BasicSocket#close_write

static VALUE
bsock_close_write(VALUE sock)
{
    rb_io_t *fptr;

    GetOpenFile(sock, fptr);
    if (!(fptr->mode & FMODE_READABLE)) {
        return rb_io_close(sock);
    }
    /* Flush any buffered (sync == false) data before shutting down the
     * write side. Otherwise the buffered bytes are silently dropped here,
     * and a subsequent #close would try to flush them into the now
     * shutdown(SHUT_WR) socket and fail with EPIPE. This matches the
     * write-only branch above, which flushes via rb_io_close(). */
    rb_io_flush(sock);
    shutdown(fptr->fd, SHUT_WR);
    fptr->mode &= ~FMODE_WRITABLE;

    return Qnil;
}