1 /**
 2 * Create a new netconn (of a specific type) that has a callback function.
 3 * The corresponding pcb is also created.
 4 *
 5 * @param t the type of ‘connection‘ to create (@see enum netconn_type)
 6 * @param proto the IP protocol for RAW IP pcbs
 7 * @param callback a function to call on status changes (RX available, TX‘ed)
 8 * @return a newly allocated struct netconn or
 9 *         NULL on memory error
10 */
11 struct netconn*
12 netconn_new_with_proto_and_callback(enum netconn_type t, u8_t proto, netconn_callback callback)
13 {
14   struct netconn *conn;
15   struct api_msg msg;
16 
17   conn = netconn_alloc(t, callback); // 开辟一个netconn
18   if (conn != NULL ) {
19     msg.function = do_newconn; // do_newconn这个函数以msg的形式送给tcpip_thread()去处理，我们随后会分析。这里需要知道do_newconn会开辟一个pcb，并和已有的conn绑定。
20     msg.msg.msg.n.proto = proto;
21     msg.msg.conn = conn;
22     TCPIP_APIMSG(&msg); 
23 
24     if (conn->err != ERR_OK) {
25       LWIP_ASSERT("freeing conn without freeing pcb", conn->pcb.tcp == NULL);
26       LWIP_ASSERT("conn has no op_completed", conn->op_completed != SYS_SEM_NULL);
27       LWIP_ASSERT("conn has no recvmbox", conn->recvmbox != SYS_MBOX_NULL);
28       LWIP_ASSERT("conn->acceptmbox shouldn‘t exist", conn->acceptmbox == SYS_MBOX_NULL);
29       sys_sem_free(conn->op_completed);
30       sys_mbox_free(conn->recvmbox);
31       memp_free(MEMP_NETCONN, conn);
32       return NULL;
33     }
34   }
35   return conn;
36 }

api_msg做了什么

1     msg.function = do_newconn; 
2     msg.msg.msg.n.proto = proto;
3     msg.msg.conn = conn;
4     TCPIP_APIMSG(&msg); 

 

首先来看TCPIP_APIMSG这个宏做了什么：

1 #define TCPIP_APIMSG(m)       tcpip_apimsg(m)

 

 1 /**
 2 * Call the lower part of a netconn_* function
 3 * This function is then running in the thread context
 4 * of tcpip_thread and has exclusive access to lwIP core code.
 5 *
 6 * @param apimsg a struct containing the function to call and its parameters
 7 * @return ERR_OK if the function was called, another err_t if not
 8 */
 9 err_t
10 tcpip_apimsg(struct api_msg *apimsg)
11 {
12   struct tcpip_msg msg;
13  
14   if (mbox != SYS_MBOX_NULL) {
15     msg.type = TCPIP_MSG_API; // 随后在tcpip_thread()里解析这个msg时需要根据这个type确定走哪个分支
16     msg.msg.apimsg = apimsg;
17     sys_mbox_post(mbox, &msg); // mbox是一个全局mailbox，实际上是一个数组，元素是void*型指针，在tcpip_init里被初始化。这里把msg地址放到mbox里
18     sys_arch_sem_wait(apimsg->msg.conn->op_completed, 0);
19     return ERR_OK;
20   }
21   return ERR_VAL;
22 }

 

至此，一个TCPIP_MSG_API type的msg被放到了mbox这个mailbox里，接下来tcpip_thread要从这个mailbox里取msg并对其进行处理，主要就是调用msg里的function。如下：

 

 1 /**
 2 * The main lwIP thread. This thread has exclusive access to lwIP core functions
 3 * (unless access to them is not locked). Other threads communicate with this
 4 * thread using message boxes.
 5 *
 6 * It also starts all the timers to make sure they are running in the right
 7 * thread context.
 8 *
 9 * @param arg unused argument
10 */
11 static void
12 tcpip_thread(void *arg)
13 {
14   struct tcpip_msg *msg;
15   LWIP_UNUSED_ARG(arg);
16 
17 ......................
18 
19   LOCK_TCPIP_CORE();
20   while (1) {                          /* MAIN Loop */
21     sys_mbox_fetch(mbox, (void *)&msg);
22     switch (msg->type) {
23 #if LWIP_NETCONN
24     case TCPIP_MSG_API:
25       LWIP_DEBUGF(TCPIP_DEBUG, ("tcpip_thread: API message %p\n", (void *)msg));
26       msg->msg.apimsg->function(&(msg->msg.apimsg->msg)); // 这个function就是netconn_write()函数里赋值的do_newconn
27       break;
28 #endif /* LWIP_NETCONN */
29 
30 ..............
31 
32     default:
33       break;
34     }
35   }
36 }

 

注意tcpip_thread()函数在tcpip.c（component\common\network\lwip\lwip_v1.3.2\src\api）里，可以认为是lwip api层的函数，只不过虽然名字叫api，但应用层并不是直接调用，应用层实际上是借mailbox与其交互的。当然用户并不知道mailbox的存在，应用层只需要直接调用send()这个lwip api，后续放入mailbox以及tcpip_thread从mailbox取走，都是lwip自己完成的。

1 void
2 do_newconn(struct api_msg_msg *msg)
3 {
4    if(msg->conn->pcb.tcp == NULL) {
5      pcb_new(msg);
6    }
7 ..........
8 }

 1 static err_t
 2 pcb_new(struct api_msg_msg *msg)
 3 {
 4 ....................
 5    /* Allocate a PCB for this connection */
 6    switch(NETCONNTYPE_GROUP(msg->conn->type)) {
 7 ...................
 8 #if LWIP_TCP
 9    case NETCONN_TCP:
10      msg->conn->pcb.tcp = tcp_new();  // 新建一个tcp_pcb结构体，并把这个pcb与conn绑定起来
11      if(msg->conn->pcb.tcp == NULL) {
12        msg->conn->err = ERR_MEM;
13        break;
14      }
15      setup_tcp(msg->conn);
16      break;
17 #endif /* LWIP_TCP */
18 .................
19    }
20 ................
21 }

 1 /**
 2 * Allocate a new socket for a given netconn.
 3 *
 4 * @param newconn the netconn for which to allocate a socket
 5 * @return the index of the new socket; -1 on error
 6 */
 7 static int
 8 alloc_socket(struct netconn *newconn)
 9 {
10   int i;
11 
12   /* Protect socket array */
13   sys_sem_wait(socksem);
14 
15   /* allocate a new socket identifier */
16   for (i = 0; i < NUM_SOCKETS; ++i) {
17     if (!sockets[i].conn) { // 从系统socket列表：sockets[]里寻找还没有被使用的
18       sockets[i].conn       = newconn; // 找出一个未被使用的socket结构体，作为用户调用socket() API申请到的socket结构体，并把它和新建的netconn绑定。
19       sockets[i].lastdata   = NULL;
20       sockets[i].lastoffset = 0;
21       sockets[i].rcvevent   = 0;
22       sockets[i].sendevent  = 1; /* TCP send buf is empty */
23       sockets[i].flags      = 0;
24       sockets[i].err        = 0;
25       sys_sem_signal(socksem);
26       return i; // 仅返回一个int型的i，即用户看不到这个socket结构体，只能socket结构体在socket列表里的index值，用户能使用的也就是这个int值
27     }
28   }
29   sys_sem_signal(socksem);
30   return -1;
31 }

1 /** The global array of available sockets */
2 static struct lwip_socket sockets[NUM_SOCKETS];