Lwip协议栈的实现目的，无非是要上层用来实现app的socket编程。好，我们就从socket开始。为了兼容性，lwip的socket应该也是提供标准的socket接口函数，恩，没错，在src\include\lwip\socket.h文件中可以看到下面的宏定义：
#if LWIP_COMPAT_SOCKETS
#define accept(a,b,c) lwip_accept(a,b,c)
#define bind(a,b,c) lwip_bind(a,b,c)
#define shutdown(a,b) lwip_shutdown(a,b)
#define closesocket(s) lwip_close(s)
#define connect(a,b,c) lwip_connect(a,b,c)
#define getsockname(a,b,c) lwip_getsockname(a,b,c)
#define getpeername(a,b,c) lwip_getpeername(a,b,c)
#define setsockopt(a,b,c,d,e) lwip_setsockopt(a,b,c,d,e)
#define getsockopt(a,b,c,d,e) lwip_getsockopt(a,b,c,d,e)
#define listen(a,b) lwip_listen(a,b)
#define recv(a,b,c,d) lwip_recv(a,b,c,d)
#define recvfrom(a,b,c,d,e,f) lwip_recvfrom(a,b,c,d,e,f)
#define send(a,b,c,d) lwip_send(a,b,c,d)
#define sendto(a,b,c,d,e,f) lwip_sendto(a,b,c,d,e,f)
#define socket(a,b,c) lwip_socket(a,b,c)
#define select(a,b,c,d,e) lwip_select(a,b,c,d,e)
#define ioctlsocket(a,b,c) lwip_ioctl(a,b,c)
#if LWIP_POSIX_SOCKETS_IO_NAMES
#define read(a,b,c) lwip_read(a,b,c)
#define write(a,b,c) lwip_write(a,b,c)
#define close(s) lwip_close(s)
先不说实际的实现函数，光看这些定义的宏，就是标准socket所必须有的接口。
接着看这些实际的函数实现。这些函数实现在src\api\socket.c中。先看下接受连接的函数，这个是tcp的
原型：int lwip_accept(int s, struct sockaddr *addr, socklen_t *addrlen)
可以看到这里的socket类型参数 s，实际上是个int型
在这个函数中的第一个函数调用是sock = get_socket(s);
这里的sock变量类型是lwip_socket，定义如下：
/** Contains all internal pointers and states used for a socket */
struct lwip_socket {
/** sockets currently are built on netconns, each socket has one netconn */
struct netconn *conn;
/** data that was left from the previous read */
struct netbuf *lastdata;
/** offset in the data that was left from the previous read */
u16_t lastoffset;
/** number of times data was received, set by event_callback(),
tested by the receive and select functions */
u16_t rcvevent;
/** number of times data was received, set by event_callback(),
tested by select */
u16_t sendevent;
/** socket flags (currently, only used for O_NONBLOCK) */
u16_t flags;
/** last error that occurred on this socket */
int err;
};
好，这个结构先不管它，接着看下get_socket函数的实现【也是在src\api\socket.c文件中】，在这里我们看到这样一条语句sock = &sockets[s];很明显，返回值也是这个sock，它是根据传进来的序列号在sockets数组中找到对应的元素并返回该元素的地址。好了，那么这个sockets数组是在哪里被赋值了这些元素的呢？
进行到这里似乎应该从标准的socket编程的开始，也就是socket函数讲起，那我们就顺便看一下。它对应的实际实现是下面这个函数
Int lwip_socket(int domain, int type, int protocol)【src\api\socket.c】
这个函数根据不同的协议类型，也就是函数中的type参数，创建了一个netconn结构体的指针，接着就是用这个指针作为参数调用了alloc_socket函数，下面具体看下这个函数的实现
static int alloc_socket(struct netconn *newconn)
{
int i;
/* Protect socket array */
sys_sem_wait(socksem);
/* allocate a new socket identifier */
for (i = 0; i < NUM_SOCKETS; ++i) {
if (!sockets[i].conn) {
sockets[i].conn = newconn;
sockets[i].lastdata = NULL;
sockets[i].lastoffset = 0;
sockets[i].rcvevent = 0;
sockets[i].sendevent = 1; /* TCP send buf is empty */
sockets[i].flags = 0;
sockets[i].err = 0;
sys_sem_signal(socksem);
return i;
}
}
sys_sem_signal(socksem);
return -1;
}
对了，就是这个时候对全局变量sockets数组的元素赋值的。
既然都来到这里了，那就顺便看下netconn结构的情况吧。它的学名叫netconn descriptor
/** A netconn descriptor */
struct netconn
{
/** type of the netconn (TCP, UDP or RAW) */
enum netconn_type type;
/** current state of the netconn */
enum netconn_state state;
/** the lwIP internal protocol control block */
union {
struct ip_pcb *ip;
struct tcp_pcb *tcp;
struct udp_pcb *udp;
struct raw_pcb *raw;
} pcb;
/** the last error this netconn had */
err_t err;
/** sem that is used to synchroneously execute functions in the core context */
sys_sem_t op_completed;
/** mbox where received packets are stored until they are fetched
by the netconn application thread (can grow quite big) */
sys_mbox_t recvmbox;
/** mbox where new connections are stored until processed
by the application thread */
sys_mbox_t acceptmbox;
/** only used for socket layer */
int socket;
#if LWIP_SO_RCVTIMEO
/** timeout to wait for new data to be received
(or connections to arrive for listening netconns) */
int recv_timeout;
#endif /* LWIP_SO_RCVTIMEO */
#if LWIP_SO_RCVBUF
/** maximum amount of bytes queued in recvmbox */
int recv_bufsize;
#endif /* LWIP_SO_RCVBUF */
u16_t recv_avail;
/** TCP: when data passed to netconn_write doesn't fit into the send buffer,
this temporarily stores the message. */
struct api_msg_msg *write_msg;
/** TCP: when data passed to netconn_write doesn't fit into the send buffer,
this temporarily stores how much is already sent. */
int write_offset;
#if LWIP_TCPIP_CORE_LOCKING
/** TCP: when data passed to netconn_write doesn't fit into the send buffer,
this temporarily stores whether to wake up the original application task
if data couldn't be sent in the first try. */
u8_t write_delayed;
#endif /* LWIP_TCPIP_CORE_LOCKING */
/** A callback function that is informed about events for this netconn */
netconn_callback callback;
};【src\include\lwip\api.h】
到此，对这个结构都有些什么，做了一个大概的了解。
下面以SOCK_STREAM类型为例，看下netconn的new过程：
在lwip_socket函数中有
case SOCK_DGRAM:
conn = netconn_new_with_callback( (protocol == IPPROTO_UDPLITE) ?
