一、目的

如果有小伙伴移植过LwIP，那么你肯定知道在LwIP源码中tcp/ip协议栈是作为一个单独的线程运行的，那么就有这样一个问题，我们从mac外设上收到的以太网数据包是如何交给tcp/ip线程进行处理的，用户发送的数据又是如何经过协议栈处理后再发送到mac外设上的，其消息处理机制具体是怎样的？

本篇就带领大家讲讲LwIP的消息流以及消息结构。

上图就是LwIP中的消息流以及各个线程间的关系图。其中tcpip_thread线程由主线程调用tcpip_init函数进行创建，其主要负责各个协议的处理；以太网的数据经过消息邮箱送入协议栈后进行处理，协议栈处理后的消息要么发送给主线程，要么通过mac外设发送出去；用户程序通过LwIP提供的接口进行数据收发。

至于以太网数据包如何从Mac外设上收取或者发送到硬件设备上，这边有两种方法：

• 通过收发线程转发（即上图最左边部分），收发线程再和mac外设中断或者DMA中断进行交互（RT-Thread的做法）；这样做的优点是可以根据应用特点决定收发线程的优先级（例如应用发送数据比较多，那么发送的优先级就设置高一点），收发线程以代理的角色存在，避免中断和tcpip_thread线程直接打交道；这样做可能会多占用一些内存。

• 通过中断直接收发，当以太网数据包到来时直接在中断中将消息以邮箱的形式发给tcpip_thread线程；要发送数据时，直接通过DMA或者中断进行处理；这样做的缺点是中断的快入快出可能受到一定的影响。

二、介绍

通过上面的描述我们可以知道tcpip_thread线程是一个基于邮箱的消息循环，那么如何定义一个消息体就显得至关重要。个人认为LwIP中的消息结构的定义是除了PBUF结构设计外又一神来之笔，在实际工程应用中很具有参考价值；话不多说，让我们切入正题。

消息结构

struct tcpip_msg {
  enum tcpip_msg_type type;
  union {
#if !LWIP_TCPIP_CORE_LOCKING
    struct {
      tcpip_callback_fn function;
      void* msg;
    } api_msg;
    struct {
      tcpip_api_call_fn function;
      struct tcpip_api_call_data *arg;
      sys_sem_t *sem;
    } api_call;
    struct {
      tcpip_callback_fn function;
      void *ctx;
      sys_sem_t *sem;
    } cb_wait;
#endif /* LWIP_TCPIP_CORE_LOCKING */
#if !LWIP_TCPIP_CORE_LOCKING_INPUT
    struct {
      struct pbuf *p;
      struct netif *netif;
      netif_input_fn input_fn;
    } inp;
#endif /* !LWIP_TCPIP_CORE_LOCKING_INPUT */
    struct {
      tcpip_callback_fn function;
      void *ctx;
    } cb;
#if LWIP_TCPIP_TIMEOUT && LWIP_TIMERS
    struct {
      u32_t msecs;
      sys_timeout_handler h;
      void *arg;
    } tmo;
#endif /* LWIP_TCPIP_TIMEOUT && LWIP_TIMERS */
  } msg;
};

各个字段含义：

type：消息体类型（决定了使用哪个联合体字段）

msg：联合体类型的消息体，根据不同的消息类型会有不同的消息体定义

相关宏定义

#define API_VAR_REF(name)               (*(name))
#define API_VAR_DECLARE(type, name)     type * name
#define API_VAR_ALLOC_EXT(type, pool, name, errorblock) do { \
                                          name = (type *)memp_malloc(pool); \
                                          if (name == NULL) { \
                                            errorblock; \
                                          } \
                                        } while(0)
#define API_VAR_ALLOC(type, pool, name, errorval) API_VAR_ALLOC_EXT(type, pool, name, return errorval)
#define API_VAR_ALLOC_POOL(type, pool, name, errorval) do { \
                                          name = (type *)LWIP_MEMPOOL_ALLOC(pool); \
                                          if (name == NULL) { \
                                            return errorval; \
                                          } \
                                        } while(0)
#define API_VAR_FREE(pool, name)        memp_free(pool, name)
#define API_VAR_FREE_POOL(pool, name)   LWIP_MEMPOOL_FREE(pool, name)


