FD_ISSET

It is necessary to implement the FD_ISSET Berkeley macro using a supporting function: __WSAFDIsSet(). It is the responsibility of a Windows Sockets implementation to make this available as part of the Windows Sockets API. Unlike the other functions exported by a Windows Sockets DLL, however, this function is not intended to be invoked directly by Windows Sockets applications: it should be used only to support the FD_ISSET macro. The source code for this function is listed below:

int FAR
__WSAFDIsSet(SOCKET fd, fd_set FAR *set)
{
    int i = set->count;

    while (i--)
        if (set->fd_arrayφi∙ == fd)
            return 1;

    return 0;
}