c - The denial-of-service in unix-network-programming 3e of chapter 6.8 -

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i reading unix network programming 3e, , when coming denial of service in chapter 6.8, have question why read block? codes in book using system call 'select' follow:

listenfd = socket(af_inet, sock_stream, 0);  bzero(&servaddr, sizeof(servaddr)); servaddr.sin_family = af_inet; servaddr.sin_addr.s_addr = htonl(inaddr_any); servaddr.sin_port = htons(serv_port);  bind(listenfd, (sa *)&servaddr, sizeof(servaddr));  signal(sigchld, sig_chld);  listen(listenfd, listenq);  maxfd = listenfd; maxi = -1; (i = 0; < fd_setsize; i++) {     client[i] = -1; } fd_zero(&allset); fd_set(listenfd, &allset);  ( ; ; ) {     rset = allset;     nready = select(maxfd + 1, &rset, 0, 0, 0);      if (fd_isset(listenfd, &rset))     {         clilen = sizeof(cliaddr);         again:         if ((connfd = accept(listenfd, (sa *)&cliaddr, &clilen)) < 0)         {             if (errno == eintr || errno == econnaborted)             {                 goto again;             } else             {                 err_sys("accept error");             }         }          (i = 0; < fd_setsize; i++)         {             if (client[i] < 0)             {                 client[i] = connfd;                 break;             }         }         if (i == fd_setsize)         {             err_quit("too many clients");         }         fd_set(connfd, &allset);         if (connfd > maxfd)         {             maxfd = connfd;         }         if (i > maxi)         {             maxi = i;         }         if (--nready == 0)         {             continue;         }     }      (i = 0; <= maxi; i++)     {         if ((sockfd = client[i]) < 0)         {             continue;         }         if (fd_isset(sockfd, &rset))         {             if ((n = read(sockfd, buf, maxline)) == 0)             {                 // book says if first send byte(not '\n'), read block, confusing me                 close(sockfd);                 fd_clr(sockfd, &allset);                 client[i] = -1;             }             else             {                 writen(sockfd, buf, n);             }             if (--nready == 0)             {                 break;             }         }     } }

and use client code:

char sendline[maxline], recvline[maxline]; while (fgets(sendline, maxline, fp) != null) {     writen(sockfd, sendline, 1);     sleep(10);     writen(sockfd, sendline + 1, strlen(sendline) - 1);      if (readline(sockfd, recvline, maxline) == 0)     {         err_quit("strcli: server terminate prematurely");     }      fputs(recvline, stdout); }

and when start 2 clients, , type in 1 line first client, while server not blocked when typing second client.

sorry, want simplify, use read instead of read. , i'm unclear why read in server side block when client write byte first(not '\n') in above example. book says waits '\n', while think read doesn't need '\n' return.





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