sys_arch interface for lwIP 0.5

Author: Adam Dunkels

The operating system emulation layer provides a common interface
between the lwIP code and the underlying operating system kernel. The
general idea is that porting lwIP to new architectures requires only
small changes to a few header files and a new sys_arch
implementation. It is also possible to do a sys_arch implementation
that does not rely on any underlying operating system.

The sys_arch provides semaphores and mailboxes to lwIP. For the full
lwIP functionality, multiple threads support can be implemented in the
sys_arch, but this is not required for the basic lwIP
functionality. Previous versions of lwIP required the sys_arch to
implement timer scheduling as well but as of lwIP 0.5 this is
implemented in a higher layer.

Semaphores can be either counting or binary - lwIP works with both
kinds. Mailboxes are used for message passing and can be implemented
either as a queue which allows multiple messages to be posted to a
mailbox, or as a rendez-vous point where only one message can be
posted at a time. lwIP works with both kinds, but the former type will
be more efficient. A message in a mailbox is just a pointer, nothing
more. 

Semaphores are represented by the type "sys_sem_t" which is typedef'd
in the sys_arch.h file. Mailboxes are equivalently represented by the
type "sys_mbox_t". lwIP does not place any restrictions on how
sys_sem_t or sys_mbox_t are represented internally.

The following functions must be implemented by the sys_arch:

- void sys_init(void)

  Is called to initialize the sys_arch layer.

- sys_sem_t sys_sem_new(u8_t count)

  Creates and returns a new semaphore. The "count" argument specifies
  the initial state of the semaphore.

- void sys_sem_free(sys_sem_t sem)

  Deallocates a semaphore. 

- void sys_sem_signal(sys_sem_t sem)

  Signals a semaphore.

- u16_t sys_arch_sem_wait(sys_sem_t sem, u16_t timeout)

  Blocks the thread while waiting for the semaphore to be
  signaled. If the "timeout" argument is non-zero, the thread should
  only be blocked for the specified time (measured in
  milliseconds).

  If the timeout argument is non-zero, the return value is the amount
  of time spent waiting for the semaphore to be signaled. If the
  semaphore wasn't signaled within the specified time, the return
  value is zero. If the thread didn't have to wait for the semaphore
  (i.e., it was already signaled), care must be taken to ensure that
  the function does not return a zero value since this is used to
  indicate that a timeout occured. A suitable way to implement this is
  to check if the time spent waiting is zero and if so, the value 1 is
  returned.

  Notice that lwIP implements a function with a similar name,
  sys_sem_wait(), that uses the sys_arch_sem_wait() function.

- sys_mbox_t sys_mbox_new(void)

  Creates an empty mailbox.

- void sys_mbox_free(sys_mbox_t mbox)

  Deallocates a mailbox. If there are messages still present in the
  mailbox when the mailbox is deallocated, it is an indication of a
  programming error in lwIP and the developer should be notified.

- void sys_mbox_post(sys_mbox_t mbox, void *msg)

  Posts the "msg" to the mailbox.

- u16_t sys_arch_mbox_fetch(sys_mbox_t mbox, void **msg, u16_t timeout)

  Blocks the thread until a message arrives in the mailbox, but does
  not block the thread longer than "timeout" milliseconds (similar to
  the sys_arch_sem_wait() function). The "msg" argument is a result
  parameter that is set by the function (i.e., by doing "*msg =
  ptr"). The "msg" parameter maybe NULL to indicate that the message
  should be dropped.

  The return values are the same as for the sys_arch_sem_wait()
  function and the function must not return zero even if a message was
  present in the mailbox and the time spent waiting was zero
  milliseconds. 

  Note that a function with a similar name, sys_mbox_fetch(), is
  implemented by lwIP. 
  
- struct sys_timeouts *sys_arch_timeouts(void)

  Returns a pointer to the per-thread sys_timeouts structure. In lwIP,
  each thread has a list of timeouts which is repressented as a linked
  list of sys_timeout structures. The sys_timeouts structure holds a
  pointer to a linked list of timeouts. This function is called by
  the lwIP timeout scheduler and must not return a NULL value. 

  In a single threadd sys_arch implementation, this function will
  simply return a pointer to a global sys_timeouts variable stored in
  the sys_arch module.
  
If threads are supported by the underlying operating system and if
such functionality is needed in lwIP, the following function will have
to be implemented as well:

- void sys_thread_new(void (* thread)(void *arg), void *arg)

  Starts a new thread that will begin its execution in the function
  "thread()". The "arg" argument will be passed as an argument to the
  thread() function.