sys_arch:

sys.c：系统抽象层，其中会调用和os相关的部分，如sys_arch_timeouts, sys_arch_mbox_fetch等，在lwip中是以sys_arch_命名开头的,

"为了方便 LwIP 移植，属于操作系统的函数调用及数据结构并没有在代码中直接使用， 
而是用操作系统模拟层来代替对这些函数的使用。操作系统模拟层使用统一的接口提供定时器、进程同步及消息传递机制等诸如此类的系统服务。原则上，移植 LwIP ，只需针对目标 操作系统修改模拟层实现即可。 
    TCP  用到的定时器功能由操作系统模拟层提供。这个定时器是一个时间间隔至少为 200ms 的单脉冲定时器（one-shot timer，单脉冲定时器，指的是当时钟启动时，它把存储寄存器的值复 制到计数器中，然后晶体的每一个脉冲使计数器减 1。减至 0  时，产生一个中断，并停止工作，直至软件 重新启动它，译者注），当时间溢出发生时就会调用一个已注册的函数。 
    进程同步机制仅提供了信号量。即使信号量不被底层的操作系统支持也可以使用其它基 本的同步方式来模拟，比如条件变量或者加锁。 
    消息传递通过一个简单机制来实现，它使用一个被称作邮箱的抽象方法。邮箱有两种操 作：邮递（post ）与提取（fetch ），邮递操作不会阻塞进程；相反，投递到邮箱的消息被操 作系统模拟层放到队列中直至其它进程将它们取出。即使底层的操作系统本身并不支持邮箱 机制，采用信号量的方式也是很容易实现的。"

sys_arch_timeouts

/* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
/* 获取lwip进程定时器链表,需要移植 */
/* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
struct sys_timeouts *sys_arch_timeouts(void);

sys.c:

sys_timeout

/**
 * Create a one-shot timer (aka timeout). Timeouts are processed in the
 * following cases:
 * - while waiting for a message using sys_mbox_fetch()
 * - while waiting for a semaphore using sys_sem_wait() or sys_sem_wait_timeout()
 * - while sleeping using the inbuilt sys_msleep()
 *
 * @param msecs time in milliseconds after that the timer should expire
 * @param h callback function to call when msecs have elapsed
 * @param arg argument to pass to the callback function
 */
/* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
/* lwip进程中创建定时器，被arp_timer igmp_timer等调用 */
/* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
void
sys_timeout(u32_t msecs, sys_timeout_handler h, void *arg)
{
  struct sys_timeouts *timeouts;
  struct sys_timeo *timeout, *t;

  timeout = memp_malloc(MEMP_SYS_TIMEOUT);
  if (timeout == NULL) {
    LWIP_ASSERT("sys_timeout: timeout != NULL", timeout != NULL);
    return;
  }
  timeout->next = NULL;
  timeout->h = h;
  timeout->arg = arg;
  timeout->time = msecs;

  /* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
  /* 获取lwip进程定时器链表 */
  /* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
  timeouts = sys_arch_timeouts();

  LWIP_DEBUGF(SYS_DEBUG, ("sys_timeout: %p msecs=%"U32_F" h=%p arg=%pn",
    (void *)timeout, msecs, *(void**)&h, (void *)arg));

  if (timeouts == NULL) {
    LWIP_ASSERT("sys_timeout: timeouts != NULL", timeouts != NULL);
    return;
  }

  if (timeouts->next == NULL) {
    timeouts->next = timeout;
    return;
  }

  /* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
  /* 将此次timeout定时器加入lwip进程定时器链 */
  /* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
  if (timeouts->next->time > msecs) {
    timeouts->next->time -= msecs;
    timeout->next = timeouts->next;
    timeouts->next = timeout;
  } else {
    for(t = timeouts->next; t != NULL; t = t->next) {
      timeout->time -= t->time;
      if (t->next == NULL || t->next->time > timeout->time) {
        if (t->next != NULL) {
          t->next->time -= timeout->time;
        }
        timeout->next = t->next;
        t->next = timeout;
        break;
      }
    }
  }
}

sys_untimeout

/**
 * Go through timeout list (for this task only) and remove the first matching
 * entry, even though the timeout has not triggered yet.
 *
 * @note This function only works as expected if there is only one timeout
 * calling 'h' in the list of timeouts.
 *
 * @param h callback function that would be called by the timeout
 * @param arg callback argument that would be passed to h
*/
/* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
/* 将h和arg匹配的定时器删除 */
/* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
sys_untimeout(sys_timeout_handler h, void *arg)
{
  struct sys_timeouts *timeouts;
  struct sys_timeo *prev_t, *t;

