为什么select的超时不准确?

takaya

我的代码是
tval.tv_sec=5;
tval.tv_usec=0;
....
send(sockfd,(char *)&Req,sizeof(RequestStruct<T>,0);
if (select(fdmax+1, &read_fds, NULL, NULL, &tval)==0) //超时
        {
                puts ("re init time out ";
        }
明明设置了超时为5秒,但执行的结果是根本等不到5秒就报超进了,通常只有几十毫秒后就报超时.

这是为什么呢?怎么才能让select按指定的时间来报超时呢?

思一克

代码贴的不全，无法看出出了什么情况

takaya

sockfd=socket(AF_INET,SOCK_STREAM,0);
if(sockfd<0)
        {
        printf("Socket  Error: Create Socket  failed\n";
        return ;
        }   
memset(&addr,0,sizeof(struct sockaddr_in));
addr.sin_family=AF_INET;
addr.sin_port=htons(PORT);
addr.sin_addr.s_addr=inet_addr(IP);
memset(&addr.sin_zero,0,;
memset(&Req,0,sizeof(Req));
if (connect(sockfd, (struct sockaddr *)&addr,sizeof(struct sockaddr)) == -1)
        {
            printf("Can not Connect to %s:%d\n",IP,PORT);
        return ;
     }
FD_ZERO(&read_fds);
FD_SET(sockfd, &read_fds);
fdmax = sockfd;
tval.tv_sec=5;
tval.tv_usec=0;


......
send(sockfd,(char *)&Req,sizeof(RequestStruct<T>,0);
if (select(fdmax+1, &read_fds, NULL, NULL, &tval)==0) //超时
        {
                puts ("time out ";

        }
else
        {
        if ((nbytes=recv(sockfd, (char *)&Rsp, sizeof(ResponseStruct<T>, 0)) <= 0)
                {
                  return -3;
            }
           else
                   {
                   return 1;
                                }
}

yuxh

在不同的系统中对tval的处理是不一样的，有的系统调用后会修改tval的值，在每次select前都重新赋值吧

精简指令

以下部分摘自<Linux Manual Document >;
"
On Linux, the function select modifies timeout to reflect the amount of time not slept; most other implementations do not do this. This causes problems both when Linux code which reads timeout is ported to other operating systems, and when code is ported to Linux that reuses a struct timeval for multiple selects in a loop without reinitializing it. Consider timeout to
be undefined after select returns.
"