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124 lines
4.8 KiB
Plaintext
124 lines
4.8 KiB
Plaintext
sys_arch interface for lwIP 0.5
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Author: Adam Dunkels
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The operating system emulation layer provides a common interface
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between the lwIP code and the underlying operating system kernel. The
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general idea is that porting lwIP to new architectures requires only
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small changes to a few header files and a new sys_arch
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implementation. It is also possible to do a sys_arch implementation
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that does not rely on any underlying operating system.
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The sys_arch provides semaphores and mailboxes to lwIP. For the full
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lwIP functionality, multiple threads support can be implemented in the
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sys_arch, but this is not required for the basic lwIP
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functionality. Previous versions of lwIP required the sys_arch to
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implement timer scheduling as well but as of lwIP 0.5 this is
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implemented in a higher layer.
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Semaphores can be either counting or binary - lwIP works with both
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kinds. Mailboxes are used for message passing and can be implemented
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either as a queue which allows multiple messages to be posted to a
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mailbox, or as a rendez-vous point where only one message can be
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posted at a time. lwIP works with both kinds, but the former type will
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be more efficient. A message in a mailbox is just a pointer, nothing
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more.
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Semaphores are represented by the type "sys_sem_t" which is typedef'd
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in the sys_arch.h file. Mailboxes are equivalently represented by the
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type "sys_mbox_t". lwIP does not place any restrictions on how
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sys_sem_t or sys_mbox_t are represented internally.
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The following functions must be implemented by the sys_arch:
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- void sys_init(void)
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Is called to initialize the sys_arch layer.
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- sys_sem_t sys_sem_new(u8_t count)
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Creates and returns a new semaphore. The "count" argument specifies
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the initial state of the semaphore.
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- void sys_sem_free(sys_sem_t sem)
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Deallocates a semaphore.
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- void sys_sem_signal(sys_sem_t sem)
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Signals a semaphore.
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- u16_t sys_arch_sem_wait(sys_sem_t sem, u16_t timeout)
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Blocks the thread while waiting for the semaphore to be
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signaled. If the "timeout" argument is non-zero, the thread should
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only be blocked for the specified time (measured in
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milliseconds).
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If the timeout argument is non-zero, the return value is the amount
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of time spent waiting for the semaphore to be signaled. If the
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semaphore wasn't signaled within the specified time, the return
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value is zero. If the thread didn't have to wait for the semaphore
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(i.e., it was already signaled), care must be taken to ensure that
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the function does not return a zero value since this is used to
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indicate that a timeout occured. A suitable way to implement this is
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to check if the time spent waiting is zero and if so, the value 1 is
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returned.
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Notice that lwIP implements a function with a similar name,
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sys_sem_wait(), that uses the sys_arch_sem_wait() function.
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- sys_mbox_t sys_mbox_new(void)
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Creates an empty mailbox.
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- void sys_mbox_free(sys_mbox_t mbox)
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Deallocates a mailbox. If there are messages still present in the
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mailbox when the mailbox is deallocated, it is an indication of a
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programming error in lwIP and the developer should be notified.
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- void sys_mbox_post(sys_mbox_t mbox, void *msg)
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Posts the "msg" to the mailbox.
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- u16_t sys_arch_mbox_fetch(sys_mbox_t mbox, void **msg, u16_t timeout)
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Blocks the thread until a message arrives in the mailbox, but does
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not block the thread longer than "timeout" milliseconds (similar to
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the sys_arch_sem_wait() function). The "msg" argument is a result
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parameter that is set by the function (i.e., by doing "*msg =
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ptr"). The "msg" parameter maybe NULL to indicate that the message
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should be dropped.
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The return values are the same as for the sys_arch_sem_wait()
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function and the function must not return zero even if a message was
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present in the mailbox and the time spent waiting was zero
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milliseconds.
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Note that a function with a similar name, sys_mbox_fetch(), is
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implemented by lwIP.
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- struct sys_timeouts *sys_arch_timeouts(void)
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Returns a pointer to the per-thread sys_timeouts structure. In lwIP,
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each thread has a list of timeouts which is repressented as a linked
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list of sys_timeout structures. The sys_timeouts structure holds a
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pointer to a linked list of timeouts. This function is called by
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the lwIP timeout scheduler and must not return a NULL value.
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In a single threadd sys_arch implementation, this function will
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simply return a pointer to a global sys_timeouts variable stored in
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the sys_arch module.
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If threads are supported by the underlying operating system and if
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such functionality is needed in lwIP, the following function will have
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to be implemented as well:
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- void sys_thread_new(void (* thread)(void *arg), void *arg)
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Starts a new thread that will begin its execution in the function
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"thread()". The "arg" argument will be passed as an argument to the
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thread() function.
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