+ * io_plan_arg - get a conn's io_plan_arg for a given direction.
+ * @conn: the connection.
+ * @dir: IO_IN or IO_OUT.
+ *
+ * This is how an io helper gets scratch space to store into; you must call
+ * io_set_plan() when you've initialized it.
+ *
+ * Example:
+ * // Simple helper to read a single char.
+ * static int do_readchar(int fd, struct io_plan_arg *arg)
+ * {
+ * return read(fd, arg->u1.cp, 1) <= 0 ? -1 : 1;
+ * }
+ *
+ * struct io_plan *io_read_char_(struct io_conn *conn, char *in,
+ * struct io_plan *(*next)(struct io_conn*,void*),
+ * void *arg)
+ * {
+ * struct io_plan_arg *arg = io_get_plan_arg(conn, IO_IN);