select函数有时会改变第二个参数readfds的值吗

Question

这是代码：

FD_ZERO(&t_set);
num = 0;
maxfd = 0;

while (num < MAX)
{
    if ((sockfd = connectSocket(ptr)))
        ;
    //printf("connect to server succeed!\n%d\n", fd);
    else
    {
        fprintf(stderr, "connect error\n");
        exit(1);
    }

    if (!writeSocket(sockfd, ptr))
        ;
        //printf("write to server succeed!\n");
    else
    {
        fprintf(stderr, "write error\n");
        exit(1);
    }
    if (maxfd < sockfd)
        maxfd = sockfd;
    FD_SET(sockfd, &t_set);
    printf("%d\n", t_set);
    memset(fileName, 0, BUFSIZE);
    sprintf(fileName,"%d.html", sockfd);
    if ((filefd = open(fileName, O_WRONLY|O_CREAT|O_APPEND, S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH)) == -1)
    {
        printf("create file failed!\n");
        return -1;
    }
    ff[num].sockfd = sockfd;
    ff[num].filefd = filefd;
    ++num;
}
printf(">------------------->\n");  
while (1) 
{
    //sleep(1);
    tv.tv_sec = 1;
    tv.tv_usec = 0;
    h = 0;
    printf("%d\n", t_set);           //print the t_set before select
    h = select(maxfd + 1, &t_set, NULL, NULL, &tv);
    printf("%d\n", t_set);           //print the t_set after select
    printf(">------------------->\n");  
    //fd_set is an array of long;
    //if use printf("%d %d", t_set, h); secondly may print the second value of t_set , not h;
    if (h < 0)
    {
        fprintf(stderr, "select error");
        return -1;
    }
    if (h == 0)
    {
        fprintf(stderr, "select return 0\n");
        break;
    }
    else
    {
        int n = 0;
        while (!FD_ISSET(ff[n].sockfd, &t_set))
            ++n;
        if (readSocket(ff[n].sockfd, ff[n].filefd))
        {
            FD_CLR(ff[n].sockfd, &t_set);
            printf("===================== %d\n", n);
        }
    }

}

num = 0;
while (num++ < MAX)
    close(ff[num].filefd);

输出：

8           // initial of t_set
40
168
680
2728
>------------------->
2728       // the two value were print before and after the select;
680
>------------------->
680
680
>------------------->
680
680
>------------------->
680
680

打印t_set的值，我发现在第一个和最后一个循环调用select后t_set的值被改变了。如果在select前加上sleep(1)，设置tv_set.sec = 0，tv_usec = 0，用sleep timeout代替select超时，t_set执行正常。还有一个问题，上面的例子无法监听到最后一个socket，使用sleep change也可以修复问题；

如有任何建议，我们将不胜感激！

Answer 1

来自 select(2) 手册页：

"On exit, the sets [that is, readfs, writefds, and exceptfds] are modified in place to indicate which file descriptors actually changed status."

您只使用了readfs，但应该被select系统调用修改。

select函数有时会改变第二个参数readfds的值吗

Does the select function change the value of the second argument readfds sometimes

sockets

linux

select

sleep