1 Joystick API Documentation -*-Text-*-
2
3 Ragnar Hojland Espinosa
4 <ragnar@macula.net>
5
6 7 Aug 1998
7
8 $Id: joystick-api.txt,v 1.2 2001/05/08 21:21:23 vojtech Exp $
9
10 1. Initialization
11 ~~~~~~~~~~~~~~~~~
12
13 Open the joystick device following the usual semantics (that is, with open).
14 Since the driver now reports events instead of polling for changes,
15 immediately after the open it will issue a series of synthetic events
16 (JS_EVENT_INIT) that you can read to check the initial state of the
17 joystick.
18
19 By default, the device is opened in blocking mode.
20
21 int fd = open ("/dev/js0", O_RDONLY);
22
23
24 2. Event Reading
25 ~~~~~~~~~~~~~~~~
26
27 struct js_event e;
28 read (fd, &e, sizeof(struct js_event));
29
30 where js_event is defined as
31
32 struct js_event {
33 __u32 time; /* event timestamp in milliseconds */
34 __s16 value; /* value */
35 __u8 type; /* event type */
36 __u8 number; /* axis/button number */
37 };
38
39 If the read is successful, it will return sizeof(struct js_event), unless
40 you wanted to read more than one event per read as described in section 3.1.
41
42
43 2.1 js_event.type
44 ~~~~~~~~~~~~~~~~~
45
46 The possible values of ``type'' are
47
48 #define JS_EVENT_BUTTON 0x01 /* button pressed/released */
49 #define JS_EVENT_AXIS 0x02 /* joystick moved */
50 #define JS_EVENT_INIT 0x80 /* initial state of device */
51
52 As mentioned above, the driver will issue synthetic JS_EVENT_INIT ORed
53 events on open. That is, if it's issuing a INIT BUTTON event, the
54 current type value will be
55
56 int type = JS_EVENT_BUTTON | JS_EVENT_INIT; /* 0x81 */
57
58 If you choose not to differentiate between synthetic or real events
59 you can turn off the JS_EVENT_INIT bits
60
61 type &= ~JS_EVENT_INIT; /* 0x01 */
62
63
64 2.2 js_event.number
65 ~~~~~~~~~~~~~~~~~~~
66
67 The values of ``number'' correspond to the axis or button that
68 generated the event. Note that they carry separate numeration (that
69 is, you have both an axis 0 and a button 0). Generally,
70
71 number
72 1st Axis X 0
73 1st Axis Y 1
74 2nd Axis X 2
75 2nd Axis Y 3
76 ...and so on
77
78 Hats vary from one joystick type to another. Some can be moved in 8
79 directions, some only in 4, The driver, however, always reports a hat as two
80 independent axis, even if the hardware doesn't allow independent movement.
81
82
83 2.3 js_event.value
84 ~~~~~~~~~~~~~~~~~~
85
86 For an axis, ``value'' is a signed integer between -32767 and +32767
87 representing the position of the joystick along that axis. If you
88 don't read a 0 when the joystick is `dead', or if it doesn't span the
89 full range, you should recalibrate it (with, for example, jscal).
90
91 For a button, ``value'' for a press button event is 1 and for a release
92 button event is 0.
93
94 Though this
95
96 if (js_event.type == JS_EVENT_BUTTON) {
97 buttons_state ^= (1 << js_event.number);
98 }
99
100 may work well if you handle JS_EVENT_INIT events separately,
101
102 if ((js_event.type & ~JS_EVENT_INIT) == JS_EVENT_BUTTON) {
103 if (js_event.value)
104 buttons_state |= (1 << js_event.number);
105 else
106 buttons_state &= ~(1 << js_event.number);
107 }
108
109 is much safer since it can't lose sync with the driver. As you would
110 have to write a separate handler for JS_EVENT_INIT events in the first
111 snippet, this ends up being shorter.
112
113
114 2.4 js_event.time
115 ~~~~~~~~~~~~~~~~~
116
117 The time an event was generated is stored in ``js_event.time''. It's a time
118 in milliseconds since ... well, since sometime in the past. This eases the
119 task of detecting double clicks, figuring out if movement of axis and button
120 presses happened at the same time, and similar.
121
122
123 3. Reading
124 ~~~~~~~~~~
125
126 If you open the device in blocking mode, a read will block (that is,
127 wait) forever until an event is generated and effectively read. There
128 are two alternatives if you can't afford to wait forever (which is,
129 admittedly, a long time;)
130
131 a) use select to wait until there's data to be read on fd, or
132 until it timeouts. There's a good example on the select(2)
133 man page.
134
135 b) open the device in non-blocking mode (O_NONBLOCK)
136
137
138 3.1 O_NONBLOCK
139 ~~~~~~~~~~~~~~
140
141 If read returns -1 when reading in O_NONBLOCK mode, this isn't
142 necessarily a "real" error (check errno(3)); it can just mean there
143 are no events pending to be read on the driver queue. You should read
144 all events on the queue (that is, until you get a -1).
145
146 For example,
147
148 while (1) {
149 while (read (fd, &e, sizeof(struct js_event)) > 0) {
150 process_event (e);
151 }
152 /* EAGAIN is returned when the queue is empty */
153 if (errno != EAGAIN) {
154 /* error */
155 }
156 /* do something interesting with processed events */
157 }
158
159 One reason for emptying the queue is that if it gets full you'll start
160 missing events since the queue is finite, and older events will get
161 overwritten.
162
163 The other reason is that you want to know all what happened, and not
164 delay the processing till later.
