Is it appropriate to traverse “readfds(fdsets)” directly when using select in winsock2？

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.