Dear Marc,

Just realized there's a memory leak in many Gambit libraries, such as the sqlite FFI - their u8vector allocation in C boils down to:

___result = ___EXT(___alloc_scmobj)(___sU8VECTOR,len,___MOVABLE0);



Studying ##make-u8vector of _kernel.scm provides some inspiration: It

 1) Shows that it's fine to use ___STILL

 2) Uses  ___still_obj_refcount_dec ,

 3) Has its C code in a ##c-code and not a ##c-lambda

 4) Has its surrounding Scheme code in (##declare (not interrupts-enabled))

 5) Surrounds the ___alloc_scmobj call by some magic (___FRAME_STORE_RA(___R0)___W_ALL ___R_ALL ___SET_R0(___FRAME_FETCH_RA) )

(##make-u8vector provides a code path for small objects apart from this, though the use regarded here should be for u8vectors of nontrivial size so going with the codepath for large objects should always be fine.)


By applying 1) and 2) I get something that works but can't know if it's really correct and safe:


How do you allocate an u8vector in C code in a way that does not leak memory, and that safely transfers the new object reference from the C world to the Scheme world?



Thanks!
Mikael


How to reproduce
Running a test that repeats

((c-lambda () scheme-object "___result = ___EXT(___alloc_scmobj)(___sU8VECTOR,10000,___MOVABLE0);"))

over and over e.g.

(##gc-report-set! #t)
(for-each (lambda (x) ((c-lambda () scheme-object "___result = ___EXT(___alloc_scmobj)(___sU8VECTOR,10000,___MOVABLE0);")))
          (string->list (make-string 100000))
(##gc)

shows that this memory leaks all the allocated memory (!).


Fix, duno if safe & complete

(##gc-report-set! #t)
(for-each (lambda (x) ((c-lambda () scheme-object "___result = ___EXT(___alloc_scmobj)(___sU8VECTOR,10000,___STILL);
                                                   if (!___FIXNUMP(___result)) ___still_obj_refcount_dec (___result);
                                                   ")))
          (string->list (make-string 100000))
(##gc)


Copy of ##make-u8vector of _kernel.scm for reference

(define-prim (##make-u8vector k #!optional (fill (macro-absent-obj)))
  (##declare (not interrupts-enabled))
  (let ((v (##c-code #<<end-of-code

long i;
long n = ___INT(___ARG1);
long words = ___WORDS(n) + 1;
___SCMOBJ result;
if (n > ___CAST(___WORD, ___LMASK>>___LF))
  result = ___FIX(___HEAP_OVERFLOW_ERR); /* requested object is too big! */
else if (words > ___MSECTION_BIGGEST)
  {
    ___FRAME_STORE_RA(___R0)
    ___W_ALL
    result = ___alloc_scmobj (___sU8VECTOR, n, ___STILL);
    ___R_ALL
    ___SET_R0(___FRAME_FETCH_RA)
    if (!___FIXNUMP(result))
      ___still_obj_refcount_dec (result);
  }
else
  {
    ___BOOL overflow = 0;
    ___hp += words;
    if (___hp > ___ps->heap_limit)
      {
        ___FRAME_STORE_RA(___R0)
        ___W_ALL
        overflow = ___heap_limit () && ___garbage_collect (0);
        ___R_ALL
        ___SET_R0(___FRAME_FETCH_RA)
      }
    else
      ___hp -= words;
    if (overflow)
      result = ___FIX(___HEAP_OVERFLOW_ERR);
    else
      {
        result = ___TAG(___hp, ___tSUBTYPED);
        ___HEADER(result) = ___MAKE_HD_BYTES(n, ___sU8VECTOR);
        ___hp += words;
      }
  }
if (!___FIXNUMP(result) && ___ARG2 != ___ABSENT)
  {
    for (i=0; i<n; i++)
      ___U8VECTORSET(result,___FIX(i),___ARG2)
  }
___RESULT = result;

end-of-code

            k
            fill)))
    (if (##fixnum? v)
      (begin
        (##raise-heap-overflow-exception)
        (##make-u8vector k fill))
      v)))