165
166 Why can get the queue full? Because you don't empty the queue as
167 mentioned, or because too much time elapses from one read to another
168 and too many events to store in the queue get generated. Note that
169 high system load may contribute to space those reads even more.
170
171 If time between reads is enough to fill the queue and lose an event,
172 the driver will switch to startup mode and next time you read it,
173 synthetic events (JS_EVENT_INIT) will be generated to inform you of
174 the actual state of the joystick.
175
176 [As for version 1.2.8, the queue is circular and able to hold 64
177 events. You can increment this size bumping up JS_BUFF_SIZE in
178 joystick.h and recompiling the driver.]
179
180
181 In the above code, you might as well want to read more than one event
182 at a time using the typical read(2) functionality. For that, you would
183 replace the read above with something like
184
185 struct js_event mybuffer[0xff];
186 int i = read (fd, mybuffer, sizeof(struct mybuffer));
187
188 In this case, read would return -1 if the queue was empty, or some
189 other value in which the number of events read would be i /
190 sizeof(js_event) Again, if the buffer was full, it's a good idea to
191 process the events and keep reading it until you empty the driver queue.
192
193
194 4. IOCTLs
195 ~~~~~~~~~
196
197 The joystick driver defines the following ioctl(2) operations.
198
199 /* function 3rd arg */
200 #define JSIOCGAXES /* get number of axes char */
201 #define JSIOCGBUTTONS /* get number of buttons char */
202 #define JSIOCGVERSION /* get driver version int */
203 #define JSIOCGNAME(len) /* get identifier string char */
204 #define JSIOCSCORR /* set correction values &js_corr */
205 #define JSIOCGCORR /* get correction values &js_corr */
206
207 For example, to read the number of axes
208
209 char number_of_axes;
210 ioctl (fd, JSIOCGAXES, &number_of_axes);
211
212
213 4.1 JSIOGCVERSION
214 ~~~~~~~~~~~~~~~~~
215
216 JSIOGCVERSION is a good way to check in run-time whether the running
217 driver is 1.0+ and supports the event interface. If it is not, the
218 IOCTL will fail. For a compile-time decision, you can test the
219 JS_VERSION symbol
220
221 #ifdef JS_VERSION
222 #if JS_VERSION > 0xsomething
223
224
225 4.2 JSIOCGNAME
226 ~~~~~~~~~~~~~~
227
228 JSIOCGNAME(len) allows you to get the name string of the joystick - the same
229 as is being printed at boot time. The 'len' argument is the length of the
230 buffer provided by the application asking for the name. It is used to avoid
231 possible overrun should the name be too long.
232
233 char name[128];
234 if (ioctl(fd, JSIOCGNAME(sizeof(name)), name) < 0)
235 strncpy(name, "Unknown", sizeof(name));
236 printf("Name: %s\n", name);
237
238
239 4.3 JSIOC[SG]CORR
240 ~~~~~~~~~~~~~~~~~
241
242 For usage on JSIOC[SG]CORR I suggest you to look into jscal.c They are
243 not needed in a normal program, only in joystick calibration software
244 such as jscal or kcmjoy. These IOCTLs and data types aren't considered
245 to be in the stable part of the API, and therefore may change without
246 warning in following releases of the driver.
247
248 Both JSIOCSCORR and JSIOCGCORR expect &js_corr to be able to hold
249 information for all axis. That is, struct js_corr corr[MAX_AXIS];
250
251 struct js_corr is defined as
252
253 struct js_corr {
254 __s32 coef[8];
255 __u16 prec;
256 __u16 type;
257 };
258
259 and ``type''
260
261 #define JS_CORR_NONE 0x00 /* returns raw values */
262 #define JS_CORR_BROKEN 0x01 /* broken line */
263
264
265 5. Backward compatibility
266 ~~~~~~~~~~~~~~~~~~~~~~~~~
267
268 The 0.x joystick driver API is quite limited and its usage is deprecated.
269 The driver offers backward compatibility, though. Here's a quick summary:
270
271 struct JS_DATA_TYPE js;
272 while (1) {
273 if (read (fd, &js, JS_RETURN) != JS_RETURN) {
274 /* error */
275 }
276 usleep (1000);
277 }
278
279 As you can figure out from the example, the read returns immediately,
280 with the actual state of the joystick.
281
282 struct JS_DATA_TYPE {
283 int buttons; /* immediate button state */
284 int x; /* immediate x axis value */
285 int y; /* immediate y axis value */
286 };
287
288 and JS_RETURN is defined as
289
290 #define JS_RETURN sizeof(struct JS_DATA_TYPE)
291
292 To test the state of the buttons,
293
294 first_button_state = js.buttons & 1;
295 second_button_state = js.buttons & 2;
296
297 The axis values do not have a defined range in the original 0.x driver,
298 except for that the values are non-negative. The 1.2.8+ drivers use a
299 fixed range for reporting the values, 1 being the minimum, 128 the
300 center, and 255 maximum value.
301
302 The v0.8.0.2 driver also had an interface for 'digital joysticks', (now
303 called Multisystem joysticks in this driver), under /dev/djsX. This driver
304 doesn't try to be compatible with that interface.
305
306
307 6. Final Notes
308 ~~~~~~~~~~~~~~
309
310 ____/| Comments, additions, and specially corrections are welcome.
311 \ o.O| Documentation valid for at least version 1.2.8 of the joystick
312 =(_)= driver and as usual, the ultimate source for documentation is
313 U to "Use The Source Luke" or, at your convenience, Vojtech ;)
314
315 - Ragnar
316 EOF
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