那些年踩过的坑---Socket源地址为0

问题相关代码如下：

while(1){
    recvfrom(sockfd, readbuff, UDP_CMD_BUFF_LENGTH - 1, 0, (struct sockaddr *)&Glob_remot_88_port_add, &len);
    PRINT("[recevudp]len %d IP is %s\n", len, inet_ntoa(Glob_remot_88_port_add.sin_addr));
}

这个函数， 第一次接受打印出的发送方IP地址总是0.0.0.0~。困扰了很久，网上搜了下解决办法：在recvfrom之前加一句： `len = sizeof(struct sockaddr);就OK了！！！

原来自己没有给len赋值导致错误发生。于是又阅读了下man手册：

The argumentaddrlen is a value-result argument, which the caller should initialize before the call to the size of the buffer associated with src_addr, and modified on return to indicate the actual size of the source address. The returned address is truncated if the buffer pro-vided is too small; in this case, addrlen will return a value greater than was supplied to the call.
意思是说在调用之前应该把addrlen赋值为src_addr的大小，调用结束通过addrlen返回实际的src_addr的大小。