lwIP(V1.3.0) RAW API函数源码分析1----tcp_new()函数

向右看齐 2022-09-28 15:28 66阅读 0赞

位于:lwip/src/core/tcp.c

原型: struct tcp\_pcb \* **tcp\_new** (void)

功能:常见一个TCP协议控制块但并不把它放到任何TCP PCB列表,直到使用tcp\_bind()函数绑定.

代码:

/\*\* \* Creates a new TCP protocol control block but doesn't place it on \* any of the TCP PCB lists. \* The pcb is not put on any list until binding using tcp\_bind(). \* \* @internal: Maybe there should be a idle TCP PCB list where these \* PCBs are put on. Port reservation using tcp\_bind() is implemented but \* allocated pcbs that are not bound can't be killed automatically if wanting \* to allocate a pcb with higher prio (@see tcp\_kill\_prio()) \* \* @return a new tcp\_pcb that initially is in state CLOSED \*/ struct tcp\_pcb \* tcp\_new(void) \{ return tcp\_alloc(TCP\_PRIO\_NORMAL); //分配一个新的tcp\_pcb结构,正常优先级 \}

分析:

1. TCP\_PRIO\_NORMAL:新pcb的优先级

在tcp.h中有以下定义:

\#define TCP\_PRIO\_MIN1

\#define TCP\_PRIO\_NORMAL 64

\#define TCP\_PRIO\_MAX127

2.tcp\_alloc函数:分配一个新的tcp\_pcb结构.

该函数源代码:

/\*\* \* Allocate a new tcp\_pcb structure. \* \* @param prio priority for the new pcb \* @return a new tcp\_pcb that initially is in state CLOSED \*/ struct tcp\_pcb \* tcp\_alloc(u8\_t prio) \{ struct tcp\_pcb \*pcb; u32\_t iss; pcb = memp\_malloc(MEMP\_TCP\_PCB); //申请内存 if (pcb == NULL) \{ //没能成功分配内存 /\* Try killing oldest connection in TIME-WAIT. \*/ LWIP\_DEBUGF(TCP\_DEBUG, ("tcp\_alloc: killing off oldest TIME-WAIT connection/n")); tcp\_kill\_timewait(); //尝试结束最老的连接 /\* Try to allocate a tcp\_pcb again. \*/ pcb = memp\_malloc(MEMP\_TCP\_PCB); //再次尝试分配一个tcp\_pcb if (pcb == NULL) \{ /\* Try killing active connections with lower priority than the new one. \*/ tcp\_kill\_prio(prio);//尝试结束一个活动的但优先级低于要申请的tcp\_pcb连接 /\* Try to allocate a tcp\_pcb again. \*/ pcb = memp\_malloc(MEMP\_TCP\_PCB); \} \} if (pcb != NULL) \{ //如果申请到一个pcb memset(pcb, 0, sizeof(struct tcp\_pcb)); //将分配到的内存块设置成0 pcb->prio = TCP\_PRIO\_NORMAL; pcb->snd\_buf = TCP\_SND\_BUF; //用于发送的有效数据缓冲区大小(单位:字节) // TCP\_SND\_BUF在opt.h中定义,默认256 pcb->snd\_queuelen = 0; //用于发送的有效数据缓冲区空间大小(单位:tep\_segs) pcb->rcv\_wnd = TCP\_WND; //接收器窗口由rcv\_wnd字段保存并且字段值是在将要发送的TCP段中获取的 // TCP\_WND在opt.h中定义,默认2048 pcb->rcv\_ann\_wnd = TCP\_WND; //宣称的接收窗口 pcb->tos = 0; //服务类型 pcb->ttl = TCP\_TTL; //生存时间 //TCP\_TTL在opt.h中定义,默认255 /\* The send MSS(发送段的最大容量) is updated when an MSS option is received. \*/ pcb->mss = (TCP\_MSS > 536) ? 536 : TCP\_MSS; // TCP\_MSS在opt.h中定义,默认128，是一个非常保守的默认值. pcb->rto = 3000 / TCP\_SLOW\_INTERVAL; //超时重传时间 // TCP\_SLOW\_INTERVAL在tcp.h中定义,默认值是500 pcb->sa = 0; //平均往返时间 pcb->sv = 3000 / TCP\_SLOW\_INTERVAL; //平均时间差 pcb->rtime = -1; //重传时间 pcb->cwnd = 1; //连接的当前阻塞窗口值 iss = tcp\_next\_iss(); //为新连接计算一个初始序号,最初值为6510 pcb->snd\_wl2 = iss; pcb->snd\_nxt = iss; //下一个要发送的序号 pcb->snd\_max = iss; //最高发送序号 pcb->lastack = iss; //收到的最后一个ACK包的序列号 pcb->snd\_lbb = iss; //传输队列最后一个字节的顺序编号 pcb->tmr = tcp\_ticks; //当指定的时间到时,连接应取消 pcb->polltmr = 0; //轮询时间 \#if LWIP\_CALLBACK\_API //在opt.h中定义,为1表示直接使用回调函数 pcb->recv = tcp\_recv\_null; // tcp\_recv\_null:默认接收回调函数 \#endif /\* LWIP\_CALLBACK\_API \*/ /\* Init KEEPALIVE timer 初始化持续连接时间\*/ pcb->keep\_idle = TCP\_KEEPIDLE\_DEFAULT; //默认两小时,定义在tcp.h中 \#if LWIP\_TCP\_KEEPALIVE //在opt.h中定义,默认为0 pcb->keep\_intvl = TCP\_KEEPINTVL\_DEFAULT; pcb->keep\_cnt = TCP\_KEEPCNT\_DEFAULT; \#endif /\* LWIP\_TCP\_KEEPALIVE \*/ pcb->keep\_cnt\_sent = 0; // KEEPALIVE计数器 \} return pcb; \}

总结一下, tcp\_alloc函数通过动态申请一块内存并初始化它,之后将这块内存的首地址返回给tcp\_new()函数.