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                      Joystick API Documentation                -*-Text-*-

                        Ragnar Hojland Espinosa

                              7 Aug 1998

        $Id: joystick-api.txt,v 1.2 2001/05/08 21:21:23 vojtech Exp $

1. Initialization

~~~~~~~~~~~~~~~~~

Open the joystick device following the usual semantics (that is, with open).

Since the driver now reports events instead of polling for changes,

immediately after the open it will issue a series of synthetic events

(JS_EVENT_INIT) that you can read to check the initial state of the

joystick.

By default, the device is opened in blocking mode.

        int fd = open ("/dev/js0", O_RDONLY);

2. Event Reading

~~~~~~~~~~~~~~~~

        struct js_event e;

        read (fd, &e, sizeof(struct js_event));

where js_event is defined as

        struct js_event {

                __u32 time;     /* event timestamp in milliseconds */

                __s16 value;    /* value */

                __u8 type;      /* event type */

                __u8 number;    /* axis/button number */

        };

If the read is successful, it will return sizeof(struct js_event), unless

you wanted to read more than one event per read as described in section 3.1.

2.1 js_event.type

~~~~~~~~~~~~~~~~~

The possible values of ``type'' are

        #define JS_EVENT_BUTTON         0x01    /* button pressed/released */

        #define JS_EVENT_AXIS           0x02    /* joystick moved */

        #define JS_EVENT_INIT           0x80    /* initial state of device */

As mentioned above, the driver will issue synthetic JS_EVENT_INIT ORed

events on open. That is, if it's issuing a INIT BUTTON event, the

current type value will be

        int type = JS_EVENT_BUTTON | JS_EVENT_INIT;     /* 0x81 */

If you choose not to differentiate between synthetic or real events

you can turn off the JS_EVENT_INIT bits

        type &= ~JS_EVENT_INIT;                          /* 0x01 */

2.2 js_event.number

~~~~~~~~~~~~~~~~~~~

The values of ``number'' correspond to the axis or button that

generated the event. Note that they carry separate numeration (that

is, you have both an axis 0 and a button 0). Generally,

                        number

        1st Axis X      0

        1st Axis Y      1

        2nd Axis X      2

        2nd Axis Y      3

        ...and so on

Hats vary from one joystick type to another. Some can be moved in 8

directions, some only in 4, The driver, however, always reports a hat as two

independent axis, even if the hardware doesn't allow independent movement.

2.3 js_event.value

~~~~~~~~~~~~~~~~~~

For an axis, ``value'' is a signed integer between -32767 and +32767

representing the position of the joystick along that axis. If you

don't read a 0 when the joystick is `dead', or if it doesn't span the

full range, you should recalibrate it (with, for example, jscal).

For a button, ``value'' for a press button event is 1 and for a release

button event is 0.

Though this

        if (js_event.type == JS_EVENT_BUTTON) {

                buttons_state ^= (1 << js_event.number);

        }

may work well if you handle JS_EVENT_INIT events separately,

        if ((js_event.type & ~JS_EVENT_INIT) == JS_EVENT_BUTTON) {

                if (js_event.value)

                        buttons_state |= (1 << js_event.number);

                else

                        buttons_state &= ~(1 << js_event.number);

        }

is much safer since it can't lose sync with the driver. As you would

have to write a separate handler for JS_EVENT_INIT events in the first

snippet, this ends up being shorter.

2.4 js_event.time

~~~~~~~~~~~~~~~~~

The time an event was generated is stored in ``js_event.time''. It's a time

in milliseconds since ... well, since sometime in the past.  This eases the

task of detecting double clicks, figuring out if movement of axis and button

presses happened at the same time, and similar.

3. Reading

~~~~~~~~~~

If you open the device in blocking mode, a read will block (that is,

wait) forever until an event is generated and effectively read. There

are two alternatives if you can't afford to wait forever (which is,

admittedly, a long time;)

        a) use select to wait until there's data to be read on fd, or

           until it timeouts. There's a good example on the select(2)

           man page.

        b) open the device in non-blocking mode (O_NONBLOCK)

3.1 O_NONBLOCK

~~~~~~~~~~~~~~

If read returns -1 when reading in O_NONBLOCK mode, this isn't

necessarily a "real" error (check errno(3)); it can just mean there

are no events pending to be read on the driver queue. You should read

all events on the queue (that is, until you get a -1).

For example,

        while (1) {

                while (read (fd, &e, sizeof(struct js_event)) > 0) {

                        process_event (e);

                }

                /* EAGAIN is returned when the queue is empty */

                if (errno != EAGAIN) {

                        /* error */

                }

                /* do something interesting with processed events */

        }

One reason for emptying the queue is that if it gets full you'll start

missing events since the queue is finite, and older events will get

overwritten.

The other reason is that you want to know all what happened, and not

delay the processing till later.

