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- Joystick API Documentation -*-Text-*-
-
- Ragnar Hojland Espinosa
- <ragnar@macula.net>
-
- 7 Aug 1998
-
-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