from linux-2.6.23.1/fs/read_write.c

#if 0
    /*
     * We need to debate whether we can enable this or not. The
     * man page documents EAGAIN return for the output at least,
     * and the application is arguably buggy if it doesn't expect
     * EAGAIN on a non-blocking file descriptor.
     */
    if (in_file->f_flags & O_NONBLOCK)
        fl = SPLICE_F_NONBLOCK;
#endif