  timeouts = sys_arch_timeouts();

  if (timeouts == NULL) {
    LWIP_ASSERT("sys_untimeout: timeouts != NULL", timeouts != NULL);
    return;
  }
  if (timeouts->next == NULL) {
    return;
  }

  for (t = timeouts->next, prev_t = NULL; t != NULL; prev_t = t, t = t->next) {
    if ((t->h == h) && (t->arg == arg)) {
      /* We have a match */
      /* Unlink from previous in list */
      if (prev_t == NULL) {
        timeouts->next = t->next;
      } else {
        prev_t->next = t->next;
      }
      /* If not the last one, add time of this one back to next */
      if (t->next != NULL) {
        t->next->time += t->time;
      }
      memp_free(MEMP_SYS_TIMEOUT, t);
      return;
    }
  }
  return;
}

sys_sem_wait

/**
 * Wait (forever) for a semaphore to become available.
 * While waiting, timeouts (for this thread) are processed.
 *
 * @param sem semaphore to wait for
 */
/* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
/* forever的等待一个semaphore,在等待过程中，lwip进程上的定时器会被刷新 */
/* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
void
sys_sem_wait(sys_sem_t sem)
{
  u32_t time_needed;
  struct sys_timeouts *timeouts;
  struct sys_timeo *tmptimeout;
  sys_timeout_handler h;
  void *arg;

 again:

  timeouts = sys_arch_timeouts();

  /* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
  /* 如果lwip进程没有定时器，则sys_arch_sem_wait(sem, 0)中的参数为0,一直等待这个信号量 */
  /* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
  if (!timeouts || !timeouts->next) {
    sys_arch_sem_wait(sem, 0);
  } else {
    /* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
    /* 否则找到最小的一个定时器时间，以此为超时参数，调用sys_arch_sem_wait */
    /* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
    if (timeouts->next->time > 0) {
	  /* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
	  /* time_needed:表明获得这个信号量需要的时间 */
	  /* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
      time_needed = sys_arch_sem_wait(sem, timeouts->next->time);
    } else {
      time_needed = SYS_ARCH_TIMEOUT;
    }

	/* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
	/* SYS_ARCH_TIMEOUT表明在指定时间未获得信号量,说明未从邮箱获得报文，继续等待(forever的缘由) */
	/* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
    if (time_needed == SYS_ARCH_TIMEOUT) {
      /* If time == SYS_ARCH_TIMEOUT, a timeout occured before a message
        could be fetched. We should now call the timeout handler and
        deallocate the memory allocated for the timeout. */
      tmptimeout = timeouts->next;
      timeouts->next = tmptimeout->next;
      h = tmptimeout->h;
      arg = tmptimeout->arg;
      memp_free(MEMP_SYS_TIMEOUT, tmptimeout);
      if (h != NULL) {
        LWIP_DEBUGF(SYS_DEBUG, ("ssw h=%p(%p)n", *(void**)&h, (void *)arg));
        h(arg);
      }

      /* We try again to fetch a message from the mbox. */
      goto again;
    } else {
      /* If time != SYS_ARCH_TIMEOUT, a message was received before the timeout
         occured. The time variable is set to the number of
         milliseconds we waited for the message. */
      /* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
      /* 若在超时之前获得信号量，则刷新定时器链表下一个时间 */
	  /* ?:这里有一个问题，为什么刷新的只是链中第一个定时器的时间呢，其它的不用刷新吗?contact me:forsakening@sina.cn */
      /* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
      if (time_needed < timeouts->next->time) {
        timeouts->next->time -= time_needed;
      } else {
        timeouts->next->time = 0;
      }
    }
  }
}

sys_sem_wait_timeout

/**
 * Wait for a semaphore with timeout (specified in ms)
 *
 * @param sem semaphore to wait
 * @param timeout timeout in ms (0: wait forever)
 * @return 0 on timeout, 1 otherwise
 */
/* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
/* 带有超时的信号量等待函数 */
/* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
int
sys_sem_wait_timeout(sys_sem_t sem, u32_t timeout)
{
  struct sswt_cb sswt_cb;

  sswt_cb.psem = &sem;
  sswt_cb.timeflag = 0;

