sendto函数的坑

　　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。