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 am trying to write a dynamic module.

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.

Now, of course, I could 'fix it' by creating a lisp function that always
passes an argument, but I wonder if this is the 'only' solution. Does the
module API provide a way to check for the 'optional' argument?

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