C`
  The window displaying buffer C, if any.
`ediff-control-frame`
  A dedicated frame displaying the control buffer, if it exists. It is non-nil only if Ediff uses the multiframe display, i.e., when the control buffer is in its own frame.

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