学步园

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:

/**<br /> * Set the state of the connection to be LISTEN, which means that it<br /> * is able to accept incoming connections. The protocol control block<br /> * is reallocated in order to consume less memory. Setting the<br /> * connection to LISTEN is an irreversible process.<br /> *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,<br /> 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<br /> * @param pcb the original tcp_pcb<br /> * @param backlog the incoming connections queue limit传入连接的队列限制<br /> * @return tcp_pcb used for listening, consumes less memory.<br /> *<br /> * @note The original tcp_pcb is freed(原始的tcp_pcb会被释放). This function therefore has to be<br /> * called like this:<br /> * tpcb = tcp_listen(tpcb);<br /> */<br /> struct tcp_pcb *<br /> tcp_listen_with_backlog(struct tcp_pcb *pcb, u8_t backlog)<br /> {<br /> struct tcp_pcb_listen *lpcb;</p> <p> LWIP_UNUSED_ARG(backlog); // #define LWIP_UNUSED_ARG(x) (void)x<br /> LWIP_ERROR("tcp_listen: pcb already connected", pcb->state == CLOSED, return NULL);</p> <p> /* already listening? 是否应经处于监听?*/<br /> if (pcb->state == LISTEN) {<br /> return pcb;<br /> }<br /> lpcb = memp_malloc(MEMP_TCP_PCB_LISTEN);//申请一块内存<br /> if (lpcb == NULL) {<br /> return NULL;<br /> }<br /> lpcb->callback_arg = pcb->callback_arg;<br /> lpcb->local_port = pcb->local_port;<br /> lpcb->state = LISTEN; //置监听标志位<br /> lpcb->so_options = pcb->so_options;<br /> lpcb->so_options |= SOF_ACCEPTCONN;<br /> lpcb->ttl = pcb->ttl;<br /> lpcb->tos = pcb->tos;<br /> ip_addr_set(&lpcb->local_ip, &pcb->local_ip);<br /> TCP_RMV(&tcp_bound_pcbs, pcb);<br /> memp_free(MEMP_TCP_PCB, pcb); //释放原来的(原始的) PCB<br /> #if LWIP_CALLBACK_API<br /> lpcb->accept = tcp_accept_null; //默认回调函数<br /> #endif /* LWIP_CALLBACK_API */<br /> #if TCP_LISTEN_BACKLOG<br /> lpcb->accepts_pending = 0;<br /> lpcb->backlog = (backlog ? backlog : 1);<br /> #endif /* TCP_LISTEN_BACKLOG */<br /> TCP_REG(&tcp_listen_pcbs.listen_pcbs, lpcb); //将连接放入已监听队列<br /> return (struct tcp_pcb *)lpcb;<br /> }<br />

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.