如何正确使用套接字上的select（）？

I'm trying to wrap my head around calling select on sockets and I can't understand what I'm doing wrong.

我试图绕着调用socket上的select来解决我无法理解我做错了什么。

setup_server_socket calls bind and listen and sets the socket to nonblocking mode.

setup_server_socket调用bind和listen并将套接字设置为非阻塞模式。

The following code blocks on the select call it seems, not moving forward to FD_ISSET. I tried connecting a client and it seems to succeed but select never returns anything.

以下代码在select调用上阻塞，而不是向前移动到FD_ISSET。我尝试连接客户端，它似乎成功但选择永远不会返回任何东西。

What's the proper way to do this?

这样做的正确方法是什么？

...
int listenfd = setup_server_socket( serverPort );

if( -1 == listenfd )
    return 1;

fd_set read_fds;
FD_ZERO(&read_fds);
int fdmax = listenfd;


// loop forever
while( 1 )
{
    if (select(fdmax+1, &read_fds, NULL,NULL,NULL) == -1){
        perror("select");
        exit(4);
    }

    for (int i = 0; i<= fdmax; i++){
        printf("Testing: %d, %d\n", i, FD_ISSET(i,&read_fds));

    }return 0;
...

2 个解决方案

#1

Read several times select(2), poll(2) (BTW, you should prefer poll to the obsolete select, which don't handle file descriptors bigger than __FD_SETSIZE, i.e. 1024 on my Linux/Debian/x86-64 system).

多次读取select（2），poll（2）（BTW，你应该更喜欢轮询到过时的select，它不能处理大于__FD_SETSIZE的文件描述符，即我的Linux / Debian / x86-64系统上的1024）。

Then:

然后：

fd_set read_fds;
FD_ZERO(&read_fds);
int fdmax = listenfd;
FD_SET(listenfd, &read_fds);

should go inside the while(1) loop, before calling select. BTW, I recommend using poll instead of select

应该在调用select之前进入while（1）循环。顺便说一句，我建议使用poll而不是select

#2

here is one (trimmed) example of using select()

这是使用select（）的一个（修剪）示例

INT32    selectStatus;                                 /* select() return code */

char     tempreport[ 256 ] = {'\0'};

struct   timeval tv;

fd_set   fdread;
//fd_set   fdwrite;
//fd_set   fdexcep;



// note:
//  must try to read report until no report available 
// so have latest report in buffer
do
{
    /* Note: timeout must be (re)set every time before call to select() */
    tv.tv_sec = 1;
    tv.tv_usec = 0;


    FD_ZERO(&fdread);
    FD_SET( FD, &fdread );

    selectStatus = select(FD+1, &fdread, NULL, NULL, &tv);

    switch( selectStatus )
    {
        case -1:
            ....
            break;

        case 0:
            // timeout, I.E. nothing to read
            ....
            break;

        default: /* available to read */

            memset(tempreport, 0x00, sizeof(tempreport) );
            readStatus = read_UDP_socket( FD, tempreport, sizeof(tempreport), &readCount );

            break;
    } // end switch( selectStatus )
} while( (0 < selectStatus)&&(eRS_Success == readStatus ) );
 // exit loop on select timeout or select error or read failure

notes:

笔记：

eRS_Success is an entry from an enum used for various return codes
read_UDP_socket() is a local function

#1