#define TCPIP_MSG_VAR_REF(name)     API_VAR_REF(name)
#define TCPIP_MSG_VAR_DECLARE(name) API_VAR_DECLARE(struct tcpip_msg, name)
#define TCPIP_MSG_VAR_ALLOC(name)   API_VAR_ALLOC(struct tcpip_msg, MEMP_TCPIP_MSG_API, name, ERR_MEM)
#define TCPIP_MSG_VAR_FREE(name)    API_VAR_FREE(MEMP_TCPIP_MSG_API, name)

#define API_VAR_DECLARE(type, name)     type * name

声明一个type类型的指针变量，例如

API_VAR_DECLARE(int, a);
//等同于
int *a;

#define API_VAR_REF(name)               (*(name))

解引用一个指针类型，例如

API_VAR_REF(a)；
//等同于
*a;

#define API_VAR_ALLOC_EXT(type, pool, name, errorblock) do { \
                                          name = (type *)memp_malloc(pool); \
                                          if (name == NULL) { \
                                            errorblock; \
                                          } \
                                        } while(0)
#define API_VAR_ALLOC(type, pool, name, errorval) API_VAR_ALLOC_EXT(type, pool, name, return errorval)
#define API_VAR_ALLOC_POOL(type, pool, name, errorval) do { \
                                          name = (type *)LWIP_MEMPOOL_ALLOC(pool); \
                                          if (name == NULL) { \
                                            return errorval; \
                                          } \
                                        } while(0)

这几个宏定义都是从特定内存池中分配一个内存块，区别在于API_VAR_ALLOC通过memp_t类型来指定内存池，API_VAR_ALLOC_POOL通过内存池描述来分配

#define TCPIP_MSG_VAR_REF(name)     API_VAR_REF(name)
#define TCPIP_MSG_VAR_DECLARE(name) API_VAR_DECLARE(struct tcpip_msg, name)
#define TCPIP_MSG_VAR_ALLOC(name)   API_VAR_ALLOC(struct tcpip_msg, MEMP_TCPIP_MSG_API, name, ERR_MEM)
#define TCPIP_MSG_VAR_FREE(name)    API_VAR_FREE(MEMP_TCPIP_MSG_API, name)

上面几个宏就是用于操作tcpip_msg结构体变量的，举例

TCPIP_MSG_VAR_DECLARE(foo);  //声明struct tcpip_msg类型的指针变量foo
//等同于
struct tcpip_msg *foo;

TCPIP_MSG_VAR_REF(foo);   //struct tcpip_msg类型的指针变量foo解引用
//等同于
*foo;

TCPIP_MSG_VAR_ALLOC(foo);  //从MEMP_TCPIP_MSG_API内存池中分配消息结构，并赋值给foo指针
//等同于
API_VAR_ALLOC(struct tcpip_msg, MEMP_TCPIP_MSG_API, foo, ERR_MEM)

TCPIP_MSG_VAR_FREE(foo);  //将foo占用的内存释放会内存池
//等同于
API_VAR_FREE(MEMP_TCPIP_MSG_API, foo);

在介绍各个具体消息体之前我们先对消息类型进行说明

enum tcpip_msg_type {
#if !LWIP_TCPIP_CORE_LOCKING
  TCPIP_MSG_API,
  TCPIP_MSG_API_CALL,
#endif /* !LWIP_TCPIP_CORE_LOCKING */
#if !LWIP_TCPIP_CORE_LOCKING_INPUT
  TCPIP_MSG_INPKT,
#endif /* !LWIP_TCPIP_CORE_LOCKING_INPUT */
#if LWIP_TCPIP_TIMEOUT && LWIP_TIMERS
  TCPIP_MSG_TIMEOUT,
  TCPIP_MSG_UNTIMEOUT,
#endif /* LWIP_TCPIP_TIMEOUT && LWIP_TIMERS */
  TCPIP_MSG_CALLBACK,
  TCPIP_MSG_CALLBACK_STATIC,
  TCPIP_MSG_CALLBACK_STATIC_WAIT
};

