Documentation/input/joystick-api.txt

   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