Why shutdown a socket can't let the select() return?

问题

In my program, there is a thread blocking on a listen socket, which waits for other connections, and the code likes this:

{
    ......
    FD_ZERO(&fd_sets);
    FD_SET(sock_fd, &fd_sets);

    ret_val = select(sock_fd + 1, &fd_sets, NULL, NULL, NULL);

    if (ret_val > 0)
    {
         accept(sock_fd, NULL, NULL);
         ......
    }
    else
    {
        ......
    }

Per my understandings, if in other thread, shutdown the socket, and the code likes this:

{
    ......
    shutdown(sock_fd, SHUT_RD);
    ......
}  

I think the select() in the previous thread should return. But after testing, I find the select() is still in blocking.

Why shutdown a socket can't let the select() return?

来源：https://stackoverflow.com/questions/18352039/why-shutdown-a-socket-cant-let-the-select-return