NETCONN_UDPLITE : NETCONN_UDP, event_callback);
#define netconn_new_with_callback(t, c) netconn_new_with_proto_and_callback(t, 0, c)
简略实现如下：
struct netconn*
netconn_new_with_proto_and_callback(enum netconn_type t, u8_t proto, netconn_callback callback)
{
struct netconn *conn;
struct api_msg msg;
conn = netconn_alloc(t, callback);
if (conn != NULL )
{
msg.function = do_newconn;
msg.msg.msg.n.proto = proto;
msg.msg.conn = conn;
TCPIP_APIMSG(&msg);
}
return conn;
}
主要就看TCPIP_APIMSG了，这个宏有两个定义，一个是LWIP_TCPIP_CORE_LOCKING的，一个非locking的。分别分析这两个不同类型的函数
* Call the lower part of a netconn_* function
* This function has exclusive access to lwIP core code by locking it
* before the function is called.
err_t tcpip_apimsg_lock(struct api_msg *apimsg)【这个是可以locking的】
{
LOCK_TCPIP_CORE();
apimsg->function(&(apimsg->msg));
UNLOCK_TCPIP_CORE();
return ERR_OK;
}
* Call the lower part of a netconn_* function
* This function is then running in the thread context
* of tcpip_thread and has exclusive access to lwIP core code.
err_t tcpip_apimsg(struct api_msg *apimsg)【此为非locking的】
{
struct tcpip_msg msg;
if (mbox != SYS_MBOX_NULL) {
msg.type = TCPIP_MSG_API;
msg.msg.apimsg = apimsg;
sys_mbox_post(mbox, &msg);
sys_arch_sem_wait(apimsg->msg.conn->op_completed, 0);
return ERR_OK;
}
return ERR_VAL;
}
其实，功能都是一样的，都是要对apimsg->function函数的调用。只是途径不一样而已。看看它们的功能说明就知道了。这么来说apimsg->function的调用很重要了。从netconn_new_with_proto_and_callback函数的实现，可以知道这个function就是do_newconn
Void do_newconn(struct api_msg_msg *msg)
{
if(msg->conn->pcb.tcp == NULL) {
pcb_new(msg);
}
/* Else? This "new" connection already has a PCB allocated. */
/* Is this an error condition? Should it be deleted? */
/* We currently just are happy and return. */
TCPIP_APIMSG_ACK(msg);
}
还是看TCP的，在pcb_new函数中有如下代码：
case NETCONN_TCP:
msg->conn->pcb.tcp = tcp_new();
if(msg->conn->pcb.tcp == NULL) {
msg->conn->err = ERR_MEM;
break;
}
setup_tcp(msg->conn);
break;
我们知道在这里建立了这个tcp的连接。至于这个超级牛的函数，以后再做介绍。
嗯，还是回过头来接着看accept函数吧。
Sock获得了，接着就是newconn = netconn_accept(sock->conn);通过mbox取得新的连接。粗略的估计了一下，这个新的连接应该和listen有关系。那就再次打断一下，看看那个listen操作。
lwip_listen --à netconn_listen_with_backlog--à do_listen--à
tcp_arg(msg->conn->pcb.tcp, msg->conn);
tcp_accept(msg->conn->pcb.tcp, accept_function);//注册了一个接受函数
* Accept callback function for TCP netconns.
* Allocates a new netconn and posts that to conn->acceptmbox.
static err_t accept_function(void *arg, struct tcp_pcb *newpcb, err_t err)
{
struct netconn *newconn;
struct netconn *conn;
conn = (struct netconn *)arg;
/* We have to set the callback here even though
* the new socket is unknown. conn->socket is marked as -1. */
newconn = netconn_alloc(conn->type, conn->callback);
if (newconn == NULL) {
return ERR_MEM;
}
newconn->pcb.tcp = newpcb;
setup_tcp(newconn);
newconn->err = err;
/* Register event with callback */
API_EVENT(conn, NETCONN_EVT_RCVPLUS, 0);
if (sys_mbox_trypost(conn->acceptmbox, newconn) != ERR_OK)
{
/* When returning != ERR_OK, the connection is aborted in tcp_process(),
so do nothing here! */
newconn->pcb.tcp = NULL;
netconn_free(newconn);
return ERR_MEM;
}
return ERR_OK;
}
对了，accept函数中从mbox中获取的连接就是这里放进去的。
再回到accept中来，取得了新的连接，接下来就是分配sock了，再然后，再然后？再然后就等用户来使用接收、发送数据了。
到此整个APP层，也就是传输层以上对socket的封装讲完了。在最后再总结一些整个路径的调用情况吧
