[ikarus-users] vector objects internals: how is the length stored

[Marco Maggi]

Ciao,

  while it is clear that, at  the C level, to allocate a new
bytevector one has to do:

ikpcb     pcb;
long int  size;
ikptr     bv;
uint8_t * data_area;

pcb  = ...;
size = ...;
bv   = ik_safe_alloc(pcb, align(disp_bytevector_data+size))
       + bytevector_tag;

ref(bv, off_bytevector_length) = fix(size);
data_area = bv + off_bytevector_data;

where "size" is the number  of bytes in the data area stored
in the bytevector memory block as a fixnum ("fix(size)"), it
is not  clear to  me how  the size is  stored in  the vector
objects.

  For  example this  function  exists (and  it  seems to  be
used):

ikptr 
ikrt_make_vector1(ikptr len, ikpcb* pcb){
  int intlen = (int)len;
  if(is_fixnum(len) && (intlen >= 0)){
    ikptr s = ik_safe_alloc(pcb, align(len + disp_vector_data));
    ref(s, 0) = len;
    memset((char*)(long)(s+disp_vector_data), 0, len);
    return s+vector_tag;
  } else {
    return 0;
  }
}
  
here, first  "len" is tested with "fixnum()"  which seems to
indicate that "len"  is an "ikptr" value with  with a fixnum
tag  (2 least  significant  bits  set to  zero,  on 32  bits
platforms); but then the number  of bytes to allocate on the
heap (before alignment normalisation) is computed as:

   len + disp_vector_data

where "len"  is used raw, without  "unfix(len)"; finally the
length is stored in the vector memory block: 

  ref(s, 0) = len;

  I still  have not reverse  engineered in my mind  the way,
for example, VECTOR-LENGTH is  implemented, so I only have a
partial understanding of how  things go; has someone already
understood this?

TIA
-- 
Marco Maggi