關于select()的困惑-有解無憂

我現在忙了2天，仍然不明白。這段代碼做了什么select()？

我知道如果有一個可以接受的傳入連接，copy.fd_array[]它將包含ListenSocket但當 while 回圈重復時它仍然存在。那么我們如何知道客戶端是否斷開連接呢？fd_set copy通話后包含什么select()？

fd_set master;
FD_ZERO(&master);
FD_SET(ListenSocket, &master);

while (1)
{
    fd_set copy = master;

    select(FD_SETSIZE, &copy, NULL, NULL, NULL);

    for (int i = 0; i < FD_SETSIZE; i  )
    {    
        // If new connection
        if (FD_ISSET(ListenSocket, &copy)) 
        {
            printf("[ ] New connection\n");
            // Accept connection
            SOCKET AcceptedClient = accept(ListenSocket, NULL, NULL);
            FD_SET(AcceptedClient, &master);

            // Send welcome message to client
            char buff[128] = "Hello Client!";
            send(AcceptedClient, buff, sizeof(buff), 0);
        }
    }
}

uj5u.com熱心網友回復：

我現在忙了2天，仍然不明白。

難怪你看不懂代碼：示例中的代碼是胡說八道。

檢查ListenSocket應該在for回圈外完成。并且FD_ISSET還必須檢查使用接受的連接accept。

回圈內的正確代碼while如下所示：

fd_set copy = master;

select(FD_SETSIZE, &copy, NULL, NULL, NULL);

// If new connection
if (FD_ISSET(ListenSocket, &copy)) 
{
    ...
}

for (int i = 0; i < FD_SETSIZE; i  )
{    
    // If an existing connection has data
    // or the connection has been closed
    if ((i != ListenSocket) && FD_ISSET(i, &copy)) 
    {
        nBytes = recv(i, buffer, maxBytes, 0);
        // Connection dropped
        if(nBytes < 1)
        {
            close(i); // other OSs (Linux, MacOS ...)
            // closesocket(i); // Windows
            FD_CLR(i, &master);
        }
        // Data received
        else
        {
            ...
        }
    }
}

我知道如果有一個可以接受的傳入連接，copy.fd_array[]它將包含ListenSocket但當while回圈重復時它仍然存在。
在 select() 呼叫之后 fd_set 副本包含什么？

首先：在呼叫之前select()，copy.fd_array[]必須包含您感興趣的所有套接字句柄。這意味著它必須包含ListenSocket所有由回傳的句柄accept()。

master.fd_array[]包含所有這些句柄，因此fd_set copy = master;將確保copy.fd_array[]也包含所有這些句柄。

select()（NULL作為最后一個引數）將等到至少一個套接字變為“可用”。這意味著它將等待至少以下條件之一為真：

接受的連接被accept()對方關閉
使用接受的連接accept()具有可以接收的資料
有一個可以使用的新連接accept(ListenSocket...)

As soon as one condition is fulfilled, select() removes all other handles from copy.fd_array[]:

ListenSocket is removed from copy.fd_array[] if there is no incoming connection
A handle returned by accept() is removed from that array if the connection has neither been closed nor new data is available

If two events happen the same time, copy.fd_array[] will contain more than one handle.

You use FD_ISSET() to check if some handle is still in the array.

So how do we know if a client is disconnected?

When you detect FD_ISSET(i, &copy) for a value i that has been returned by accept(), you must call recv() (under Linux read() would also work):

If recv() returns 0 (or negative in the case of errors), the other computer has dropped the connection. You must call close() (closesocket() on Windows) and remove the handle from copy.fd_array[] (this means: you must remove it from master.fd_array[] because of the line fd_set copy = master;).

If recv() returns a positive value, this is the number of bytes that have been received.

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