linux - select 函数可能会更新 linux 中的超时参数。为什么它是 'may update'？

Question

我的操作系统是 Centos 6.4。

对于 select，linux 程序员手册说:“select 可能会更新超时参数以指示还剩多少时间……”

我想知道为什么它是“可能”而不是“必须”？有系统版本还是内核版本？

我做一个测试。test_code01.cpp

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>
#include <resolv.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/time.h>
#include <pthread.h>

#define BUFSIZE 1024

struct timeval g_timeout;

int main(int argc, char* argv[])
{
    int sock_clientfd, ret_recvsize, i;
    struct sockaddr_in dest, mine;
    char buffer[BUFSIZE + 1];

    // create socket fd
    if ((sock_clientfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        perror("Socket");
        exit(EXIT_FAILURE);
    }

    // init server address that client will connetct to.
    bzero(&dest, sizeof(dest));
    dest.sin_family = AF_INET;
    dest.sin_port = htons(9567);
    if(argc != 2)
    {
        printf("Usage: %s <dest ip>\n", argv[0]);
        printf("Usage: %s 127.0.0.1\n", argv[0]);
        return -1;
    }

    printf("-----\n");

    if (inet_aton(argv[1], (struct in_addr *) &dest.sin_addr.s_addr) == 0)
    {
        perror(argv[1]);
        exit(1);
    }

    // connect to server
    printf("will connect!\n");
    if (connect(sock_clientfd, (struct sockaddr *) &dest, sizeof(dest)) != 0)
    {
        perror("Connect ");
        exit(EXIT_FAILURE);
    }

    int ret_select = 0;
    fd_set readfds;
    FD_SET(sock_clientfd, &readfds);

    g_timeout.tv_sec = 3;
    g_timeout.tv_usec = 0;

    while(1)
    {
        ret_select = select(sock_clientfd + 1, &readfds, NULL, NULL, &g_timeout);

        // Becasue select can update timeout, we need to set value for timeout.
        // g_timeout.tv_sec = 3;
        // g_timeout.tv_usec = 0;

        // Recover sock_clienfd state
        FD_SET(sock_clientfd, &readfds);

        if(ret_select == 0)
        {
            printf("select wait timeout.\n");
            continue;
        }

        bzero(buffer, BUFSIZE + 1);
        ret_recvsize = recv(sock_clientfd, buffer, BUFSIZE, 0);
        if(ret_recvsize > 0)
        {
            printf("get %d message:%s", strlen(buffer), buffer);
            ret_recvsize=0;
        }
        else
        {
            printf("no data from server\n");
        }
    }

    // close sock_clientfd
    close(sock_clientfd);

    return 0;
}

我执行这个test_code01.cpp，结果select只有第一次等待3秒，我认为是select修改超时。所以我修改代码如下:

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>
#include <resolv.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/time.h>
#include <pthread.h>

#define BUFSIZE 1024

// in order to facilitate the ovservation data
// the timeout is set to global varible
struct timeval g_timeout;

void* thredproc_looktimeout(void*)
{
    while(1)
    {
        printf("left time: %ds %dms\n", g_timeout.tv_sec, g_timeout.tv_usec);
    }
}

int main(int argc, char* argv[])
{
    int sock_clientfd, ret_recvsize, i;
    struct sockaddr_in dest, mine;
    char buffer[BUFSIZE + 1];

    // create socket fd
    if ((sock_clientfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        perror("Socket");
        exit(EXIT_FAILURE);
    }

    // init server address that client will connetct to.
    bzero(&dest, sizeof(dest));
    dest.sin_family = AF_INET;
    dest.sin_port = htons(9567);
    if(argc != 2)
    {
        printf("Usage: %s <dest ip>\n", argv[0]);
        printf("Usage: %s 127.0.0.1\n", argv[0]);
        return -1;
    }

    printf("-----\n");

    if (inet_aton(argv[1], (struct in_addr *) &dest.sin_addr.s_addr) == 0)
    {
        perror(argv[1]);
        exit(1);
    }

    // connect to server
    printf("will connect!\n");
    if (connect(sock_clientfd, (struct sockaddr *) &dest, sizeof(dest)) != 0)
    {
        perror("Connect ");
        exit(EXIT_FAILURE);
    }

    int ret_select = 0;
    fd_set readfds;
    FD_SET(sock_clientfd, &readfds);
    printf("fdset add fd start+++\n");
    for(int i = 0; i < 2000; i++)
    {
        FD_SET(1025+i, &readfds);
    }
    printf("%d--%s\n", errno, strerror(errno));
    printf("fdset add 2000 fd finish...\n");

    g_timeout.tv_sec = 3;
    g_timeout.tv_usec = 0;


    /**
     *  look timeout left time by create thread
     **/
    pthread_t tid;
    int ret_pthreadcreate = pthread_create(&tid, NULL, thredproc_looktimeout, NULL);
    printf("pthread_create return value is %d\n", ret_pthreadcreate);

    while(1)
    {
        ret_select = select(sock_clientfd + 1, &readfds, NULL, NULL, &g_timeout);

        // Becasue select can update timeout, we need to set value for timeout.
        g_timeout.tv_sec = 3;
        g_timeout.tv_usec = 0;

        // Recover sock_clienfd state
        FD_SET(sock_clientfd, &readfds);

        if(ret_select == 0)
        {
            printf("select wait timeout.\n");
            continue;
        }

        bzero(buffer, BUFSIZE + 1);
        ret_recvsize = recv(sock_clientfd, buffer, BUFSIZE, 0);
        if(ret_recvsize > 0)
        {
            printf("get %d message:%s", strlen(buffer), buffer);
            ret_recvsize=0;
        }
        else
        {
            printf("no data from server\n");
        }
    }

    // close sock_clientfd
    close(sock_clientfd);

    return 0;
}

我认为我们应该在 thredproc_looktimeout 中看到超时的变化。但没有变化。我不知道为什么？

Answer 1

手册页意味着返回超时可能会更新以指示剩余的时间量。这绝对不意味着在等待仍在进行时会更新超时。

即使是，您也无法检测到它。您尝试使用线程将无法工作，因为它没有任何类型的同步。除其他问题外，编译器可以优化 while 循环以仅读取一次 g_timeout。并且没有任何可以使用的同步机制。