gamepad_tool.cpp

#include <cstdlib>
#include <cstring>
#include <string>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <unistd.h>
#include <fcntl.h>
#include <libudev.h>
#include <libevdev-1.0/libevdev/libevdev.h>


#define LBITS (sizeof(long) * 8)
#define NLONGS(x) (((x) - 1) / LBITS + 1)
#define TEST_BIT(v, i) !!((v)[(i) / LBITS] & (1L << ((i) % LBITS)))

static void writeLEShort(std::ostream& s, unsigned short v) {
    s << std::setw(2) << (v & 0xff) << std::setw(2) << ((v >> 8) & 0xff);
}
std::string getGUID(struct libevdev* edev) {
    std::stringstream ss;
    ss << std::hex << std::setfill('0');
    writeLEShort(ss, (unsigned short) libevdev_get_id_bustype(edev));
    writeLEShort(ss, 0);
    writeLEShort(ss, (unsigned short) libevdev_get_id_vendor(edev));
    writeLEShort(ss, 0);
    writeLEShort(ss, (unsigned short) libevdev_get_id_product(edev));
    writeLEShort(ss, 0);
    writeLEShort(ss, (unsigned short) libevdev_get_id_version(edev));
    writeLEShort(ss, 0);
    return ss.str();
}

inline bool isHat(int index) {
    return (index >= ABS_HAT0X && index <= ABS_HAT3Y);
}
void handleDev(struct udev_device* dev) {
    const char* val = udev_device_get_property_value(dev, "ID_INPUT_JOYSTICK");
    if (val != nullptr && strcmp(val, "1") == 0) {
        const char* devPath = udev_device_get_devnode(dev);
        if (devPath == nullptr)
            return;

        int fd = open(devPath, O_RDONLY | O_NONBLOCK);
        struct libevdev* edev = nullptr;
        int err = libevdev_new_from_fd(fd, &edev);
        if (err) {
            printf("libevdev_new_from_fd error %i (%s)\n", err, devPath);
            close(fd);
            return;
        }
        printf("%s\n", getGUID(edev).c_str());
        unsigned long abs[NLONGS(ABS_CNT)];
        int len = ioctl(fd, EVIOCGBIT(EV_ABS, sizeof(abs)), abs);
        if (len < 0)
            throw std::runtime_error("Failed to get joystick abs");
        len *= 8;
        int nextId = 0;
        for (size_t i = 0; i < len; i++) {
            if (!TEST_BIT(abs, i)) {
                continue;
            }
            const input_absinfo* absinfo = libevdev_get_abs_info(edev, i);
            if (absinfo == nullptr)
                continue;
            int id = isHat(i) ? 0 : (nextId++);
            if (!isHat(i))
              printf("id=%i min=%i max=%i flat=%i fuzz=%i\n", id, absinfo->minimum, absinfo->maximum, absinfo->flat, absinfo->fuzz);
        }
        libevdev_free(edev);
    }
}

int main() {
    struct udev* udev = udev_new();
    struct udev_enumerate* enumerate = udev_enumerate_new(udev);
    udev_enumerate_add_match_subsystem(enumerate, "input");
    udev_enumerate_scan_devices(enumerate);
    struct udev_list_entry* devices = udev_enumerate_get_list_entry(enumerate);
    struct udev_list_entry* dev_list_entry;
    udev_list_entry_foreach(dev_list_entry, devices) {
        const char* path = udev_list_entry_get_name(dev_list_entry);
        struct udev_device* dev = udev_device_new_from_syspath(udev, path);
        if (dev == nullptr)
            continue;
        handleDev(dev);
        udev_device_unref(dev);
    }
    udev_enumerate_unref(enumerate);
    return 0;
}

ln 59: for (size_t i = 0; i < len; i++) {

should probably be "i <= len", id=0 doesn't print a value in the loop.
on my usb joystick
id=0 min=-32768 max=32767 flat=128 fuzz=16

I don't think the suggested patch matches much sense, albeit this doesn't mean there's no bug in the code -- I'm not confident whether the result of ioctl is even supposed to be used now that I am reviewing the code; maybe it should be replaced with a generic loop to NLONGS(ABS_CNT) * 8.