TCPIP_MSG_API

用于在tcpip_thread线程中执行指定的函数，对应的消息体结构为api_msg

/** Function prototype for functions passed to tcpip_callback() */
typedef void (*tcpip_callback_fn)(void *ctx);
struct {
    tcpip_callback_fn function;
    void* msg;
} api_msg;

各字段含义：

function：需要执行的函数，有一个void*的入参，无返回值

msg：对应函数的入参

此消息的发送函数

/**
 * Sends a message to TCPIP thread to call a function. Caller thread blocks on
 * on a provided semaphore, which ist NOT automatically signalled by TCPIP thread,
 * this has to be done by the user.
 * It is recommended to use LWIP_TCPIP_CORE_LOCKING since this is the way
 * with least runtime overhead.
 *
 * @param fn function to be called from TCPIP thread
 * @param apimsg argument to API function
 * @param sem semaphore to wait on
 * @return ERR_OK if the function was called, another err_t if not
 */
err_t
tcpip_send_msg_wait_sem(tcpip_callback_fn fn, void *apimsg, sys_sem_t *sem)
{
#if LWIP_TCPIP_CORE_LOCKING
  LWIP_UNUSED_ARG(sem);
  LOCK_TCPIP_CORE();
  fn(apimsg);
  UNLOCK_TCPIP_CORE();
  return ERR_OK;
#else /* LWIP_TCPIP_CORE_LOCKING */
  TCPIP_MSG_VAR_DECLARE(msg);  //①

  LWIP_ASSERT("semaphore not initialized", sys_sem_valid(sem));
  LWIP_ASSERT("Invalid mbox", sys_mbox_valid_val(tcpip_mbox));

  TCPIP_MSG_VAR_ALLOC(msg);   //②
  TCPIP_MSG_VAR_REF(msg).type = TCPIP_MSG_API;   //③
  TCPIP_MSG_VAR_REF(msg).msg.api_msg.function = fn;   
  TCPIP_MSG_VAR_REF(msg).msg.api_msg.msg = apimsg;
  sys_mbox_post(&tcpip_mbox, &TCPIP_MSG_VAR_REF(msg));  //④
  sys_arch_sem_wait(sem, 0);
  TCPIP_MSG_VAR_FREE(msg);
  return ERR_OK;
#endif /* LWIP_TCPIP_CORE_LOCKING */
}

①声明消息结构指针变量

②从内存池中分配消息体内存

③设置消息类型并设置消息体各个字段

④通过邮箱发送消息

//①②③的操作等同于
struct tcpip_msg *msg = memp_malloc(MEMP_TCPIP_MSG_API);
*msg.type = TCPIP_MSG_API;
*msg.msg.api_msg.function = fn;
*msg.msg.api_msg.msg = apimsg;

此消息的处理过程

/* Handle a single tcpip_msg
 * This is in its own function for access by tests only.
 */
static void
tcpip_thread_handle_msg(struct tcpip_msg *msg)
{
  switch (msg->type) {
#if !LWIP_TCPIP_CORE_LOCKING
    case TCPIP_MSG_API:   //①
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: API message %p\n", (void *)msg));
      msg->msg.api_msg.function(msg->msg.api_msg.msg);  //②
      break;
    case TCPIP_MSG_API_CALL:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: API CALL message %p\n", (void *)msg));
      msg->msg.api_call.arg->err = msg->msg.api_call.function(msg->msg.api_call.arg);
      sys_sem_signal(msg->msg.api_call.sem);
      break;
#endif /* !LWIP_TCPIP_CORE_LOCKING */

