[Chicken-users] define-external and abort

[Caolan McMahon]

I have been doing some work on the LMDB bindings
(https://github.com/iraikov/chicken-lmdb) and submitted a patch for it's
error-handling. Ivan and I are not 100% sure what I've done is safe, so
I'd appreciate your expertise.

Normally, I have my C bindings return an error code and then in scheme I would 
create a condition and call (abort ...). The LMDB binding, however, does more 
work in C.

Currently, the LMDB error handling looks like this:

```
static void chicken_ThrowException(C_word value) C_noret;
static void chicken_ThrowException(C_word value)
{
  char *aborthook = C_text("\003sysabort");

  C_word *a = C_alloc(C_SIZEOF_STRING(strlen(aborthook)));
  C_word abort = C_intern2(&a, aborthook);

  abort = C_block_item(abort, 0);
  if (C_immediatep(abort))
    Chicken_Panic(C_text("`##sys#abort' is not defined"));

#if defined(C_BINARY_VERSION) && (C_BINARY_VERSION >= 8)
  C_word rval[3] = { abort, C_SCHEME_UNDEFINED, value };
  C_do_apply(3, rval);
#else
  C_save(value);
  C_do_apply(1, abort, C_SCHEME_UNDEFINED);
#endif
}

void chicken_lmdb_exception (int code, int msglen, const char *msg) 
{
  C_word *a;
  C_word scmmsg;
  C_word list;

  a = C_alloc (C_SIZEOF_STRING (msglen) + C_SIZEOF_LIST(2));
  scmmsg = C_string2 (&a, (char *) msg);
  list = C_list(&a, 2, C_fix(code), scmmsg);
  chicken_ThrowException(list);
}
```

Where error codes from LMDB functions are passed to `chicken_lmdb_exception` 
from C.

I wanted to update this to create a composite condition that could be used with 
condition-case and friends. I replaced the two functions above with a 
define-external:

```
(define-external (chicken_lmdb_exception (int code)) void
  (abort
   (make-composite-condition
    (make-property-condition 'exn 'message (error-code-string code))
    (make-property-condition 'lmdb)
    (make-property-condition (error-code-symbol code)))))
```

The rest of the C code still calls `chicken_lmdb_exception` as before.

My question: Is this functionally equivalent to the original C
implementation in terms of calling abort?


-- 
Caolan