C: select()下部分代码不执行

Question

我有这样的东西:

#define QUIT_TIME 5
int main(int argc, char **argv) {
        //... SOCKETS STUFF ....
    fdmax = parentfd;


    while (notdone) {

        //Set the timers
        waitd.tv_sec = 1;
        waitd.tv_usec = 0;

        FD_ZERO(&tempreadfds);
        FD_ZERO(&tempwritefds);

        FD_ZERO(&readfds);          /* initialize the read fd set */
        FD_ZERO(&writefds);         /* initialize the write fd set */

        FD_SET(parentfd, &readfds); /* add listener socket fd */
        FD_SET(0, &readfds);        /* add stdin fd (0) */

        tempreadfds = readfds; //make a copy
        tempwritefds = writefds; //make a copy

        if (select(fdmax+1, &tempreadfds, &tempwritefds, (fd_set*) 0, &waitd) < 0) {
            error("ERROR in select");
        }

        for(i = 1; i <= fdmax; i++) {

            if(FD_ISSET(i, &readfds)) {
                if(i == parentfd) {
                //This is a new connection
                childfd = accept(parentfd, (struct sockaddr *) &clientaddr, &clientlen);
                if (childfd < 0)
                    error("ERROR on accept");

                InitializeDataStructures(childfd);

                FD_SET(childfd, &readfds); //add to the master set
                if(childfd > fdmax)
                    fdmax = childfd;
            } else {
                //Existing connection
                if((nBytes = read(i, connections[i].buffer, MAXBUFFER)) <= 0) {
                    if(nBytes == 0) {
                        //Connection closed
                        printf("Socket %d hung up\n", read_write_loop);
                    } else {
                        error("\nReceive error\n");
                    }

                    FD_CLR(i, &readfds);
                } else {
                    //We have some data from the connection
                                            //... Manipulate the buffer
                    //Handle the message
                }
            }
            }

            if(FD_ISSET(i, &writefds)) {
                                .....
                FD_CLR(i, &writefds);
            }

            //Timer checking
            if(connections[i].active) {
                gettimeofday(&TimeNow, NULL);
                timeval_diff(&Interval, &TimeNow, &connections[i].TimeConnected);
                printf("*_*_*__*_*_*__*_*_*_*_* difference is %ld seconds, %ld microseconds\n",
                         Interval.tv_sec,
                         Interval.tv_usec
                        );
                if(Interval.tv_sec >= QUIT_TIME) {
                    printf("Timer elapsed!!\n");
                }
            }

        }
    }

    /* clean up */
    printf("Terminating server.\n");
    close(parentfd);
    return 0;
}

void InitializeDataStructures(int i) {

    clients[i].active = YES;
    clients[i].fd = i;
    //Initialize other members of the structure
}

long long timeval_diff(struct timeval *difference, timeval *end_time, struct timeval *start_time) {
      struct timeval temp_diff;

      if(difference==NULL)
        difference=&temp_diff;

      difference->tv_sec =end_time->tv_sec -start_time->tv_sec ;
      difference->tv_usec=end_time->tv_usec-start_time->tv_usec;

      while(difference->tv_usec<0)
      {
        difference->tv_usec+=1000000;
        difference->tv_sec -=1;
      }

      return 1000000LL*difference->tv_sec + difference->tv_usec;

    }

我期望在执行期间至少打印一次“Timer elapsed”行(TimeConnected 被初始化为 if 条件之一)，但由于某种原因，它永远不会打印出来。我认为我的 while 循环应该继续打印它...有人知道我是否在某个地方搞砸了吗？

编辑:实际上，我正在使用计时器在超时后断开时间。我刚刚观察到，如果另一个客户端连接到服务器，它会打印“计时器已过”。我确实将最终参数传递给了 select，但不确定为什么它没有任何效果。

感谢bdk!!如果您有兴趣了解我在这段代码中遇到的“愚蠢”错误，请详细阅读下面的讨论...这是一个明显的错误，我忽略了...这一切都是因为教程中的一句话:“选择修改你的原始描述符”。

更改列表:

• 请注意，一组 FD_ZERO 语句被错误地放置在 while 循环内
• FD_ISSET 正在传递 readfds 和 writefds，而不是 tempreadfds 和 tempwritefds...

