1 Joystick API Documentation -*-Text-*-
3 Ragnar Hojland Espinosa
11 Open the joystick device following the usual semantics (that is, with open).
12 Since the driver now reports events instead of polling for changes,
13 immediately after the open it will issue a series of synthetic events
14 (JS_EVENT_INIT) that you can read to check the initial state of the
17 By default, the device is opened in blocking mode.
19 int fd = open ("/dev/input/js0", O_RDONLY);
26 read (fd, &e, sizeof(struct js_event));
28 where js_event is defined as
31 __u32 time; /* event timestamp in milliseconds */
32 __s16 value; /* value */
33 __u8 type; /* event type */
34 __u8 number; /* axis/button number */
37 If the read is successful, it will return sizeof(struct js_event), unless
38 you wanted to read more than one event per read as described in section 3.1.
44 The possible values of ``type'' are
46 #define JS_EVENT_BUTTON 0x01 /* button pressed/released */
47 #define JS_EVENT_AXIS 0x02 /* joystick moved */
48 #define JS_EVENT_INIT 0x80 /* initial state of device */
50 As mentioned above, the driver will issue synthetic JS_EVENT_INIT ORed
51 events on open. That is, if it's issuing a INIT BUTTON event, the
52 current type value will be
54 int type = JS_EVENT_BUTTON | JS_EVENT_INIT; /* 0x81 */
56 If you choose not to differentiate between synthetic or real events
57 you can turn off the JS_EVENT_INIT bits
59 type &= ~JS_EVENT_INIT; /* 0x01 */
65 The values of ``number'' correspond to the axis or button that
66 generated the event. Note that they carry separate numeration (that
67 is, you have both an axis 0 and a button 0). Generally,
76 Hats vary from one joystick type to another. Some can be moved in 8
77 directions, some only in 4, The driver, however, always reports a hat as two
78 independent axis, even if the hardware doesn't allow independent movement.
84 For an axis, ``value'' is a signed integer between -32767 and +32767
85 representing the position of the joystick along that axis. If you
86 don't read a 0 when the joystick is `dead', or if it doesn't span the
87 full range, you should recalibrate it (with, for example, jscal).
89 For a button, ``value'' for a press button event is 1 and for a release
94 if (js_event.type == JS_EVENT_BUTTON) {
95 buttons_state ^= (1 << js_event.number);
98 may work well if you handle JS_EVENT_INIT events separately,
100 if ((js_event.type & ~JS_EVENT_INIT) == JS_EVENT_BUTTON) {
102 buttons_state |= (1 << js_event.number);
104 buttons_state &= ~(1 << js_event.number);
107 is much safer since it can't lose sync with the driver. As you would
108 have to write a separate handler for JS_EVENT_INIT events in the first
109 snippet, this ends up being shorter.
115 The time an event was generated is stored in ``js_event.time''. It's a time
116 in milliseconds since ... well, since sometime in the past. This eases the
117 task of detecting double clicks, figuring out if movement of axis and button
118 presses happened at the same time, and similar.
124 If you open the device in blocking mode, a read will block (that is,
125 wait) forever until an event is generated and effectively read. There
126 are two alternatives if you can't afford to wait forever (which is,
127 admittedly, a long time;)
129 a) use select to wait until there's data to be read on fd, or
130 until it timeouts. There's a good example on the select(2)
133 b) open the device in non-blocking mode (O_NONBLOCK)
139 If read returns -1 when reading in O_NONBLOCK mode, this isn't
140 necessarily a "real" error (check errno(3)); it can just mean there
141 are no events pending to be read on the driver queue. You should read
142 all events on the queue (that is, until you get a -1).
147 while (read (fd, &e, sizeof(struct js_event)) > 0) {
150 /* EAGAIN is returned when the queue is empty */
151 if (errno != EAGAIN) {
154 /* do something interesting with processed events */
157 One reason for emptying the queue is that if it gets full you'll start
158 missing events since the queue is finite, and older events will get
161 The other reason is that you want to know all what happened, and not
162 delay the processing till later.
164 Why can get the queue full? Because you don't empty the queue as
165 mentioned, or because too much time elapses from one read to another
166 and too many events to store in the queue get generated. Note that
167 high system load may contribute to space those reads even more.
