原文网址：https://blog.csdn.net/zhzht19861011/article/details/6191480
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: [cpp] view plain copy
- /**
- * 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.
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