在winsock2中使用select时直接遍历readfds(fdsets)是否合适？

Question

I am studying select api in winsock2. fdRead is the fd_sets contains all the sockets that are able to read. I found in most arcticles readfds is not traversed directly. Instead, fdSocket is traversed and judged by FD_ISSET(FD_ISSET(fdSocket.fd_array[i], &fdRead)). I have tried both two methods and both works. So my question is: why not traversing readfds directly?

fd_set fdSocket;
FD_ZERO(&fdSocket);
FD_SET(sock_listen, &fdSocket);//add sock_listen to fdSocket
while (true)
{
    fd_set fdRead = fdSocket;
    if (select(NULL, &fdRead, NULL, NULL, NULL) <= 0) break;
    for (int i = 0; i < (int)fdSocket.fd_count; ++i)
    {
        if (FD_ISSET(fdSocket.fd_array[i], &fdRead))
        {
            if (fdSocket.fd_array[i] == sock_listen)
            {
                //do something
            }
            else
            {
                //do something
            }
        }
    }
}

fd_set fdSocket;
FD_ZERO(&fdSocket);
FD_SET(sock_listen, &fdSocket);//add sock_listen to fdSocket
while (true)
{
    fd_set fdRead = fdSocket;
    if (select(NULL, &fdRead, NULL, NULL, NULL) < 0) break;
    for (int i = 0; i < (int)fdRead.fd_count; ++i)
    {
        if (fdRead.fd_array[i] == sock_listen)
        {
            //do something
        }
        else
        {
            //do something
        }
    }
}

Answer 1

It's a portability measure and an attempt to make code look similar to POSIX spec. fd_set allowed to vary in implementation

Answer 2

Directly accessing the contents of an fd_set struct is not portable across platforms (neither is directly copying fd_set structs, either). The fact that fd_set uses a plain C-style array on Windows is just an implementation detail .

If you need to write portable socket code, using FD_ISSET() , FD_COPY() and other related macros is the portable solution. Otherwise, don't use select() at all, use (e)poll() ( WSAPoll() on Windows) instead, which allows you to manage your own array of sockets that the function simply updates with status info when called.