mysql> \d $$
mysql> DROP PROCEDURE IF EXISTS p5 $$
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> CREATE PROCEDURE p5 (IN v_in INT, INOUT v_io INT, OUT v_out INT)
    ->   BEGIN
    ->     SELECT v_in, v_io, v_out;
    ->     SET v_in = 10;
    ->     SET v_io = 10;
    ->     SET v_out = 10;
    ->     SELECT v_in, v_io, v_out;
    ->   END;
    -> $$
Query OK, 0 rows affected (0.00 sec)

mysql> \d ;
mysql> SET @a = 5; SET @b = 5; SET @c = 5;
Query OK, 0 rows affected (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

mysql> CALL p5(@a, @b, @c);
+------+------+-------+
| v_in | v_io | v_out |
+------+------+-------+
|    5 |    5 |  NULL | 
+------+------+-------+
1 row in set (0.00 sec)

+------+------+-------+
| v_in | v_io | v_out |
+------+------+-------+
|   10 |   10 |    10 | 
+------+------+-------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @a, @b, @c;
+------+------+------+
| @a   | @b   | @c   |
+------+------+------+
| 5    | 10   | 10   | 
+------+------+------+
1 row in set (0.00 sec)