Am 04.11.22 um 14:22 schrieb Ævar Arnfjörð Bjarmason:
> When the OPT_SUBCOMMAND() API was implemented in [1] it did so by
> adding a new "subcommand_fn" member to "struct option", rather than
> allowing the user of the API to pick the type of the function.
>
> An advantage of mandating that "parse_opt_subcommand_fn" must be used
> is that we'll get type checking for the function we're passing in, a
> disadvantage is that we can't convert e.g. "builtin/bisect--helper.c"
> easily to it, as its callbacks need their own argument.
>
> Let's generalize this interface, while leaving in place a small hack
> to give the existing API users their type safety. We assign to
> "typecheck_subcommand_fn", but don't subsequently use it for
> anything. Instead we use the "defval" and "value" members.
>
> A subsequent commit will add a OPT_SUBCOMMAND() variant where the
> "callback" isn't our default "parse_options_pick_subcommand" (and that
> caller won't be able to use the type checking).
>
> 1. fa83cc834da (parse-options: add support for parsing subcommands,
> 2022-08-19)
>
> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>
> ---
> parse-options.c | 9 ++++++---
> parse-options.h | 25 +++++++++++++++++++++----
> 2 files changed, 27 insertions(+), 7 deletions(-)
>
> diff --git a/parse-options.c b/parse-options.c
> index a1ec932f0f9..1d9e46c9dc7 100644
> --- a/parse-options.c
> +++ b/parse-options.c
> @@ -427,7 +427,8 @@ static enum parse_opt_result parse_subcommand(const char *arg,
> for (; options->type != OPTION_END; options++)
> if (options->type == OPTION_SUBCOMMAND &&
> !strcmp(options->long_name, arg)) {
> - *(parse_opt_subcommand_fn **)options->value = options->subcommand_fn;
> + if (options->callback(options, arg, 0))
> + BUG("OPT_SUBCOMMAND callback returning non-zero");
> return PARSE_OPT_SUBCOMMAND;
> }
>
> @@ -506,8 +507,10 @@ static void parse_options_check(const struct option *opts)
> "That case is not supported yet.");
> break;
> case OPTION_SUBCOMMAND:
> - if (!opts->value || !opts->subcommand_fn)
> - optbug(opts, "OPTION_SUBCOMMAND needs a value and a subcommand function");
> + if (!opts->value || !opts->callback)
> + optbug(opts, "OPTION_SUBCOMMAND needs a value and a callback function");
> + if (opts->ll_callback)
> + optbug(opts, "OPTION_SUBCOMMAND uses callback, not ll_callback");
> if (!subcommand_value)
> subcommand_value = opts->value;
> else if (subcommand_value != opts->value)
> diff --git a/parse-options.h b/parse-options.h
> index b6ef86e0d15..61e3016c3fc 100644
> --- a/parse-options.h
> +++ b/parse-options.h
> @@ -128,19 +128,24 @@ typedef int parse_opt_subcommand_fn(int argc, const char **argv,
> * the option takes optional argument.
> *
> * `callback`::
> - * pointer to the callback to use for OPTION_CALLBACK
> + * pointer to the callback to use for OPTION_CALLBACK and OPTION_SUBCOMMAND.
> *
> * `defval`::
> * default value to fill (*->value) with for PARSE_OPT_OPTARG.
> * OPTION_{BIT,SET_INT} store the {mask,integer} to put in the value when met.
> + * OPTION_SUBCOMMAND stores the pointer the function selected for
> + * the subcommand.
> + *
> * CALLBACKS can use it like they want.
> *
> * `ll_callback`::
> * pointer to the callback to use for OPTION_LOWLEVEL_CALLBACK
> *
> * `subcommand_fn`::
> - * pointer to a function to use for OPTION_SUBCOMMAND.
> - * It will be put in value when the subcommand is given on the command line.
> + * pointer to the callback used with OPT_SUBCOMMAND() and
> + * OPT_SUBCOMMAND_F(). Internally we store the same value in
> + * `defval`. This is only here to give the OPT_SUBCOMMAND{,_F}()
> + * common case type safety.
> */
> struct option {
> enum parse_opt_type type;
> @@ -217,12 +222,24 @@ struct option {
> #define OPT_ALIAS(s, l, source_long_name) \
> { OPTION_ALIAS, (s), (l), (source_long_name) }
>
> +static inline int parse_options_pick_subcommand_cb(const struct option *option,
> + const char *arg UNUSED,
> + int unset UNUSED)
> +{
> + parse_opt_subcommand_fn *fn = (parse_opt_subcommand_fn *)option->defval;
> + *(parse_opt_subcommand_fn **)option->value = fn;
->defval is of type intptr_t and ->value is a void pointer. The result
of converting a void pointer value to an intptr_t and back is a void
pointer equal to the original pointer if I read 6.3.2.3 (Pointers,
paragraphs 5 and 6) and 7.18.1.4 (Integer types capable of holding
object pointers) in C99 correctly.
6.3.2.3 paragraph 8 says that casting between function pointers of
different type is OK and you can get your original function pointer
back and use it in a call if you convert it back to the right type.
Casting between a function pointer and an object pointer is undefined,
though. They don't have to be of the same size, so a function pointer
doesn't have to fit into an intptr_t. I wouldn't be surprised if CHERI
(https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/) was an actual
example of that.
Why is this trickery needed? Above you write that callbacks in
builtin/bisect--helper.c can't use subcommand_fn because they need
their own argument. Can we extend subcommand_fn or use a global
variable to pass that extra thing instead? The latter may be ugly, but
at least it's valid C..
> + return 0;
> +}
> +
> #define OPT_SUBCOMMAND_F(l, v, fn, f) { \
> .type = OPTION_SUBCOMMAND, \
> .long_name = (l), \
> .value = (v), \
> .flags = (f), \
> - .subcommand_fn = (fn) }
> + .defval = (intptr_t)(fn), \
> + .subcommand_fn = (fn), \
> + .callback = parse_options_pick_subcommand_cb, \
Getting the address of an inline function feels weird, but the compiler
is free to emit to ignore that keyword and will provide an addressable
function object here.
> +}
> #define OPT_SUBCOMMAND(l, v, fn) OPT_SUBCOMMAND_F((l), (v), (fn), 0)
>
> /*
