Reads joystick/gamepad events on Linux and displays them.

joystick.c

/**
 * Author: Jason White
 *
 * Description:
 * Reads joystick/gamepad events and displays them.
 *
 * Compile:
 * gcc joystick.c -o joystick
 *
 * Run:
 * ./joystick [/dev/input/jsX]
 *
 * See also:
 * https://www.kernel.org/doc/Documentation/input/joystick-api.txt
 */
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <linux/joystick.h>


/**
 * Reads a joystick event from the joystick device.
 *
 * Returns 0 on success. Otherwise -1 is returned.
 */
int read_event(int fd, struct js_event *event)
{
    ssize_t bytes;

    bytes = read(fd, event, sizeof(*event));

    if (bytes == sizeof(*event))
        return 0;

    /* Error, could not read full event. */
    return -1;
}

/**
 * Returns the number of axes on the controller or 0 if an error occurs.
 */
size_t get_axis_count(int fd)
{
    __u8 axes;

    if (ioctl(fd, JSIOCGAXES, &axes) == -1)
        return 0;

    return axes;
}

/**
 * Returns the number of buttons on the controller or 0 if an error occurs.
 */
size_t get_button_count(int fd)
{
    __u8 buttons;
    if (ioctl(fd, JSIOCGBUTTONS, &buttons) == -1)
        return 0;

    return buttons;
}

/**
 * Current state of an axis.
 */
struct axis_state {
    short x, y;
};

/**
 * Keeps track of the current axis state.
 *
 * NOTE: This function assumes that axes are numbered starting from 0, and that
 * the X axis is an even number, and the Y axis is an odd number. However, this
 * is usually a safe assumption.
 *
 * Returns the axis that the event indicated.
 */
size_t get_axis_state(struct js_event *event, struct axis_state axes[3])
{
    size_t axis = event->number / 2;

    if (axis < 3)
    {
        if (event->number % 2 == 0)
            axes[axis].x = event->value;
        else
            axes[axis].y = event->value;
    }

    return axis;
}

int main(int argc, char *argv[])
{
    const char *device;
    int js;
    struct js_event event;
    struct axis_state axes[3] = {0};
    size_t axis;

    if (argc > 1)
        device = argv[1];
    else
        device = "/dev/input/js0";

    js = open(device, O_RDONLY);

    if (js == -1)
        perror("Could not open joystick");

    /* This loop will exit if the controller is unplugged. */
    while (read_event(js, &event) == 0)
    {
        switch (event.type)
        {
            case JS_EVENT_BUTTON:
                printf("Button %u %s\n", event.number, event.value ? "pressed" : "released");
                break;
            case JS_EVENT_AXIS:
                axis = get_axis_state(&event, axes);
                if (axis < 3)
                    printf("Axis %zu at (%6d, %6d)\n", axis, axes[axis].x, axes[axis].y);
                break;
            default:
                /* Ignore init events. */
                break;
        }
        
        fflush(stdout);
    }

    close(js);
    return 0;
}

For more modern version and not requiring you to be root for reading gamepad input in linux using evdev generic input event interface with liburing and libevdev libraries, go to my repo https://github.com/e2dk4r/gamepad