工作代码:

#define QUIT_TIME 5
int main(int argc, char **argv) {
        //... SOCKETS STUFF ....
    fdmax = parentfd;


    FD_ZERO(&readfds);          /* initialize the read fd set */
    FD_ZERO(&writefds);         /* initialize the write fd set */

    while (notdone) {

        //Set the timers
        waitd.tv_sec = 1;
        waitd.tv_usec = 0;

        FD_ZERO(&tempreadfds);
        FD_ZERO(&tempwritefds);


        FD_SET(parentfd, &readfds); /* add listener socket fd */
        FD_SET(0, &readfds);        /* add stdin fd (0) */

        tempreadfds = readfds; //make a copy
        tempwritefds = writefds; //make a copy

        if (select(fdmax+1, &tempreadfds, &tempwritefds, (fd_set*) 0, &waitd) < 0) {
            error("ERROR in select");
        }

        for(i = 1; i <= fdmax; i++) {

            if(FD_ISSET(i, &tempreadfds)) {
                if(i == parentfd) {
                //This is a new connection
                childfd = accept(parentfd, (struct sockaddr *) &clientaddr, &clientlen);
                if (childfd < 0)
                    error("ERROR on accept");

                InitializeDataStructures(childfd);

                FD_SET(childfd, &readfds); //add to the master set
                if(childfd > fdmax)
                    fdmax = childfd;
            } else {
                //Existing connection
                if((nBytes = read(i, connections[i].buffer, MAXBUFFER)) <= 0) {
                    if(nBytes == 0) {
                        //Connection closed
                        printf("Socket %d hung up\n", read_write_loop);
                    } else {
                        error("\nReceive error\n");
                    }

                    FD_CLR(i, &readfds);
                } else {
                    //We have some data from the connection
                                            //... Manipulate the buffer
                    //Handle the message
                }
            }
            }

            if(FD_ISSET(i, &tempwritefds)) {
                                .....
                FD_CLR(i, &writefds);
            }

            //Timer checking
            if(connections[i].active) {
                gettimeofday(&TimeNow, NULL);
                timeval_diff(&Interval, &TimeNow, &connections[i].TimeConnected);
                printf("*_*_*__*_*_*__*_*_*_*_* difference is %ld seconds, %ld microseconds\n",
                         Interval.tv_sec,
                         Interval.tv_usec
                        );
                if(Interval.tv_sec >= QUIT_TIME) {
                    printf("Timer elapsed!!\n");
                }
            }

        }
    }

    /* clean up */
    printf("Terminating server.\n");
    close(parentfd);
    return 0;
}

void InitializeDataStructures(int i) {

    clients[i].active = YES;
    clients[i].fd = i;
    //Initialize other members of the structure
}

long long timeval_diff(struct timeval *difference, timeval *end_time, struct timeval *start_time) {
      struct timeval temp_diff;

      if(difference==NULL)
        difference=&temp_diff;

      difference->tv_sec =end_time->tv_sec -start_time->tv_sec ;
      difference->tv_usec=end_time->tv_usec-start_time->tv_usec;

      while(difference->tv_usec<0)
      {
        difference->tv_usec+=1000000;
        difference->tv_sec -=1;
      }

      return 1000000LL*difference->tv_sec + difference->tv_usec;

    }

Answer 1

看看你选择的循环参数，它们对我来说看起来很可疑。主要是您在 tempreadfd 和 tempwritefd 上调用 select，但是当您调用 FD_ISSET 时，您将其传递给 readfd 和 writefd。在调用 select 之前，您使用 FD_SET 来设置您感兴趣的所有 fd。由于这些变量没有被发送到 select，因此未触发的 fd 不会被屏蔽。因此，您将在所有描述符上检测到“事件”。该接受描述符上确实没有任何事件，因此它会阻塞，直到有新客户端连接为止。

至少这是我的猜测。