引用网址：https://blog.csdn.net/zhzht19861011/article/details/6186669
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: [cpp] view plain copy
- /**
- * 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_MIN 1 #define TCP_PRIO_NORMAL 64 #define TCP_PRIO_MAX 127 2.tcp_alloc函数:分配一个新的tcp_pcb结构. 该函数源代码: [cpp] view plain copy
- /**
- * 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.
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