builtin API
===========

Adding a new built-in
---------------------

There are 4 things to do to add a built-in command implementation to
Git:

. Define the implementation of the built-in command `foo` with
  signature:

        int cmd_foo(int argc, const char **argv, const char *prefix);

. Add the external declaration for the function to `builtin.h`.

. Add the command to the `commands[]` table defined in `git.c`.
  The entry should look like:

        { "foo", cmd_foo, <options> },
+
where options is the bitwise-or of:

`RUN_SETUP`::

        Make sure there is a Git directory to work on, and if there is a
        work tree, chdir to the top of it if the command was invoked
        in a subdirectory.  If there is no work tree, no chdir() is
        done.

`USE_PAGER`::

        If the standard output is connected to a tty, spawn a pager and
        feed our output to it.

`NEED_WORK_TREE`::

        Make sure there is a work tree, i.e. the command cannot act
        on bare repositories.
        This only makes sense when `RUN_SETUP` is also set.

. Add `builtin/foo.o` to `BUILTIN_OBJS` in `Makefile`.

Additionally, if `foo` is a new command, there are 3 more things to do:

. Add tests to `t/` directory.

. Write documentation in `Documentation/git-foo.txt`.

. Add an entry for `git-foo` to `command-list.txt`.

. Add an entry for `/git-foo` to `.gitignore`.


How a built-in is called
------------------------

The implementation `cmd_foo()` takes three parameters, `argc`, `argv,
and `prefix`.  The first two are similar to what `main()` of a
standalone command would be called with.

When `RUN_SETUP` is specified in the `commands[]` table, and when you
were started from a subdirectory of the work tree, `cmd_foo()` is called
after chdir(2) to the top of the work tree, and `prefix` gets the path
to the subdirectory the command started from.  This allows you to
convert a user-supplied pathname (typically relative to that directory)
to a pathname relative to the top of the work tree.

The return value from `cmd_foo()` becomes the exit status of the
command.