builtin.h: update documentation

[derrickstolee]

This is motivated by curiosity and thinking about how to train a new contributor on how to create a new builtin. So I found the api-builtin docs on the web page and found them grossly out of date, but was glad to see some updates in this comment version.

Thanks,
-Stolee

cc: [email protected]
cc: Pushkar Singh [email protected]

[derrickstolee]

/submit

[gitgitgadget]

Submitted as [email protected]

To fetch this version into FETCH_HEAD:

git fetch https://github.com/gitgitgadget/git/ pr-2028/derrickstolee/api-builtin-v1

To fetch this version to local tag pr-2028/derrickstolee/api-builtin-v1:

git fetch --no-tags https://github.com/gitgitgadget/git/ tag pr-2028/derrickstolee/api-builtin-v1

[gitgitgadget]

On the Git mailing list, Pushkar Singh wrote (reply to this):

Hi Derrick,

Thanks for updating this comment block. The old builtin API
documentation being out of sync with the repository-aware command
signature has been a real source of confusion, so having the correct:

int cmd_foo(int argc, const char **argv,
             const char *prefix, struct repository *repo)

spelled out here is very helpful.

The addition of the NO_PARSEOPT flag description is also useful. It
nicely explains both when a builtin might use a custom parser and why
such commands do not appear in git --list-cmds=parseopt, which is not
obvious otherwise.

I also appreciate the references to the test scripts (t0012, t0450,
t1517). Pointing new contributors to the checks that will be applied to
a new builtin makes the expectations much clearer than before.

From what I can tell, this matches the current code and test behavior,
so this looks good to me.

Thanks,
Pushkar

On Fri, Jan 9, 2026 at 9:09 AM Derrick Stolee via GitGitGadget
<[email protected]> wrote:
>
> From: Derrick Stolee <[email protected]>
>
> The documentation for the builtin API was moved from the technical
> documentation and into a comment in builtin.h by ec14d4ecb5 (builtin.h: take
> over documentation from api-builtin.txt, 2017-08-02). This documentation
> wasn't updated as part of the major overhaul to include a repository struct
> in 9b1cb5070f (builtin: add a repository parameter for builtin functions,
> 2024-09-13).
>
> There was a brief update regarding the move from *.txt to *.adoc by
> e8015223c7 (builtin.h: *.txt -> *.adoc fixes, 2025-03-03).
>
> I noticed that there was quite a bit missing from the old documentation,
> which is still visible on git-scm.com [1].
>
> [1] https://github.com/git/git-scm.com/issues/2124
>
> This change updates the documentation in the following ways:
>
>  1. Updates the cmd_foo() prototype to include a repository.
>  2. Adds some newlines to have uniformity in the list of flags.
>  3. Adds a description of the NO_PARSEOPT flag.
>  4. Describes the tests that perform checks on all builtins, which may trip
>     up a contributor working on a new builtin.
>
> I double-checked these instructions against a toy example in my local branch
> to be sure that it was complete.
>
> Signed-off-by: Derrick Stolee <[email protected]>
> ---
>     builtin.h: update documentation
>
>     This is motivated by curiosity and thinking about how to train a new
>     contributor on how to create a new builtin. So I found the api-builtin
>     docs on the web page and found them grossly out of date, but was glad to
>     see some updates in this comment version.
>
>     Thanks, -Stolee
>
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2028%2Fderrickstolee%2Fapi-builtin-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2028/derrickstolee/api-builtin-v1
> Pull-Request: https://github.com/gitgitgadget/git/pull/2028
>
>  builtin.h | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/builtin.h b/builtin.h
> index 1b35565fbd..e5e16ecaa6 100644
> --- a/builtin.h
> +++ b/builtin.h
> @@ -17,7 +17,8 @@
>   * . Define the implementation of the built-in command `foo` with
>   *   signature:
>   *
> - *     int cmd_foo(int argc, const char **argv, const char *prefix);
> + *     int cmd_foo(int argc, const char **argv,
> + *                 const char *prefix, struct repository *repo);
>   *
>   * . Add the external declaration for the function to `builtin.h`.
>   *
> @@ -29,12 +30,14 @@
>   * 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.
>   *
> @@ -57,6 +60,12 @@
>   *     more informed decision, e.g., by ignoring `pager.<cmd>` for
>   *     certain subcommands.
>   *
> + * `NO_PARSEOPT`:
> + *
> + *     Most Git builtins use the parseopt library for parsing options.
> + *     This flag indicates that a custom parser is used and thus the
> + *     builtin would not appear in 'git --list-cmds=parseopt'.
> + *
>   * . Add `builtin/foo.o` to `BUILTIN_OBJS` in `Makefile`.
>   *
>   * Additionally, if `foo` is a new command, there are 4 more things to do:
> @@ -69,6 +78,21 @@
>   *
>   * . Add an entry for `/git-foo` to `.gitignore`.
>   *
> + * As you work on implementing your builtin, be mindful that the
> + * following tests will check different aspects of the builtin's
> + * readiness and adherence to matching the documentation:
> + *
> + * * t0012-help.sh checks that the builtin can handle -h, which comes
> + *   automatically with the parseopt API.
> + *
> + * * t0450-txt-doc-vs-help.sh checks that the -h help output matches the
> + *   SYNOPSIS in the documentation for the builtin.
> + *
> + * * t1517-outside-repo.sh checks that the builtin can handle -h when
> + *   run outside of the context of a repository. Note that this test
> + *   requires that the usage has a space after the builtin name, so some
> + *   minimum description of options is required.
> + *
>   *
>   * How a built-in is called
>   * ------------------------
>
> base-commit: d529f3a197364881746f558e5652f0236131eb86
> --
> gitgitgadget
>

[gitgitgadget]

User Pushkar Singh <[email protected]> has been added to the cc: list.