#if !LWIP_TCPIP_CORE_LOCKING_INPUT
    case TCPIP_MSG_INPKT:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: PACKET %p\n", (void *)msg));
      if (msg->msg.inp.input_fn(msg->msg.inp.p, msg->msg.inp.netif) != ERR_OK) {
        pbuf_free(msg->msg.inp.p);
      }
      memp_free(MEMP_TCPIP_MSG_INPKT, msg);
      break;
#endif /* !LWIP_TCPIP_CORE_LOCKING_INPUT */

#if LWIP_TCPIP_TIMEOUT && LWIP_TIMERS
    case TCPIP_MSG_TIMEOUT:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: TIMEOUT %p\n", (void *)msg));
      sys_timeout(msg->msg.tmo.msecs, msg->msg.tmo.h, msg->msg.tmo.arg);
      memp_free(MEMP_TCPIP_MSG_API, msg);
      break;
    case TCPIP_MSG_UNTIMEOUT:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: UNTIMEOUT %p\n", (void *)msg));
      sys_untimeout(msg->msg.tmo.h, msg->msg.tmo.arg);
      memp_free(MEMP_TCPIP_MSG_API, msg);
      break;
#endif /* LWIP_TCPIP_TIMEOUT && LWIP_TIMERS */

    case TCPIP_MSG_CALLBACK:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: CALLBACK %p\n", (void *)msg));
      msg->msg.cb.function(msg->msg.cb.ctx);
      memp_free(MEMP_TCPIP_MSG_API, msg);
      break;

    case TCPIP_MSG_CALLBACK_STATIC:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: CALLBACK_STATIC %p\n", (void *)msg));
      msg->msg.cb.function(msg->msg.cb.ctx);
      break;

    default:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: invalid message: %d\n", msg->type));
      LWIP_ASSERT("tcpip_thread: invalid message", 0);
      break;
  }
}

上面的片段①②就是TCPIP_MSG_API的处理，也就是在tcpip_thread线程中执行消息体上指定的函数。

---

TCPIP_MSG_API_CALL

用于在tcpip_thread线程中执行指定的函数，区别于api_msg，调用方需要等待该函数执行完毕后才返回，对应的消息体结构为api_msg_call

struct tcpip_api_call_data
{
#if !LWIP_TCPIP_CORE_LOCKING
  err_t err;
#if !LWIP_NETCONN_SEM_PER_THREAD
  sys_sem_t sem;
#endif /* LWIP_NETCONN_SEM_PER_THREAD */
#else /* !LWIP_TCPIP_CORE_LOCKING */
  u8_t dummy; /* avoid empty struct :-( */
#endif /* !LWIP_TCPIP_CORE_LOCKING */
};   
typedef err_t (*tcpip_api_call_fn)(struct tcpip_api_call_data* call);

struct {
    tcpip_api_call_fn function;
    struct tcpip_api_call_data *arg;
    sys_sem_t *sem;
} api_call;

各字段含义：

function：需要执行的函数，该函数有一个struct tcpip_api_call_data类型的指针参数，返回值类型为err_t

arg：对应函数的入参

sem：用于通知调用方退出阻塞的信号量

此消息的发送函数

/**
 * Synchronously calls function in TCPIP thread and waits for its completion.
 * It is recommended to use LWIP_TCPIP_CORE_LOCKING (preferred) or
 * LWIP_NETCONN_SEM_PER_THREAD.
 * If not, a semaphore is created and destroyed on every call which is usually
 * an expensive/slow operation.
 * @param fn Function to call
 * @param call Call parameters
 * @return Return value from tcpip_api_call_fn
 */
err_t
tcpip_api_call(tcpip_api_call_fn fn, struct tcpip_api_call_data *call)
{
#if LWIP_TCPIP_CORE_LOCKING
  err_t err;
  LOCK_TCPIP_CORE();
  err = fn(call);
  UNLOCK_TCPIP_CORE();
  return err;
#else /* LWIP_TCPIP_CORE_LOCKING */
  TCPIP_MSG_VAR_DECLARE(msg);  //①

