How do you define constants in an emacs dynamic module?

Question

I am trying to write a dynamic module to expose some constants in the GNU Scientific Library. I thought something like this would do it.

#include <gsl/gsl_const_mksa.h>
#include "emacs-module.h"

int plugin_is_GPL_compatible;

int emacs_module_init(struct emacs_runtime *ert)
{
  emacs_env *env = ert->get_environment(ert);

  emacs_value defconst = env->intern(env, "defconst");

  char doc[] = "Speed of light in vacuum (m/s).";
  emacs_value args[] = {
    env->intern(env, "GSL-CONST-MKSA-SPEED-OF-LIGHT"),
    env->make_float(env, GSL_CONST_MKSA_SPEED_OF_LIGHT),
    env->make_string(env, doc, sizeof doc)
    };

  env->funcall(env, defconst, 3, args);

  // This is what allows the shared library to provide a feature 
  emacs_value provide = env->intern(env, "provide");
  emacs_value provide_args[] = { env->intern(env, "gsl-constants") };
  env->funcall(env, provide, 1, provide_args);

  return 0;
}

This compiles fine with

gcc -Wall -I/usr/local/include -fPIC -c gsl-constants.c
gcc -shared -L/usr/local/include -lgsl -o gsl-constants.so gsl-constants.o

But something is not working because I get "Required feature ‘gsl-constants’ was not provided" if I try to (require 'gsl-constants).

I guess that something is wrong with the defconst part because if I comment that out, it requires fine. Does anyone know if there is another way to define constants or variables in a dynamic module?

My C isn't great, but are you sure that `sizeof doc` is what you want? Maybe you wanted the length of the string? i.e. `strlen`. Also, it's probably better to add a few `const`s when you work with literal strings. — wvxvw, Jul 09 '17 at 15:05
I might want strlen, but it also doesn't work if I just leave the third argument out, so I don't think it is related to the string. — John Kitchin, Jul 09 '17 at 15:06

score 4 · Answer 1 · answered Jul 09 '17 at 22:55

Here is what I finally got to work. @politza pointed out that you cannot use funcall on special forms (eg. macros), and apply does not work either. @politza also pointed out that you can build up a list of symbols and then eval it. That is what the code below does. Thanks for the help!

#include <gsl/gsl_const_mksa.h>
#include <string.h>
#include "emacs-module.h"

int plugin_is_GPL_compatible;

int emacs_module_init(struct emacs_runtime *ert)
{
  emacs_env *env = ert->get_environment(ert);

  emacs_value eval = env->intern(env, "eval");  
  emacs_value list = env->intern(env, "list");
  emacs_value defconst = env->intern(env, "defconst");

  emacs_value sym = env->intern(env, "GSL-CONST-MKSA-SPEED-OF-LIGHT");
  emacs_value val = env->make_float(env, GSL_CONST_MKSA_SPEED_OF_LIGHT);

  char doc[] = "Speed of light in vacuum (m/s).";
  emacs_value sdoc = env->make_string(env, doc, strlen(doc));

  emacs_value largs[] = {defconst, sym, val, sdoc};
  emacs_value qlist = env->funcall(env, list, 4, &largs);   
  emacs_value args[] = { qlist };

  env->funcall(env, eval, 1, &args);

  // This is what allows the shared library to provide a feature 
  emacs_value provide = env->intern(env, "provide");
  emacs_value provide_args[] = { env->intern(env, "gsl-constants") };
  env->funcall(env, provide, 1, provide_args);

  return 0;
}

Once compiled, you use it like this:

#+BEGIN_SRC emacs-lisp
(add-to-list 'load-path "/Users/jkitchin/vc/blogofile-jkitchin.github.com/_blog/dynamic-module/")
(require 'gsl-constants)
GSL-CONST-MKSA-SPEED-OF-LIGHT
#+END_SRC

#+RESULTS:
: 299792458.0

#+BEGIN_SRC emacs-lisp
(describe-variable 'GSL-CONST-MKSA-SPEED-OF-LIGHT)
#+END_SRC

#+RESULTS:
: GSL-CONST-MKSA-SPEED-OF-LIGHT’s value is 299792458.0
: 
:   This variable may be risky if used as a file-local variable.
: 
: Documentation:
: Speed of light in vacuum (m/s).

score 1 · Accepted Answer · answered Jul 09 '17 at 19:08

1

Apparently you can not funcall special forms. At least not the ones having unevaluated arguments, with defconst beeing one of them.

(funcall 'defconst 'foo 42 "Hey")
 => Debugger entered--Lisp error: (invalid-function defconst)

So the env->funcall(...) signals an error, which goes by unnoticed since you don't inspect its return value. Also, it seems that the following calls into the module API are silently ignored in this scenario.

answered Jul 09 '17 at 19:08

politza

3,316
14
16

That is a good tip. It looks like there is no way to do it this way. You can (funcall 'set 'sym val) but quote is a special form too, so I can't quote it in the C code. Also emacs segfaults when I try that! Thanks for pointing this out. – John Kitchin Jul 09 '17 at 21:26
I think you can build a list `(defconst ...)` and then call `eval` with it. – politza Jul 09 '17 at 21:52
That is genius and works! for the record I will post what worked below. – John Kitchin Jul 09 '17 at 22:50
I don't think need `quote` with `set`, since `env->funcall` does not (re)evaluate the arguments. – politza Jul 10 '17 at 00:07
I think I tried it both ways and it segfaulted both times IIRC. – John Kitchin Jul 10 '17 at 00:12
That should'nt happen in any case. – politza Jul 10 '17 at 00:16
Maybe not, but I also wanted to have docstrings on the constants so you know what they are, so the eval/defconst idea was a good one. – John Kitchin Jul 10 '17 at 00:25

How do you define constants in an emacs dynamic module?

2 Answers2