标签：cat 数据 地址 对比 was cas null als for
测试unix数据报套接字时，一个程序收，一个程序发，分别绑定自己的socket。结果在收的部分，返回的发送方的地址总是空的，但是返回的地址长度又是对的。
while ( 1 ) { bzero(&clientaddr, sizeof(struct sockaddr_un)); slen = 0; rn = recvfrom(fd,buf, sizeof(buf), 0 ,(struct sockaddr *)&clientaddr, slen); if ( rn == -1) { perror("recvfrom"); } buf[n] = 0; }
仔细对比unp的代码，发现 slen = 0 这行改成 slen = sizeof(strcut sockaddr_un) 结果就对了，细看man
ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *src_addr, socklen_t *addrlen);
=====================================================================================================================
If src_addr is not NULL, and the underlying protocol provides the source address, this source address is filled in. When src_addr is NULL, nothing is filled in; in this case, addrlen is not used, and should also be NULL. The argument addrlen 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 provided is too small; in this case, addrlen will return a value greater than was supplied to the call.
红色部分指出，最后一个参数addrlen是一个"值-结果"的参数，赋给它的初始值用来指定拷贝到倒数第二个参数 src_addr 里面的字节数，返回实际值是实际的源地址结构的大小。当传入的参数小于输出的值，说明返回的源地址被截断了。
上面的例子中，传入的值为0，而输出的值大于0，明显这里有问题，未拷贝任何数据到返回的源地址clientaddr。
标签：cat 数据 地址 对比 was cas null als for
原文地址：http://www.cnblogs.com/thammer/p/6517987.html