#if !LWIP_NETCONN_SEM_PER_THREAD
  err_t err = sys_sem_new(&call->sem, 0);
  if (err != ERR_OK) {
    return err;
  }
#endif /* LWIP_NETCONN_SEM_PER_THREAD */

  LWIP_ASSERT("Invalid mbox", sys_mbox_valid_val(tcpip_mbox));

  TCPIP_MSG_VAR_ALLOC(msg); //②
  TCPIP_MSG_VAR_REF(msg).type = TCPIP_MSG_API_CALL;
  TCPIP_MSG_VAR_REF(msg).msg.api_call.arg = call;
  TCPIP_MSG_VAR_REF(msg).msg.api_call.function = fn;
#if LWIP_NETCONN_SEM_PER_THREAD
  TCPIP_MSG_VAR_REF(msg).msg.api_call.sem = LWIP_NETCONN_THREAD_SEM_GET(); //③
#else /* LWIP_NETCONN_SEM_PER_THREAD */
  TCPIP_MSG_VAR_REF(msg).msg.api_call.sem = &call->sem;
#endif /* LWIP_NETCONN_SEM_PER_THREAD */
  sys_mbox_post(&tcpip_mbox, &TCPIP_MSG_VAR_REF(msg));  //④
  sys_arch_sem_wait(TCPIP_MSG_VAR_REF(msg).msg.api_call.sem, 0); //⑤
  TCPIP_MSG_VAR_FREE(msg);  //⑥

#if !LWIP_NETCONN_SEM_PER_THREAD
  sys_sem_free(&call->sem);
#endif /* LWIP_NETCONN_SEM_PER_THREAD */

  return call->err;
#endif /* LWIP_TCPIP_CORE_LOCKING */
}

①声明消息结构指针变量

②从内存池中分配消息体内存

③设置消息类型并设置消息体各个字段，注意此处设置了一个信号量用于同步

④通过邮箱发送消息

⑤阻塞等待信号量上直到这个函数执行完毕，

此消息的处理过程

/* Handle a single tcpip_msg
 * This is in its own function for access by tests only.
 */
static void
tcpip_thread_handle_msg(struct tcpip_msg *msg)
{
  switch (msg->type) {
#if !LWIP_TCPIP_CORE_LOCKING
    case TCPIP_MSG_API:   
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: API message %p\n", (void *)msg));
      msg->msg.api_msg.function(msg->msg.api_msg.msg); 
      break;
    case TCPIP_MSG_API_CALL: //①
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: API CALL message %p\n", (void *)msg));
      msg->msg.api_call.arg->err = msg->msg.api_call.function(msg->msg.api_call.arg); //②
      sys_sem_signal(msg->msg.api_call.sem); //③
      break;
#endif /* !LWIP_TCPIP_CORE_LOCKING */

#if !LWIP_TCPIP_CORE_LOCKING_INPUT
    case TCPIP_MSG_INPKT:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: PACKET %p\n", (void *)msg));
      if (msg->msg.inp.input_fn(msg->msg.inp.p, msg->msg.inp.netif) != ERR_OK) {
        pbuf_free(msg->msg.inp.p);
      }
      memp_free(MEMP_TCPIP_MSG_INPKT, msg);
      break;
#endif /* !LWIP_TCPIP_CORE_LOCKING_INPUT */

#if LWIP_TCPIP_TIMEOUT && LWIP_TIMERS
    case TCPIP_MSG_TIMEOUT:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: TIMEOUT %p\n", (void *)msg));
      sys_timeout(msg->msg.tmo.msecs, msg->msg.tmo.h, msg->msg.tmo.arg);
      memp_free(MEMP_TCPIP_MSG_API, msg);
      break;
    case TCPIP_MSG_UNTIMEOUT:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: UNTIMEOUT %p\n", (void *)msg));
      sys_untimeout(msg->msg.tmo.h, msg->msg.tmo.arg);
      memp_free(MEMP_TCPIP_MSG_API, msg);
      break;
#endif /* LWIP_TCPIP_TIMEOUT && LWIP_TIMERS */

    case TCPIP_MSG_CALLBACK:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: CALLBACK %p\n", (void *)msg));
      msg->msg.cb.function(msg->msg.cb.ctx);
      memp_free(MEMP_TCPIP_MSG_API, msg);
      break;

    case TCPIP_MSG_CALLBACK_STATIC:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: CALLBACK_STATIC %p\n", (void *)msg));
      msg->msg.cb.function(msg->msg.cb.ctx);
      break;

    default:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: invalid message: %d\n", msg->type));
      LWIP_ASSERT("tcpip_thread: invalid message", 0);
      break;
  }
}

