help with linked lists

[Peter Jay Salzman <p dirac org>]

hi there,

using sdl i draw a number of circles (electric charges) and would like
to know when the user clicks on a charge.  a basic charge is a struct:

   typedef struct {
      SDL_Surface *img;
      int magnitude;
      SDL_Rect rect;
   } ChargeStruct;

and i have a double linked list of them:

   GList *ChargeList = NULL;
	ChargeStruct puck;
	...
	ChargeList = g_list_append (ChargeList, &puck);

i'd like to take the mouse coordinates on a button click and check to
see if the click happened on a charge.  if it finds a clicked charge, it
returns a pointer to the ChargeStruct belonging to the clicked charge:


  ChargeStruct *ChargeClicked(GList *ChgList, SDL_Event event)
  {
     GList *ptr;
     int x, y;
  
     while (ptr != NULL) {
        ptr = g_list_next(ChgList);
        x = ptr->data->rect.x;
        y = ptr->data->rect.y;
        if (event.button.x > x - 10 && event.button.x < x + 10) {
           printf("click\n");
        }
     }
     return(ptr->data);
  }

i don't have any experience with GList's, so i'm grasping at straws.

ptr->data should be a pointer to a ChargeStruct.  so ptr->data->rect
should be the rect element of a ChargeStruct which is pointed to by
data.

but this function gives the compile errors:

functions.c: In function `ChargeClicked':
functions.c:21: warning: dereferencing `void *' pointer
functions.c:21: request for member `rect' in something not a structure or union
functions.c:22: warning: dereferencing `void *' pointer
functions.c:22: request for member `rect' in something not a structure or union

i feel like i'm losing my grip on what a pointer is.  the code makes
sense to me, but obviously i'm mistaken about something.

can someone help me out?  how do i access rect.x and rect.y of the
ChargeStruct pointed to by ptr->data?

thanks!
pete