Re: [Bug-guile-ncurses] [PATCH 1/2] Add reference count to ITEM structs contained by SCMs

[John Darrington]

This patch (partially) fixes a bug where a segmentation violation in the 
guile library would occur after, for example the current-item had been called.

The problem was, the native C item struct is shared between many SCM smobs.
Thus, if the garbage collector is allowed to run on any one of them, it'll
free memory which will later be needed by another.

This patch fixes the problem by using the "userptr" member as a reference count.

On Tue, May 03, 2016 at 01:22:34PM +0200, John Darrington wrote:
     * ncurses/menu_type.c (_scm_from_item, new_item, free_item): Add reference 
counting.
     ---
      ncurses/menu_type.c | 11 ++++++++++-
      1 file changed, 10 insertions(+), 1 deletion(-)
     
     diff --git a/ncurses/menu_type.c b/ncurses/menu_type.c
     index 679e9c1..712cd35 100644
     --- a/ncurses/menu_type.c
     +++ b/ncurses/menu_type.c
     @@ -88,6 +88,8 @@ gucu_new_item (SCM name, SCM description)
        abort ();
          }
      
     +  /* Use the userptr member as a reference count */
     +  set_item_userptr (c_item, 0);
        SCM ret = _scm_from_item (c_item);
      
        return ret;
     @@ -117,6 +119,9 @@ _scm_from_item (ITEM * x)
      
        assert (x == (ITEM *) SCM_SMOB_DATA (s_item));
      
     +  intptr_t refcnt = (intptr_t) item_userptr (x);
     +  set_item_userptr (x, (void *) (++refcnt));
     +  
        if (0)
          {
            fprintf (stderr, "Making <#item> smob from ITEM * %p\n", (void *) 
x);
     @@ -157,7 +162,11 @@ gc_free_item (SCM item)
      
        assert (m != NULL);
      
     -  free_item (m);
     +  intptr_t refcnt = (intptr_t) item_userptr (m);
     +  set_item_userptr (m, (void *) (--refcnt));
     +
     +  if (refcnt == 0)
     +    free_item (m);
      
        return 0;
      }
     -- 
     2.1.4

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