Programming with Joystick on Linux

Was trying to figure out a way to programatically use the Cyborg Controller lying around in the lab. My final objective is to control objects and view points in RViz. In this blog, I will explore how to get inputs from it with a C/C++ program.

cyborg-evo

The Linux kernel provides an API to control this. As everything else, a joystick is also treated as a file. You can see your device listed in /dev/input. It is actually a very easy to use API. The documentation can be found at : https://www.kernel.org/doc/Documentation/input/joystick-api.txt

Sample Program : [.cpp]

Compilation Instruction :

Just compile the file with g++, no special flags needed.


#include <stdio.h>

#include <sys/types.h>

#include <sys/stat.h>

#include <fcntl.h>

#include <unistd.h>

#include <stdlib.h>

#include <string.h>

Include the standard Linux headers.


struct js_event {

        unsigned int time;      /* event timestamp in milliseconds */

        short value;   /* value */

        unsigned char type;     /* event type */

        unsigned char number;   /* axis/button number */

};

A struct to receive an event data into.


#define JS_EVENT_BUTTON         0x01    /* button pressed/released */

#define JS_EVENT_AXIS           0x02    /* joystick moved */

#define JS_EVENT_INIT           0x80    /* initial state of device */

Joystick produces 2 types of events (struct js_event.type) viz, button events and axis events.


int main()

{

	int fd = open ("/dev/input/js0", O_RDONLY);

	if( fd < 0 )

		printf( "cannot open dev\n" );

	else

		printf( "opened success...:)\n" );

Make sure the device can be opened.


	struct js_event e;

	while( 1 ) //event loop

	{

		read( fd, &e, sizeof(e) );

		//printf( "%d %d %d %d\n", e.time, e.value, e.type, e.number );

		if( e.type == JS_EVENT_BUTTON || e.type == JS_EVENT_AXIS )

		{

			if( e.type == JS_EVENT_BUTTON )

				printf( "button#%d value:%d\n", (int) e.number, e.value );

			else

				printf( "axis#%d value:%d\n", (int) e.number, e.value );

		}

		else

		{

			printf( "Init Events\n" );

		}

	}

	return 0;

}

The while loop makes a call to read. Note that this call is a blocking call. Which means, the function read() does not return unless there is an event. This behavior is undesirable if using this code into a ros-node (spinOnce() loop). For non-blocking make the open() call as —

Caution: The non-blocking calls is actually polling the device. One should use a sleep() call in the while-loop to control the polling rate.

6 thoughts on “Programming with Joystick on Linux

  1. Is this applicable to all kind of joystick devices, and how about the different input buttons, should we use “xev” for monitoring their input id’s or something else?

  2. So I can use this source code for reading the input of an Xbox360 controller, assign it as a variable with a while an make something happen like, when “X” is pressed, start a counter or print: Hello, this is X?

  3. Hello, my question is unrelated to this specific source code but to this very subject of joysticks, I have two joysticks of the same type, so they produce the same code number or name, I want to use them both, the first one normally and the second remapped.
    thank you

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