[lwip-users] Re: [lwip] sys_timeout()

[Peter Graf]

Hi!

>while(1) {
>   sys_mbox_fetch(packet);
>   process_incoming_packet(packet);
>}
>
>Now, in the sys_mbox_fetch() function, the following pseudo code illustrates 
>what happens:
>
>sys_mbox_fetch() {
>start:
>   if(there_are_scheduled_timeouts) {
>      time_until_next_timeout = calculate_time_until_next_timeout();
>      if(mfetch_t(mailbox, time_until_next_timeout) == TIMEOUT) {
>          call_timeout_handler_for_this_timeout();
>          goto start;
>      } else {
>         return;
>      }
>   } else {
>      mfetch(mailbox);
>   }
>}

Thanks a lot for your explanation! It now seems clear.

My mistake was, that I saw sys_mbox_fetch() as an isolated function
which could be implemented just based on its own arguments.
(This way, the timeout implementation would have needed internal
OS infos!)

Maybe we should somewhere add a short specification about how
sys_mbox_fetch() is supposed to behave.

For example sys_mbox_fetch() and sys_sem_wait() could be given
a comment like this:

"This is an operation with a timeout. The timeout is not specified as an
argument to this function call, but as a global mechanism for this thread. 

The mechanism involves all calls to sys_timeout(), sys_mbox_fetch(),
and sys_sem_wait() which are made in this thread, and uses global data.
For further info read ..."

(I think your above example explains it well. Unfortunately I had
not seen something like it, although I thought I had read your docs.)

Another way to make things easier to understand might be to pass a pointer
to the data structures used by the timeout implementation, when calling
sys_mbox_fetch() or sys_sem_wait().

Thanks a lot

Peter


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