Re: What is the recommended way to find out the number of arguments passed to a module function?

[dalanicolai <dalanicolai@gmail.com>]

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(I should reply to all :)

Ah okay, well I am completely new to C, so I am still finding out things
(e.g. I have not 'actively' used ptrdiff yet).
But indeed the 'nargs' just give the number of passed args (obviously).

I had checked the 'sizeof' the args array when passing an argument and
without passing an argument,
and it was the same. Then the docs
<https://www.gnu.org/software/emacs/manual/html_node/elisp/Module-Functions.html>
mention that NARGS is the 'required' number of arguments,
I had misunderstood it a little (of course the required number is variable,
but somehow I assumed it
would be 1 in this case). So sometimes I overlook the 'obvious', and
because I had spent some time on testing
things already, I decided to just ask here.

Anyway, as usual, thanks for your quick and helpful answer!

On Wed, 10 Jan 2024 at 20:16, Eli Zaretskii <eliz@gnu.org> wrote:

> > From: dalanicolai <dalanicolai@gmail.com>
> > Date: Wed, 10 Jan 2024 20:01:09 +0100
> >
> > In the module API's 'make-function' we should pass a min-arity and a
> max-arity.
> > However, it is unclear to me what is the recommended way to check for
> the number of
> > arguments passed to some module function, as when not passing any
> argument, the 'optional'
> > argument does not seem
> > to be nil, or any emacs-value at all (I have tested if it might be a
> NULL pointer). I have tested it using a
> > 'test-module' with the following code:
> >
> >  #include <emacs-module.h>
> >  int plugin_is_GPL_compatible;
> >  static emacs_value
> >  test (emacs_env *env, ptrdiff_t nargs, emacs_value *args, void *data)
> >  {
> >    int integer = env->is_not_nil(env, args[0])? 1 : 0;
> >    return env->make_integer(env, integer);
> >    /* return args[0]; */
> >  }
> >  int
> >  emacs_module_init (struct emacs_runtime *runtime)
> >  {
> >    emacs_env *env = runtime->get_environment (runtime);
> >    emacs_value func = env->make_function (env, 0, 1, test, NULL, NULL);
> >    emacs_value symbol = env->intern (env, "test");
> >    emacs_value args[] = {symbol, func};
> >    env->funcall (env, env->intern (env, "defalias"), 2, args);
> >    return 0;
> >  }
> >
> > The 'test' function checks if the value of the argument is non-nil, and
> 'returns' a 1 if it is and a 0
> > otherwise. It works fine when passing an argument, e.g. t or nil, but
> Emacs crashes when I don't pass
> > an argument. Also, I tried to simply return the value (by replacing the
> return line with the line in the
> > comment below it), which returns the value successfully when I pass an
> argument, but again Emacs
> > crashes when I don't pass any argument.
>
> I guess I'm confused: if you call your function with zero arguments,
> then why do you expect to find anything useful in the args[] array, or
> even assume that the args[] array can be accessed?
>
> Don't you get nargs = 0 in 'test' in this case?
>

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