Why can get the queue full? Because you don't empty the queue as

mentioned, or because too much time elapses from one read to another

and too many events to store in the queue get generated. Note that

high system load may contribute to space those reads even more.

If time between reads is enough to fill the queue and lose an event,

the driver will switch to startup mode and next time you read it,

synthetic events (JS_EVENT_INIT) will be generated to inform you of

the actual state of the joystick.

[As for version 1.2.8, the queue is circular and able to hold 64

 events. You can increment this size bumping up JS_BUFF_SIZE in

 joystick.h and recompiling the driver.]

In the above code, you might as well want to read more than one event

at a time using the typical read(2) functionality. For that, you would

replace the read above with something like

        struct js_event mybuffer[0xff];

        int i = read (fd, mybuffer, sizeof(struct mybuffer));

In this case, read would return -1 if the queue was empty, or some

other value in which the number of events read would be i /

sizeof(js_event)  Again, if the buffer was full, it's a good idea to

process the events and keep reading it until you empty the driver queue.

4. IOCTLs

~~~~~~~~~

The joystick driver defines the following ioctl(2) operations.

                                /* function                     3rd arg  */

        #define JSIOCGAXES      /* get number of axes           char     */

        #define JSIOCGBUTTONS   /* get number of buttons        char     */

        #define JSIOCGVERSION   /* get driver version           int      */

        #define JSIOCGNAME(len) /* get identifier string        char     */

        #define JSIOCSCORR      /* set correction values        &js_corr */

        #define JSIOCGCORR      /* get correction values        &js_corr */

For example, to read the number of axes

        char number_of_axes;

        ioctl (fd, JSIOCGAXES, &number_of_axes);

4.1 JSIOGCVERSION

~~~~~~~~~~~~~~~~~

JSIOGCVERSION is a good way to check in run-time whether the running

driver is 1.0+ and supports the event interface. If it is not, the

IOCTL will fail. For a compile-time decision, you can test the

JS_VERSION symbol

        #ifdef JS_VERSION

        #if JS_VERSION > 0xsomething

4.2 JSIOCGNAME

~~~~~~~~~~~~~~

JSIOCGNAME(len) allows you to get the name string of the joystick - the same

as is being printed at boot time. The 'len' argument is the length of the

buffer provided by the application asking for the name. It is used to avoid

possible overrun should the name be too long.

        char name[128];

        if (ioctl(fd, JSIOCGNAME(sizeof(name)), name) < 0)

                strncpy(name, "Unknown", sizeof(name));

        printf("Name: %s\n", name);

4.3 JSIOC[SG]CORR

~~~~~~~~~~~~~~~~~

For usage on JSIOC[SG]CORR I suggest you to look into jscal.c  They are

not needed in a normal program, only in joystick calibration software

such as jscal or kcmjoy. These IOCTLs and data types aren't considered

to be in the stable part of the API, and therefore may change without

warning in following releases of the driver.

Both JSIOCSCORR and JSIOCGCORR expect &js_corr to be able to hold

information for all axis. That is, struct js_corr corr[MAX_AXIS];

struct js_corr is defined as

        struct js_corr {

                __s32 coef[8];

                __u16 prec;

                __u16 type;

        };

and ``type''

        #define JS_CORR_NONE            0x00    /* returns raw values */

        #define JS_CORR_BROKEN          0x01    /* broken line */

5. Backward compatibility

~~~~~~~~~~~~~~~~~~~~~~~~~

The 0.x joystick driver API is quite limited and its usage is deprecated.

The driver offers backward compatibility, though. Here's a quick summary:

        struct JS_DATA_TYPE js;

        while (1) {

                if (read (fd, &js, JS_RETURN) != JS_RETURN) {

                        /* error */

                }

                usleep (1000);

        }

As you can figure out from the example, the read returns immediately,

with the actual state of the joystick.

        struct JS_DATA_TYPE {

                int buttons;    /* immediate button state */

                int x;          /* immediate x axis value */

                int y;          /* immediate y axis value */

        };

and JS_RETURN is defined as

        #define JS_RETURN       sizeof(struct JS_DATA_TYPE)

To test the state of the buttons,

        first_button_state  = js.buttons & 1;

        second_button_state = js.buttons & 2;

The axis values do not have a defined range in the original 0.x driver,

except for that the values are non-negative. The 1.2.8+ drivers use a

fixed range for reporting the values, 1 being the minimum, 128 the

center, and 255 maximum value.

The v0.8.0.2 driver also had an interface for 'digital joysticks', (now

called Multisystem joysticks in this driver), under /dev/djsX. This driver

doesn't try to be compatible with that interface.

6. Final Notes

~~~~~~~~~~~~~~

____/|  Comments, additions, and specially corrections are welcome.

\ o.O|  Documentation valid for at least version 1.2.8 of the joystick

 =(_)=  driver and as usual, the ultimate source for documentation is

   U    to "Use The Source Luke" or, at your convenience, Vojtech ;)

                                        - Ragnar

EOF