teleporters code problem

[Oestreich, Charles ICFN]

void load_teleporters( void )
{
        FILE *fp;
        char *string;
        OBJ_DATA *obj;
        CHAR buf[MAX_STRING_LENGTH];
        bool tMatch = FALSE;

        sprintf(buf, "%steleporters.dat", DATA_DIR);

        if ( ( fp = fopen(buf, "r") ) == NULL )
        {
                log_string("Error: teleporters.dat file not found!");
                return;
        }

        for ( ; ; )
        {
                string = feof(fp) ? "End" : fread_word(fp);

                if (!str_cmp(string, "End") )
                break;

        switch (UPPER(string[0]))
                {
                case 'C':
                        fread_to_eol(fp);
                        tMatch = TRUE;
                        break;

                case 'T':
                        obj = create_obj(
get_obj_index(OBJ_VNUM_TELEPORTER), MAX_LEVEL );
                        obj_to_room( obj, get_room_index( fread_number(fp) )
);
                sprintf(buf, "%d %s", obj->in_room->vnum, "teleporter");
                do_function(NULL, &do_activate_teleporter, buf);
                tMatch = TRUE;
                fread_to_eol(fp);
                break;
        }

        if ( !tMatch )
                {
                        bug("Load_teleporters: no match.", 0);
                        fread_to_eol(fp);
                }
        }
        
        fclose(fp);
        return;
}

void do_activate_teleporter( CHAR_DATA *ch, char *argument )
{
    TELEPORTER_DATA *tp;
    ROOM_INDEX_DATA *in_room;
    OBJ_DATA *obj;
    char arg1[MAX_INPUT_LENGTH];

    argument = one_argument(argument, arg1);

    if ( ( in_room = get_room_index( atoi(arg1) ) ) == NULL )
    {
        log_string("Bug: invalid room vnum");
        return;
    }

    if ( ( obj = get_obj_list(ch, argument, in_room->contents) ) == NULL )
    {
        log_string("Bug: invalid teleporter object in room");
        return;
    }

    /* set teleporter object as active */
    obj->active = TRUE;
    obj->active_room = obj->in_room->vnum;

    /* add teleporter onto teleporter_list */
    tp = new_teleporter();
    tp->in_room = obj->in_room;
    tp->cost = obj->cost;
    tp->base_obj = obj;
    tp->next = teleporter_list;
    teleporter_list = tp;

    return;
}

when i leave the function with the break under the do_function(NULL,
&do_activate_teleporter, buf); the mud crashes with a sigsegv fault in
handler.c and bool can_see_obj but when i place it above the sprintf in the
case 'T' the mud loads, places the teleporters right but doesn't activate
them. i was wondering if anyone could help me at all. i'm not sure if my
problem is in the load_teleporters or do_activate_teleporters, any help
would be greatly appreciated, because this is stumping me. thanks.