Subject: Broken pointers again, and again, and .....

(This message is being sent to all CPSC 360 students)

I have seen a virtually unending stream of errors of the
following type:


   337    rps_buf_t *buf;
   338    rps_hdr_t *hdr;
   339    struct timeval tv;
   340
   341    struct  sockaddr_in remote;
   342    unsigned int addrlen = sizeof(remote);
   343
   344    dprintf("rps_sender: starting with sock %p in state %d \n",
   345                 rpsock, rpsock->state);
   346
   347    while(rpsock->state != RPS_SHUTDOWN)
   348    {
   349       // ask ring_get_item to return index of next available item in send ring
   350       dprintf("rps_sender before item wait with items %d \n", \
   351               (&rpsock->sndring)->items);
   352       item = ring_get_item(&rpsock->sndring);
   353       dprintf("rps_sender after item wait with item %d \n", item);
   354
   355       // return if the item index is -1
   356       if(item == -1)
   357          return;
   358
   359       // this code is not used in the typing speed version
   360       //while(rps_usable_window() == 0)
   361       //   wait on win_cv
   362
   363       // compute the address of the hdr element of the buffer
   364       hdr = &buf->hdr;

Note that uninitialized pointer *buf is declared in 337 and then
is used in line 364.  For a variety of reasons, a number of you
have weaknesses regarding pointer use,  but there is no reason to
continue making THIS error.

To avoid doing so you should visually audit each function of your
ENTIRE program. For each pointer declared as a local variable
(inside the function), use your text editor's find and replace
facility to find the NEXT occurrence of that variable.  (For
buf declared on line 337 the next occurrence is on line 364).

If, in this next occurence, the pointer variable either

(a) appears on the RIGHT side of an assignment statement
or
(b) is passed to another function

Then YOU HAVE MADE THIS ERROR.

It should be clear that you don't have to understand how to use
pointers correctly AT ALL to be able to IDENTIFY this problem.

You ONLY have to be able to:
(1) recognize a pointer declaration
(2) understand what the right side of an assignment statement is
and
(3) understand what a function call is

I am 110% confident that even the most Java afflicted amongst you
are quite capable of doing that!

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All that said,  I FULLY AGREE with the statement... "But,  but
Dr. Westall even if I can find the problem now, that still
doesn't mean I can fix it because I don't know how to set the
pointer correctly"

To that I say:
(1) use the notes and the msgs for relevant examples but if after
a diligent search you still can't find any
(2) ASK ME HOW to set the pointer.

We are wasting FAR too much of BOTH YOUR TIME and mine by
attempting to work with code containing FATAL problems that even
a non-programmer could identify with a little bit of effort.