Why would GLU crash at this spot?

Question

I have found that my application crashes with a null reference exception right here in sweep.c in GLU source code:

static void ConnectLeftVertex( GLUtesselator *tess, GLUvertex *vEvent )
/*
 * Purpose: connect a "left" vertex (one where both edges go right)
 * to the processed portion of the mesh.  Let R be the active region
 * containing vEvent, and let U and L be the upper and lower edge
 * chains of R.  There are two possibilities:
 *
 * - the normal case: split R into two regions, by connecting vEvent to
 *   the rightmost vertex of U or L lying to the left of the sweep line
 *
 * - the degenerate case: if vEvent is close enough to U or L, we
 *   merge vEvent into that edge chain.  The subcases are:
 *  - merging with the rightmost vertex of U or L
 *  - merging with the active edge of U or L
 *  - merging with an already-processed portion of U or L
 */
{
  ActiveRegion *regUp, *regLo, *reg;
  GLUhalfEdge *eUp, *eLo, *eNew;
  ActiveRegion tmp;

  /* assert( vEvent->anEdge->Onext->Onext == vEvent->anEdge ); */

  /* Get a pointer to the active region containing vEvent */
  tmp.eUp = vEvent->anEdge->Sym;
  /* __GL_DICTLISTKEY */ /* __gl_dictListSearch */
  regUp = (ActiveRegion *)dictKey( dictSearch( tess->dict, &tmp ));
  regLo = RegionBelow( regUp );
  eUp = regUp->eUp;
  eLo = regLo->eUp; //CRASHES RIGHT HERE SINCE RegLo is = 0x000000 FOR SOME REASON

  /* Try merging with U or L first */
  if( EdgeSign( eUp->Dst, vEvent, eUp->Org ) == 0 ) {
    ConnectLeftDegenerate( tess, regUp, vEvent );
    return;
  }

  /* Connect vEvent to rightmost processed vertex of either chain.
   * e->Dst is the vertex that we will connect to vEvent.
   */
  reg = VertLeq( eLo->Dst, eUp->Dst ) ? regUp : regLo;

  if( regUp->inside || reg->fixUpperEdge) {
    if( reg == regUp ) {
      eNew = __gl_meshConnect( vEvent->anEdge->Sym, eUp->Lnext );
      if (eNew == NULL) longjmp(tess->env,1);
    } else {
      GLUhalfEdge *tempHalfEdge= __gl_meshConnect( eLo->Dnext, vEvent->anEdge);
      if (tempHalfEdge == NULL) longjmp(tess->env,1);

      eNew = tempHalfEdge->Sym;
    }
    if( reg->fixUpperEdge ) {
      if ( !FixUpperEdge( reg, eNew ) ) longjmp(tess->env,1);
    } else {
      ComputeWinding( tess, AddRegionBelow( tess, regUp, eNew ));
    }
    SweepEvent( tess, vEvent );
  } else {
    /* The new vertex is in a region which does not belong to the polygon.
     * We don''t need to connect this vertex to the rest of the mesh.
     */
    AddRightEdges( tess, regUp, vEvent->anEdge, vEvent->anEdge, NULL, TRUE );
  }
}

It seems as though eRegLo is a NULL pointer sometimes which causes my application to crash. How could I modify the source to prevent it from crashing my application?

Thanks

Answer 1

You will have to look into the source for RegionBelow and see if and when it can return a NULL pointer. Between your call to RegionBelow and your de-reference of regLo, perform a regLo == NULL check. You can do this with an assert or by throwing an exception (in C++). Most likely, if RegionBelow can return NULL on error, there will be some other method that you can use to determine what the error was and how to handle it. You will have to consult the source code or documentation for RegionBelow to find that information.