如何通过Linux中的索引访问fd_set中的FD？

Question

For example, I can access them with fd_set.fd_array[i] in Windows..

request_list getIncomingRequests()
{
    fd_set master_set_copy = master_set;
    request_list requests;
    int socket_count = select(0, &master_set_copy, nullptr, nullptr, nullptr);
    for (int i = 0; i < socket_count; ++i)
    {
    #ifdef _WIN32
        auto req_fd = master_set_copy.fd_array[i];
    #else
        auto req_fd = master_set_copy...[i]; // ??
    #endif
        sockaddr_in req_addr;
        getsockname(req_fd, (sockaddr*)&req_addr, &addr_len);
        requests.push_back(request(req_fd, this->fd, req_addr));
    }
    return requests;
}

But fd_array doesn't exist in Linux so I need an equivalent.

Answer 1

The POSIX implementation of fd_set does not require that the structure has the field fd_array . fd_set is supposed to be an opaque data structure.

You can check which file descriptors are set by iterating through all supported descriptors ( 0 through FD_SETSIZE-1 ) and calling FD_ISSET() .

Note from the manual page:

select() can monitor only file descriptors numbers that are less than FD_SETSIZE ; poll(2) does not have this limitation. See BUGS.

Answer 2

Add a new variable, maybe max_fd , that tracks the highest numbered file descriptor in the set. Then you can do this:

request_list getIncomingRequests()
{
    fd_set master_set_copy = master_set;
    request_list requests;
    int socket_count = select(max_fd + 1, &master_set_copy, nullptr, nullptr, nullptr);
    for (int req_fd = 0; req_fd <= max_fd; ++req_fd)
    {
        if (!FD_ISSET(req_fd, &master_set_copy))
            continue;
        sockaddr_in req_addr;
        getsockname(req_fd, (sockaddr*)&req_addr, &addr_len);
        requests.push_back(request(req_fd, this->fd, req_addr));
    }
    return requests;
}

But you're probably better off using poll instead of select .