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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`::
If there is not a Git directory to work on, abort. 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.
`RUN_SETUP_GENTLY`::
If there is a Git directory, chdir as per RUN_SETUP, otherwise,
don't chdir anywhere.
`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.
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