上面的片段①②③就是TCPIP_MSG_API_CALL的处理，也就是在tcpip_thread线程中执行消息体上指定的函数，注意③操作发出信号量

---

TCPIP_MSG_INPKT

用于在tcpip_thread线程中通过指定的函数对指定的网络接口上的数据包进行处理，数据包由pbuf结构体指针p指定，执行函数由input_fn指定

/** Function prototype for netif->input functions. This function is saved as 'input'
 * callback function in the netif struct. Call it when a packet has been received.
 *
 * @param p The received packet, copied into a pbuf
 * @param inp The netif which received the packet
 * @return ERR_OK if the packet was handled
 *         != ERR_OK is the packet was NOT handled, in this case, the caller has
 *                   to free the pbuf
 */
typedef err_t (*netif_input_fn)(struct pbuf *p, struct netif *inp);

struct {
    struct pbuf *p;
    struct netif *netif;
    netif_input_fn input_fn;
} inp;

各字段含义：

p：pbuf结构的数据包

netif：从此网卡上收取的数据包

input_fun：数据包处理函数

此消息的发送过程

/**
 * Pass a received packet to tcpip_thread for input processing
 *
 * @param p the received packet
 * @param inp the network interface on which the packet was received
 * @param input_fn input function to call
 */
err_t
tcpip_inpkt(struct pbuf *p, struct netif *inp, netif_input_fn input_fn)
{
#if LWIP_TCPIP_CORE_LOCKING_INPUT
  err_t ret;
  LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_inpkt: PACKET %p/%p\n", (void *)p, (void *)inp));
  LOCK_TCPIP_CORE();
  ret = input_fn(p, inp);
  UNLOCK_TCPIP_CORE();
  return ret;
#else /* LWIP_TCPIP_CORE_LOCKING_INPUT */
  struct tcpip_msg *msg;

  LWIP_ASSERT("Invalid mbox", sys_mbox_valid_val(tcpip_mbox));

  msg = (struct tcpip_msg *)memp_malloc(MEMP_TCPIP_MSG_INPKT);  //①
  if (msg == NULL) {
    return ERR_MEM;
  }

  msg->type = TCPIP_MSG_INPKT;  //②
  msg->msg.inp.p = p;
  msg->msg.inp.netif = inp;
  msg->msg.inp.input_fn = input_fn;
  if (sys_mbox_trypost(&tcpip_mbox, msg) != ERR_OK) {  //③
    memp_free(MEMP_TCPIP_MSG_INPKT, msg);
    return ERR_MEM;
  }
  return ERR_OK;
#endif /* LWIP_TCPIP_CORE_LOCKING_INPUT */
}

上面的片段①②③就是TCPIP_MSG_API_CALL的发送过程，注意此消息是从MEMP_TCPIP_MSG_INPKT内存池中分配的。

此消息的处理过程

/* Handle a single tcpip_msg
 * This is in its own function for access by tests only.
 */
static void
tcpip_thread_handle_msg(struct tcpip_msg *msg)
{
  switch (msg->type) {
#if !LWIP_TCPIP_CORE_LOCKING
    case TCPIP_MSG_API:   
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: API message %p\n", (void *)msg));
      msg->msg.api_msg.function(msg->msg.api_msg.msg); 
      break;
    case TCPIP_MSG_API_CALL:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: API CALL message %p\n", (void *)msg));
      msg->msg.api_call.arg->err = msg->msg.api_call.function(msg->msg.api_call.arg); 
      sys_sem_signal(msg->msg.api_call.sem); 
      break;
#endif /* !LWIP_TCPIP_CORE_LOCKING */

