在lwip-x.x.x/src/core/tcp.h中定义: #define tcp_listen(pcb)  tcp_listen_with_backlog(pcb, TCP_DEFAULT_LISTEN_BACKLOG) 所以, tcp_listen(pcb) 和tcp_listen_with_backlog(pcb, TCP_DEFAULT_LISTEN_BACKLOG)是一回事. TCP_DEFAULT_LISTEN_BACKLOG在opt.h中定义,默认值为0xFF,指明最大允许的可等待TCP监听链接个数. 位于: 位于:lwip-x.x.x/src/core/tcp.c 原型: struct tcp_pcb * tcp_listen_with_backlog(struct tcp_pcb *pcb, u8_t backlog) 功能: 使指定的连接开始进入监听状态 函数源码: /** * Set the state of the connection to be LISTEN, which means that it * is able to accept incoming connections. The protocol control block * is reallocated in order to consume less memory. Setting the * connection to LISTEN is an irreversible process. *设置连接进入LISTEN状态,这意味着这个连接能够接受传入的连接,为了消耗更少的内存, 协议控制块会被重新分配.设置连接进入LISTEN状态是一个不可逆转的过程 * @param pcb the original tcp_pcb * @param backlog the incoming connections queue limit传入连接的队列限制 * @return tcp_pcb used for listening, consumes less memory. * * @note The original tcp_pcb is freed(原始的tcp_pcb会被释放). This function therefore has to be * called like this: * tpcb = tcp_listen(tpcb); */ struct tcp_pcb * tcp_listen_with_backlog(struct tcp_pcb *pcb, u8_t backlog) { struct tcp_pcb_listen *lpcb; LWIP_UNUSED_ARG(backlog); // #define LWIP_UNUSED_ARG(x) (void)x LWIP_ERROR("tcp_listen: pcb already connected", pcb->state == CLOSED, return NULL); /* already listening? 是否应经处于监听?*/ if (pcb->state == LISTEN) { return pcb; } lpcb = memp_malloc(MEMP_TCP_PCB_LISTEN);//申请一块内存 if (lpcb == NULL) { return NULL; } lpcb->callback_arg = pcb->callback_arg; lpcb->local_port = pcb->local_port; lpcb->state = LISTEN; //置监听标志位 lpcb->so_options = pcb->so_options; lpcb->so_options |= SOF_ACCEPTCONN; lpcb->ttl = pcb->ttl; lpcb->tos = pcb->tos; ip_addr_set(&lpcb->local_ip, &pcb->local_ip); TCP_RMV(&tcp_bound_pcbs, pcb); memp_free(MEMP_TCP_PCB, pcb); //释放原来的(原始的) PCB #if LWIP_CALLBACK_API lpcb->accept = tcp_accept_null; //默认回调函数 #endif /* LWIP_CALLBACK_API */ #if TCP_LISTEN_BACKLOG lpcb->accepts_pending = 0; lpcb->backlog = (backlog ? backlog : 1); #endif /* TCP_LISTEN_BACKLOG */ TCP_REG(&tcp_listen_pcbs.listen_pcbs, lpcb); //将连接放入已监听队列 return (struct tcp_pcb *)lpcb; }
分析:这个函数从原理上看也比较简单,首先是做一些必要的检查,判断原始pcb是否已经处于连接状态,如果没有则申请一块tcp_pcb类型的内存,将原始的必要的pcb内容复制到新的pcb中,设置新的pcb状态为LISTEN,释放原始的pcb,并将新pcb连接放入已监听队列.