çUDP无阻塞recvfrom的选择插座 [英] c udp non-blocking socket with recvfrom and select

问题描述

我想在有选择功能的客户端的非阻塞套接字实现。但随着预期这是行不通的。在code低于它从来没有运行到别人，RV始终为1，当没有什么是对插座应用程序停止了一会儿，继续当另一个消息是在插座上。我不希望这样的行为，我想客户端发回的信息到服务器时，没有什么插座给recvfrom上。

  FD_SET readfds;的fcntl（SD，F_SETFL，O_NONBLOCK）;
而（1）{        FD_ZERO（安培; readfds）;
        FD_SET（SD，＆安培; readfds）;        RV =选​​择（SD + 1，＆安培; readfds，NULL，NULL，NULL）;        如果（RV == 1）{            为nbytes = recvfrom的（SD，BUF，RW_SIZE，0，（结构sockaddr *）及srv_addr，＆安培; addrlen中）;
        }其他{            的printf（我从来没有在这里，所以我不能发送消息到服务器\\ n！）;
        }}
 

与timeval结构：

  FD_SET readfds;
的fcntl（SD，F_SETFL，O_NONBLOCK）;
timeval结构电视;
而（1）{        FD_ZERO（安培; readfds）;
        FD_SET（SD，＆安培; readfds）;        tv.tv_sec = 0;
        tv.tv_usec = 0;        RV =选​​择（SD + 1，＆安培; readfds，NULL，NULL，＆安培;电视）;        如果（RV == 1）{            为nbytes = recvfrom的（SD，BUF，RW_SIZE，0，（结构sockaddr *）及srv_addr，＆安培; addrlen中）;
        }其他{            的printf（我一直在这里像现在\\ n！）;
        }}
 

解决方案

您设置超时（选择的最后一个参数）为NULL，这意味着它只会返回一次数据可用的插座上（或中断）。您需要设置超时应该等待。超时可能是0，如果你不想等待，但0是指使用 timeval结构* 与 tv_sec = 0 和 tv_usec = 0 而不是使用 timeval结构* 像你这样。

I want to implement at the client side non-blocking socket with select function. But it doesn't work as expected. In the code below it never runs into else , rv is always 1 and when nothing is on the socket application stops for a while and continue when another messages is on the socket. I don't want that behavior , I want that client sends back message to the server when there is nothing on the socket to recvfrom.

fd_set readfds; 

fcntl(sd, F_SETFL, O_NONBLOCK); 


while (1) {

        FD_ZERO(&readfds);
        FD_SET(sd, &readfds);

        rv = select(sd + 1, &readfds, NULL, NULL, NULL); 

        if(rv == 1){ 

            nbytes = recvfrom(sd, buf, RW_SIZE, 0, (struct sockaddr *) &srv_addr, &addrlen); 


        } else {

            printf("I'm never here so I can't send message back to the server!\n");


        }

}

with struct timeval:

fd_set readfds; 
fcntl(sd, F_SETFL, O_NONBLOCK); 
struct timeval tv;


while (1) {

        FD_ZERO(&readfds);
        FD_SET(sd, &readfds);

        tv.tv_sec = 0;
        tv.tv_usec = 0;

        rv = select(sd + 1, &readfds, NULL, NULL, &tv); 

        if(rv == 1){ 

            nbytes = recvfrom(sd, buf, RW_SIZE, 0, (struct sockaddr *) &srv_addr, &addrlen); 


        } else {

            printf("I'm always here like now ! \n");


        }

}

解决方案

You set the timeout (last parameter of select) to NULL, which means it will only return once data are available on the socket (or interrupt). You need to set a timeout it should wait. The timeout might be 0 if you don't want to wait, but 0 means to use a struct timeval* with tv_sec=0 and tv_usec=0 and not use a struct timeval* of NULL like you did.

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