sendto函數的坑

　　測試unix數據報套接字時，一個程序收，一個程序發，分別綁定自己的socket。結果在收的部分，返回的發送方的地址總是空的，但是返回的地址長度又是對的。

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;   
}

      仔細對比unp的代碼，發現   slen = 0  這行改成   slen = sizeof(strcut sockaddr_un) 結果就對了，細看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.

　　紅色部分指出，最后一個參數addrlen是一個"值-結果"的參數，賦給它的初始值用來指定拷貝到倒數第二個參數 src_addr 里面的字節數，返回實際值是實際的源地址結構的大小。當傳入的參數小於輸出的值，說明返回的源地址被截斷了。

上面的例子中，傳入的值為0，而輸出的值大於0，明顯這里有問題，未拷貝任何數據到返回的源地址clientaddr。