169 If time between reads is enough to fill the queue and lose an event,
170 the driver will switch to startup mode and next time you read it,
171 synthetic events (JS_EVENT_INIT) will be generated to inform you of
172 the actual state of the joystick.
174 [As for version 1.2.8, the queue is circular and able to hold 64
175 events. You can increment this size bumping up JS_BUFF_SIZE in
176 joystick.h and recompiling the driver.]
179 In the above code, you might as well want to read more than one event
180 at a time using the typical read(2) functionality. For that, you would
181 replace the read above with something like
183 struct js_event mybuffer[0xff];
184 int i = read (fd, mybuffer, sizeof(struct mybuffer));
186 In this case, read would return -1 if the queue was empty, or some
187 other value in which the number of events read would be i /
188 sizeof(js_event) Again, if the buffer was full, it's a good idea to
189 process the events and keep reading it until you empty the driver queue.
195 The joystick driver defines the following ioctl(2) operations.
197 /* function 3rd arg */
198 #define JSIOCGAXES /* get number of axes char */
199 #define JSIOCGBUTTONS /* get number of buttons char */
200 #define JSIOCGVERSION /* get driver version int */
201 #define JSIOCGNAME(len) /* get identifier string char */
202 #define JSIOCSCORR /* set correction values &js_corr */
203 #define JSIOCGCORR /* get correction values &js_corr */
205 For example, to read the number of axes
208 ioctl (fd, JSIOCGAXES, &number_of_axes);
214 JSIOGCVERSION is a good way to check in run-time whether the running
215 driver is 1.0+ and supports the event interface. If it is not, the
216 IOCTL will fail. For a compile-time decision, you can test the
220 #if JS_VERSION > 0xsomething
226 JSIOCGNAME(len) allows you to get the name string of the joystick - the same
227 as is being printed at boot time. The 'len' argument is the length of the
228 buffer provided by the application asking for the name. It is used to avoid
229 possible overrun should the name be too long.
232 if (ioctl(fd, JSIOCGNAME(sizeof(name)), name) < 0)
233 strncpy(name, "Unknown", sizeof(name));
234 printf("Name: %s\n", name);
240 For usage on JSIOC[SG]CORR I suggest you to look into jscal.c They are
241 not needed in a normal program, only in joystick calibration software
242 such as jscal or kcmjoy. These IOCTLs and data types aren't considered
243 to be in the stable part of the API, and therefore may change without
244 warning in following releases of the driver.
246 Both JSIOCSCORR and JSIOCGCORR expect &js_corr to be able to hold
247 information for all axis. That is, struct js_corr corr[MAX_AXIS];
249 struct js_corr is defined as
259 #define JS_CORR_NONE 0x00 /* returns raw values */
260 #define JS_CORR_BROKEN 0x01 /* broken line */
263 5. Backward compatibility
264 ~~~~~~~~~~~~~~~~~~~~~~~~~
266 The 0.x joystick driver API is quite limited and its usage is deprecated.
267 The driver offers backward compatibility, though. Here's a quick summary:
269 struct JS_DATA_TYPE js;
271 if (read (fd, &js, JS_RETURN) != JS_RETURN) {
277 As you can figure out from the example, the read returns immediately,
278 with the actual state of the joystick.
280 struct JS_DATA_TYPE {
281 int buttons; /* immediate button state */
282 int x; /* immediate x axis value */
283 int y; /* immediate y axis value */
286 and JS_RETURN is defined as
288 #define JS_RETURN sizeof(struct JS_DATA_TYPE)
290 To test the state of the buttons,
292 first_button_state = js.buttons & 1;
293 second_button_state = js.buttons & 2;
295 The axis values do not have a defined range in the original 0.x driver,
296 except for that the values are non-negative. The 1.2.8+ drivers use a
297 fixed range for reporting the values, 1 being the minimum, 128 the
298 center, and 255 maximum value.
300 The v0.8.0.2 driver also had an interface for 'digital joysticks', (now
301 called Multisystem joysticks in this driver), under /dev/djsX. This driver
302 doesn't try to be compatible with that interface.
308 ____/| Comments, additions, and specially corrections are welcome.
309 \ o.O| Documentation valid for at least version 1.2.8 of the joystick
310 =(_)= driver and as usual, the ultimate source for documentation is
311 U to "Use The Source Luke" or, at your convenience, Vojtech ;)