  /* If timeout is zero, then just wait forever */
  /* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
  /* 为了实现超时，调用sys_timeout向lwip进程加入一个定时器 */
  /* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
  if (timeout > 0) {
    /* Create a timer and pass it the address of our flag */
    sys_timeout(timeout, sswt_handler, &sswt_cb);
  }

  /* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
  /* 由于sswt_handler实现中，若超时则释放信号量，所以调用sys_sem_wait肯定不会forever的等待 */
  /* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
  sys_sem_wait(sem);
  /* Was it a timeout? */
  if (sswt_cb.timeflag) {
    /* timeout */
    return 0;
  } else {
    /* Not a timeout. Remove timeout entry */
    sys_untimeout(sswt_handler, &sswt_cb);
    return 1;
  }
}

sswt_handler

/**
 * Timeout handler function for sys_sem_wait_timeout()
 *
 * @param arg struct sswt_cb* used to signal a semaphore and end waiting.
 */
/* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
/* 超时的时候发送一个信号量 */
/* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
static void
sswt_handler(void *arg)
{
  struct sswt_cb *sswt_cb = (struct sswt_cb *) arg;

  /* Timeout. Set flag to TRUE and signal semaphore */
  sswt_cb->timeflag = 1;
  sys_sem_signal(*(sswt_cb->psem));
}

sys_msleep

/**
 * Sleep for some ms. Timeouts are processed while sleeping.
 *
 * @param ms number of milliseconds to sleep
 */
/* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
/* 等待some ms，在其中定义一个临时信号量，并调用sys_sem_wait_timeout来实现等待 */
/* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
void
sys_msleep(u32_t ms)
{
  sys_sem_t delaysem = sys_sem_new(0);

  sys_sem_wait_timeout(delaysem, ms);

  sys_sem_free(delaysem);
}

sys_mbox_fetch

/**
 * Wait (forever) for a message to arrive in an mbox.
 * While waiting, timeouts (for this thread) are processed.
 *
 * @param mbox the mbox to fetch the message from
 * @param msg the place to store the message
 */
/* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
/* forever的在邮箱中等待一个message,在等待过程中lwip进程的定时器会被刷新 */
/* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
void
sys_mbox_fetch(sys_mbox_t mbox, void **msg)
{
  u32_t time_needed;
  struct sys_timeouts *timeouts;
  struct sys_timeo *tmptimeout;
  sys_timeout_handler h;
  void *arg;

 again:
  timeouts = sys_arch_timeouts();

  if (!timeouts || !timeouts->next) {
    UNLOCK_TCPIP_CORE();
	/* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
	/* 参数为0，永远等待邮箱中有message */
	/* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
    time_needed = sys_arch_mbox_fetch(mbox, msg, 0);
    LOCK_TCPIP_CORE();
  } else {
    if (timeouts->next->time > 0) {
      UNLOCK_TCPIP_CORE();
	  /* BEGIN: Added by zhengxiang, 2013/10/2   PN:lwip_comment */
	  /* 选用lwip进程下的第一个定时器(时间最小)作为参数，定时等待邮箱 */
	  /* END:   Added by zhengxiang, 2013/10/2   PN:lwip_comment */
      time_needed = sys_arch_mbox_fetch(mbox, msg, timeouts->next->time);
      LOCK_TCPIP_CORE();
    } else {
      time_needed = SYS_ARCH_TIMEOUT;
    }

    if (time_needed == SYS_ARCH_TIMEOUT) {
      /* If time == SYS_ARCH_TIMEOUT, a timeout occured before a message
         could be fetched. We should now call the timeout handler and
         deallocate the memory allocated for the timeout. */
      tmptimeout = timeouts->next;
      timeouts->next = tmptimeout->next;
      h   = tmptimeout->h;
      arg = tmptimeout->arg;
      memp_free(MEMP_SYS_TIMEOUT, tmptimeout);
      if (h != NULL) {
        LWIP_DEBUGF(SYS_DEBUG, ("smf calling h=%p(%p)n", *(void**)&h, arg));
        h(arg);
      }

      /* We try again to fetch a message from the mbox. */
      goto again;
    } else {
      /* If time != SYS_ARCH_TIMEOUT, a message was received before the timeout
         occured. The time variable is set to the number of
         milliseconds we waited for the message. */
      if (time_needed < timeouts->next->time) {
        timeouts->next->time -= time_needed;
      } else {
        timeouts->next->time = 0;
      }
    }
  }
}

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