please resolve a mystery for me: what is j-c-diff exactly? ;)

please resolve a mystery for me: what is j-c-diff exactly? ;)

[Yaroslav Halchenko]

Hi Junio et al,

j-c-diff is "used" in the docs within git, git-annex, and other places
discussing git.  But I failed to find it to seek for an ultimate prototypical
example of the diff command used by git ppl ;)

$> git log -Sj-c-diff -p Documentation/gitattributes.txt 
commit 2cc3167c688d1c91bc4cb9b1caa737b9d4971056
Author: Junio C Hamano <junkio@cox.net>
Date:   Mon Apr 23 00:21:02 2007 -0700

    Document "diff=driver" attribute
    
    Signed-off-by: Junio C Hamano <junkio@cox.net>

diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index 126871756d..d2edb9b14a 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -151,8 +151,34 @@ Unspecified::
        text, it is treated as text.  Otherwise it would
        generate `Binary files differ`.
 
-Any other value set to `diff` attribute is ignored and git acts
-as if the attribute is left unspecified.
+String::
+
+ Diff is shown using the specified custom diff driver.
+ The driver program is given its input using the same
+ calling convention as used for GIT_EXTERNAL_DIFF
+ program.
+
+
+Defining a custom diff driver
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The definition of a diff driver is done in `gitconfig`, not
+`gitattributes` file, so strictly speaking this manual page is a
+wrong place to talk about it.  However...
+
+To define a custom diff driver `jcdiff`, add a section to your
+`$GIT_DIR/config` file (or `$HOME/.gitconfig` file) like this:
+
+----------------------------------------------------------------
+[diff "jcdiff"]
+ command = j-c-diff
+----------------------------------------------------------------
+
+When git needs to show you a diff for the path with `diff`
+attribute set to `jcdiff`, it calls the command you specified
+with the above configuration, i.e. `j-c-diff`, with 7
+parameters, just like `GIT_EXTERNAL_DIFF` program is called.
+See gitlink:git[7] for details.

Cheers!
-- 
Yaroslav O. Halchenko
Center for Open Neuroscience     http://centerforopenneuroscience.org
Dartmouth College, 419 Moore Hall, Hinman Box 6207, Hanover, NH 03755
Phone: +1 (603) 646-9834                       Fax: +1 (603) 646-1419
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