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+                      Joystick API Documentation                -*-Text-*-
+
+                        Ragnar Hojland Espinosa
+                          <ragnar@macula.net>
+
+                              7 Aug 1998
+
+        $Id: joystick-api.txt,v 1.2 2001/05/08 21:21:23 vojtech Exp $
+
+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