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Joystick API Documentation -*-Text-*-
Ragnar Hojland Espinosa
7 Aug 1998
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
By default, the device is opened in blocking mode.
int fd = open ("/dev/input/js0", O_RDONLY);
2. Event Reading
struct js_event e;
read (fd, &e, sizeof(e));
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(e), 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,
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);
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)
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(e)) > 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
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(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.
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);
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
#if JS_VERSION > 0xsomething
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);
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