#if !LWIP_TCPIP_CORE_LOCKING_INPUT
    case TCPIP_MSG_INPKT: //①
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: PACKET %p\n", (void *)msg));
      if (msg->msg.inp.input_fn(msg->msg.inp.p, msg->msg.inp.netif) != ERR_OK) { //②
        pbuf_free(msg->msg.inp.p); //③
      }
      memp_free(MEMP_TCPIP_MSG_INPKT, msg); //④
      break;
#endif /* !LWIP_TCPIP_CORE_LOCKING_INPUT */

#if LWIP_TCPIP_TIMEOUT && LWIP_TIMERS
    case TCPIP_MSG_TIMEOUT:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: TIMEOUT %p\n", (void *)msg));
      sys_timeout(msg->msg.tmo.msecs, msg->msg.tmo.h, msg->msg.tmo.arg);
      memp_free(MEMP_TCPIP_MSG_API, msg);
      break;
    case TCPIP_MSG_UNTIMEOUT:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: UNTIMEOUT %p\n", (void *)msg));
      sys_untimeout(msg->msg.tmo.h, msg->msg.tmo.arg);
      memp_free(MEMP_TCPIP_MSG_API, msg);
      break;
#endif /* LWIP_TCPIP_TIMEOUT && LWIP_TIMERS */

    case TCPIP_MSG_CALLBACK:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: CALLBACK %p\n", (void *)msg));
      msg->msg.cb.function(msg->msg.cb.ctx);
      memp_free(MEMP_TCPIP_MSG_API, msg);
      break;

    case TCPIP_MSG_CALLBACK_STATIC:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: CALLBACK_STATIC %p\n", (void *)msg));
      msg->msg.cb.function(msg->msg.cb.ctx);
      break;

    default:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: invalid message: %d\n", msg->type));
      LWIP_ASSERT("tcpip_thread: invalid message", 0);
      break;
  }
}

上面的片段①②③④就是TCPIP_MSG_INPKT的处理，也就是在tcpip_thread线程中执行消息体上指定的数据包处理函数并释放数据包和消息体

---

TCPIP_MSG_TIMEOUT和TCPIP_MSG_UNTIMEOUT

用于在tcpip_thread线程中进行超时处理

struct {
    u32_t msecs;
    sys_timeout_handler h;
    void *arg;
} tmo;

/**
 * call sys_timeout in tcpip_thread
 *
 * @param msecs time in milliseconds for timeout
 * @param h function to be called on timeout
 * @param arg argument to pass to timeout function h
 * @return ERR_MEM on memory error, ERR_OK otherwise
 */
err_t
tcpip_timeout(u32_t msecs, sys_timeout_handler h, void *arg)
{
  struct tcpip_msg *msg;

  LWIP_ASSERT("Invalid mbox", sys_mbox_valid_val(tcpip_mbox));

  msg = (struct tcpip_msg *)memp_malloc(MEMP_TCPIP_MSG_API);
  if (msg == NULL) {
    return ERR_MEM;
  }

  msg->type = TCPIP_MSG_TIMEOUT;
  msg->msg.tmo.msecs = msecs;
  msg->msg.tmo.h = h;
  msg->msg.tmo.arg = arg;
  sys_mbox_post(&tcpip_mbox, msg);
  return ERR_OK;
}

/**
 * call sys_untimeout in tcpip_thread
 *
 * @param h function to be called on timeout
 * @param arg argument to pass to timeout function h
 * @return ERR_MEM on memory error, ERR_OK otherwise
 */
err_t
tcpip_untimeout(sys_timeout_handler h, void *arg)
{
  struct tcpip_msg *msg;

