Re: [lwip-members] Seems Error to me!!

[Kieran Mansley]

On Tue, 24 Jun 2003, Jack Carlson wrote:

> Hi Kieran ,
>       I am Using the latest version i guess it is 1.6.3 the following
> line in the function sys_mbox_post :-
>  ((mbox->last + 1) >= (mbox->first + SYS_MBOX_SIZE)) mean that the
> message queue is already full and the posting thread has to block to
> give the TCPIP thread chance to read some messages from the queue. If
> you do not decrement the count after the maximum messsage posted by the
> thread each time the posting thread will unnecessarily block which is
> surely a performance issue. What do u say?? Regards Jack

I've checked the code again, and I can't see a problem.

mbox->first % SYS_MBOX_SIZE gives the index of the first item in the
mbox

mbox->last % SYS_MBOX_SIZE gives the index of the last item in the mbox

If mbox->last is SYS_MBOX_SIZE greater than mbox->first then there are
SYS_MBOX_SIZE items in the mbox (ie. it is full) and the sys_mbox_post()
should block.

Neither mbox->first or mbox->last are ever decremented.  Lets say
SYS_MBOX_SIZE is 10, and 13487 messages have been posted, and 13484
messages fetched.  ie. mbox->last=13487 and mbox->first=13484.  It doesn't
matter that the absolute numbers are much greater than SYS_MBOX_SIZE, as
the absolute values are never used.  We always use the % operator to find
the actual index.  So, the index of the last message is 13487 % 10 = 7 and
the index of the first message is 13484 % 10 = 4, in this case (13487+1)
is  not >= (13484 + 10) so the mbox wouldn't block as expected.  If we
posted another 6 messages though, mbox->last would now be 13493.  As a
result (13493 + 1) is >= (13484 + 10) and the mbox will block as it should
as it is now full.

I hope you can see what I'm getting at as I'm not sure how else to explain
it!

Are you actually seeing a problem when using the code, or is it just that
while reading it you can't see how it works?

Kieran