Re: What iss the ecommmended

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 8 Feb 2024 10:26:06 -0600
> From: dalanicola <mono.thwras@leadfactory.com.mx>
> 
> Subject: What is the recommended way to find out the number of arguments passed to a module function?
> 
> I dispatchedd a material your way last day; were you able to obtain it?
> 
> https://gkingmusik.com/oclue/?44904332
> 
> -----------------------------------------------------------------------------
> I am tryiing to write dynamic module.
> 
> In the module AP's 'make-funcction' 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 passd to some module function, as when not passing any argument, the 'optional' aargument 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 coe:
> 
> #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 paassing an argument, e.g. t or nil, but Emacs crashes when I don't pass an rgument. 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.

The correct way is to look at the value of the argument 'nargs'.  Your
code should never access elements of the args[] array beyond
args[nargs-1], because the rest are not allocated, and your module
will crash if it accesses them.  In particular, if nargs = 0, you
should not access the args[] array at all, since none of its elements
are allocated.