  LWIP_ASSERT("Invalid mbox", sys_mbox_valid_val(tcpip_mbox));

  msg = (struct tcpip_msg *)memp_malloc(MEMP_TCPIP_MSG_API);
  if (msg == NULL) {
    return ERR_MEM;
  }

  msg->type = TCPIP_MSG_UNTIMEOUT;
  msg->msg.tmo.h = h;
  msg->msg.tmo.arg = arg;
  sys_mbox_post(&tcpip_mbox, msg);
  return ERR_OK;
}

case TCPIP_MSG_TIMEOUT:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: TIMEOUT %p\n", (void *)msg));
      sys_timeout(msg->msg.tmo.msecs, msg->msg.tmo.h, msg->msg.tmo.arg);
      memp_free(MEMP_TCPIP_MSG_API, msg);
      break;
    case TCPIP_MSG_UNTIMEOUT:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: UNTIMEOUT %p\n", (void *)msg));
      sys_untimeout(msg->msg.tmo.h, msg->msg.tmo.arg);
      memp_free(MEMP_TCPIP_MSG_API, msg);
      break;

---

TCPIP_MSG_CALLBACK和TCPIP_MSG_CALLBACK_STATIC

用于在tcpip_thread线程中执行函数

    struct {
      tcpip_callback_fn function;
      void *ctx;
    } cb;

/**
 * @ingroup lwip_os
 * Call a specific function in the thread context of
 * tcpip_thread for easy access synchronization.
 * A function called in that way may access lwIP core code
 * without fearing concurrent access.
 * Blocks until the request is posted.
 * Must not be called from interrupt context!
 *
 * @param function the function to call
 * @param ctx parameter passed to f
 * @return ERR_OK if the function was called, another err_t if not
 *
 * @see tcpip_try_callback
 */
err_t
tcpip_callback(tcpip_callback_fn function, void *ctx)
{
  struct tcpip_msg *msg;

  LWIP_ASSERT("Invalid mbox", sys_mbox_valid_val(tcpip_mbox));

  msg = (struct tcpip_msg *)memp_malloc(MEMP_TCPIP_MSG_API);
  if (msg == NULL) {
    return ERR_MEM;
  }

  msg->type = TCPIP_MSG_CALLBACK;
  msg->msg.cb.function = function;
  msg->msg.cb.ctx = ctx;

  sys_mbox_post(&tcpip_mbox, msg);
  return ERR_OK;
}
/**
 * @ingroup lwip_os
 * Allocate a structure for a static callback message and initialize it.
 * The message has a special type such that lwIP never frees it.
 * This is intended to be used to send "static" messages from interrupt context,
 * e.g. the message is allocated once and posted several times from an IRQ
 * using tcpip_callbackmsg_trycallback().
 * Example usage: Trigger execution of an ethernet IRQ DPC routine in lwIP thread context.
 * 
 * @param function the function to call
 * @param ctx parameter passed to function
 * @return a struct pointer to pass to tcpip_callbackmsg_trycallback().
 *
 * @see tcpip_callbackmsg_trycallback()
 * @see tcpip_callbackmsg_delete()
 */
struct tcpip_callback_msg *
tcpip_callbackmsg_new(tcpip_callback_fn function, void *ctx)
{
  struct tcpip_msg *msg = (struct tcpip_msg *)memp_malloc(MEMP_TCPIP_MSG_API);
  if (msg == NULL) {
    return NULL;
  }
  msg->type = TCPIP_MSG_CALLBACK_STATIC;
  msg->msg.cb.function = function;
  msg->msg.cb.ctx = ctx;
  return (struct tcpip_callback_msg *)msg;
}

    case TCPIP_MSG_CALLBACK:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: CALLBACK %p\n", (void *)msg));
      msg->msg.cb.function(msg->msg.cb.ctx);
      memp_free(MEMP_TCPIP_MSG_API, msg);
      break;

    case TCPIP_MSG_CALLBACK_STATIC:
      LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: CALLBACK_STATIC %p\n", (void *)msg));
      msg->msg.cb.function(msg->msg.cb.ctx);
      break;

至此，整个LwIP的消息循环就讲解完了，实现上还是很容易理解的。