# builtin API

## Adding a new built-in

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

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

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

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

3. 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.

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

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

1. Add tests to `t/` directory.

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

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

4. 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.