Based on kernel version 4.10.8. Page generated on 2017-04-01 14:43 EST.
1 Joystick API Documentation -*-Text-*- 2 3 Ragnar Hojland Espinosa 4 <ragnar@macula.net> 5 6 7 Aug 1998 7 8 1. Initialization 9 ~~~~~~~~~~~~~~~~~ 10 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 15 joystick. 16 17 By default, the device is opened in blocking mode. 18 19 int fd = open ("/dev/input/js0", O_RDONLY); 20 21 22 2. Event Reading 23 ~~~~~~~~~~~~~~~~ 24 25 struct js_event e; 26 read (fd, &e, sizeof(e)); 27 28 where js_event is defined as 29 30 struct js_event { 31 __u32 time; /* event timestamp in milliseconds */ 32 __s16 value; /* value */ 33 __u8 type; /* event type */ 34 __u8 number; /* axis/button number */ 35 }; 36 37 If the read is successful, it will return sizeof(e), unless you wanted to read 38 more than one event per read as described in section 3.1. 39 40 41 2.1 js_event.type 42 ~~~~~~~~~~~~~~~~~ 43 44 The possible values of ``type'' are 45 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 */ 49 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 53 54 int type = JS_EVENT_BUTTON | JS_EVENT_INIT; /* 0x81 */ 55 56 If you choose not to differentiate between synthetic or real events 57 you can turn off the JS_EVENT_INIT bits 58 59 type &= ~JS_EVENT_INIT; /* 0x01 */ 60 61 62 2.2 js_event.number 63 ~~~~~~~~~~~~~~~~~~~ 64 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, 68 69 number 70 1st Axis X 0 71 1st Axis Y 1 72 2nd Axis X 2 73 2nd Axis Y 3 74 ...and so on 75 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. 79 80 81 2.3 js_event.value 82 ~~~~~~~~~~~~~~~~~~ 83 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). 88 89 For a button, ``value'' for a press button event is 1 and for a release 90 button event is 0. 91 92 Though this 93 94 if (js_event.type == JS_EVENT_BUTTON) { 95 buttons_state ^= (1 << js_event.number); 96 } 97 98 may work well if you handle JS_EVENT_INIT events separately, 99 100 if ((js_event.type & ~JS_EVENT_INIT) == JS_EVENT_BUTTON) { 101 if (js_event.value) 102 buttons_state |= (1 << js_event.number); 103 else 104 buttons_state &= ~(1 << js_event.number); 105 } 106 107 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. 110 111 112 2.4 js_event.time 113 ~~~~~~~~~~~~~~~~~ 114 115 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. 119 120 121 3. Reading 122 ~~~~~~~~~~ 123 124 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;) 128 129 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) 131 man page. 132 133 b) open the device in non-blocking mode (O_NONBLOCK) 134 135 136 3.1 O_NONBLOCK 137 ~~~~~~~~~~~~~~ 138 139 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). 143 144 For example, 145 146 while (1) { 147 while (read (fd, &e, sizeof(e)) > 0) { 148 process_event (e); 149 } 150 /* EAGAIN is returned when the queue is empty */ 151 if (errno != EAGAIN) { 152 /* error */ 153 } 154 /* do something interesting with processed events */ 155 } 156 157 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 159 overwritten. 160 161 The other reason is that you want to know all what happened, and not 162 delay the processing till later. 163 164 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. 168 169 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. 173 174 [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.] 177 178 179 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 182 183 struct js_event mybuffer[0xff]; 184 int i = read (fd, mybuffer, sizeof(mybuffer)); 185 186 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. 190 191 192 4. IOCTLs 193 ~~~~~~~~~ 194 195 The joystick driver defines the following ioctl(2) operations. 196 197 /* 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 */ 204 205 For example, to read the number of axes 206 207 char number_of_axes; 208 ioctl (fd, JSIOCGAXES, &number_of_axes); 209 210 211 4.1 JSIOGCVERSION 212 ~~~~~~~~~~~~~~~~~ 213 214 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 217 JS_VERSION symbol 218 219 #ifdef JS_VERSION 220 #if JS_VERSION > 0xsomething 221 222 223 4.2 JSIOCGNAME 224 ~~~~~~~~~~~~~~ 225 226 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. 230 231 char name[128]; 232 if (ioctl(fd, JSIOCGNAME(sizeof(name)), name) < 0) 233 strncpy(name, "Unknown", sizeof(name)); 234 printf("Name: %s\n", name); 235 236 237 4.3 JSIOC[SG]CORR 238 ~~~~~~~~~~~~~~~~~ 239 240 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. 245 246 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]; 248 249 struct js_corr is defined as 250 251 struct js_corr { 252 __s32 coef[8]; 253 __u16 prec; 254 __u16 type; 255 }; 256 257 and ``type'' 258 259 #define JS_CORR_NONE 0x00 /* returns raw values */ 260 #define JS_CORR_BROKEN 0x01 /* broken line */ 261 262 263 5. Backward compatibility 264 ~~~~~~~~~~~~~~~~~~~~~~~~~ 265 266 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: 268 269 struct JS_DATA_TYPE js; 270 while (1) { 271 if (read (fd, &js, JS_RETURN) != JS_RETURN) { 272 /* error */ 273 } 274 usleep (1000); 275 } 276 277 As you can figure out from the example, the read returns immediately, 278 with the actual state of the joystick. 279 280 struct JS_DATA_TYPE { 281 int buttons; /* immediate button state */ 282 int x; /* immediate x axis value */ 283 int y; /* immediate y axis value */ 284 }; 285 286 and JS_RETURN is defined as 287 288 #define JS_RETURN sizeof(struct JS_DATA_TYPE) 289 290 To test the state of the buttons, 291 292 first_button_state = js.buttons & 1; 293 second_button_state = js.buttons & 2; 294 295 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. 299 300 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. 303 304 305 6. Final Notes 306 ~~~~~~~~~~~~~~ 307 308 ____/| 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 ;) 312 313 - Ragnar 314 EOF