From: Charles Mills <cmills@...>
Date: 2005-05-17T08:15:30+09:00
Subject: Re: rb_gc_mark question


Ara.T.Howard wrote:
> i reading some code attm that goes:
>
>    static VALUE convertgslmatrixToRubyMatrix(gsl_matrix *dm)
>    {
>      int i, j;
>      volatile VALUE result;
>      assert(dm);
>    //  printf("Trying to convert dm %p\n", dm);
>    //    printf("With size %d, %d\n", dm->size1, dm->size2);
>      volatile VALUE rows = rb_ary_new();
>      rb_gc_mark(rows);
>      for (i = 0; i < dm->size1; i += 1) {
>        volatile VALUE currow = rb_ary_new();
>        rb_gc_mark(currow);
>        rb_ary_push(rows, currow);
>        for (j = 0; j < dm->size2; j += 1) {
>          double val = gsl_matrix_get(dm, i, j);
>          VALUE rval = rb_float_new(val);
>          rb_gc_mark(rval);
>          rb_ary_push(currow, rval);
>        }
>      }
>      result = rb_funcall(cMatrix, rb_intern("rows"), 2, rows, Qnil);
>      rb_gc_mark(result);
>      return result;
>    }
>
>
> now, everything i know says this should be something along the lines
of:
>
>    static VALUE
>    convertgslmatrixToRubyMatrix (gsl_matrix * dm)
>    {
>      int i, j;
>      VALUE result;
>      VALUE rows = rb_ary_new ();
>      for (i = 0; i < dm->size1; i += 1)
>        {
>          VALUE currow = rb_ary_new ();
>          rb_gc_mark (currow);
>          rb_ary_push (rows, currow);
>          for (j = 0; j < dm->size2; j += 1)
>            {
>              double val = gsl_matrix_get (dm, i, j);
>              VALUE rval = rb_float_new (val);
>              rb_ary_push (currow, rval);
>            }
>        }
>      result = rb_funcall (cMatrix, rb_intern ("rows"), 2, rows,
Qnil);
>      return result;
>    }
>
> (please correct me if i'm wrong)
>
> or nasty things will happen in memory right?  what would it do to
call
> rb_gc_mark on ruby objects like this?
>

You shoud just not call rb_gc_mark at all.  Not sure if it even
matters, but it is possible that objects you mark using rb_gc_mark
outside of a mark and sweep cycle will hang around AFTER some of there
fields have been GCed - like an array whose elements have been GCed,
but is still -alive-.  This will only happen if the stack does not
overflow when marking and the only way this could be a problem is by
using the object space API.

Anyway, hopefully whoever wrote this code hopefully wasn't modivated by
my overly paranoid/incorrect past posts about the Ruby GC.

Also you could use rb_ary_new2 to save ruby from having to resize the
array since you know the size ahead of time.

-Charlie