Accessing Lisp variables from C

Accessing Lisp variables from C

[Oliver Scholz]

I decided to make a stab at porting `decode-char' to C. The better
part of it was indeed fairly trivial, but it turned out that I
obviously don't understand something very fundamental about the
interpreter. `decode-char' checks whether `utf-translate-cjk-mode' is
non-nil; if it is, it calls `utf-lookup-subst-table-for-decode'. This
one gets a translation hash table from the property
`translation-hash-table' on the symbol `utf-subst-table-for-decode'.
But before that, it checks whether the translation hash tables are
loaded, and calls `utf-translate-cjk-load-tables'.

I had the impression that in C this would look like this:


int
ucs_to_internal (ucs)
     int ucs;
{
  int c = 0;
  Lisp_Object tmp_char;

  if (! EQ (Qnil, SYMBOL_VALUE (intern ("utf-translate-cjk-mode"))))
    /* cf. `utf-lookup-subst-table-for-decode' */
    {
      if (EQ (Qnil, SYMBOL_VALUE (intern ("utf-translate-cjk-lang-env"))))
        call0 (intern ("utf-translate-cjk-load-tables"));
      tmp_char = Fgethash (ucs,
                           Fget (intern ("utf-subst-table-for-decode"),
                                 intern ("translation-hash-table")),
                           Qnil);
      if (! EQ (Qnil, tmp_char))
        {
          CHECK_NUMBER (tmp_char);
          c = XFASTINT (tmp_char);
        }
    }
[...]


But, oddly, this is not equivalent to what is supposed to be its Lisp
counterpart. `(utf-lookup-subst-table-for-decode #x9662)' returns a
nice Japanese character. This (alleged) C equivalent doesn't.

Besides, debugging is rather painful, since gdb insists on stepping
into `intern' and then into `oblookup' and then into `hash_string'
etc. I wonder what I am doing wrong here. The behaviour when stepping
is rather erratic, because for instance the gdb command "return"
causes a segfault when in `intern', but not when I step through it.
(The garbage collector is "mark and sweep", isn't it? Besides garbage
collection is triggered in the above only by `call0', right?)


    Oliver
-- 
15 Floréal an 214 de la Révolution
Liberté, Egalité, Fraternité!

Re: Accessing Lisp variables from C

[Oliver Scholz]

Why is it always that you find the solution to your problem right
after you wrote about it?

Oliver Scholz <alkibiades@gmx.de> writes:
[...]

Silly me, I need `make_number' in order to use a C int as the first
argument to Fgethash.

Still I wonder about the behaviour of gdb ...


    Oliver
-- 
15 Floréal an 214 de la Révolution
Liberté, Egalité, Fraternité!