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 error10 */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netconn18   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_t10 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 argument10 */11 static void12 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_NETCONN24     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_newconn27       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 void2 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   =http://www.mamicode.com/ 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];