inotify inconsistent with devices - c++

Question

On Linux, I am trying to detect a bluetooth controller being connected and start reading from it. I know there's SDL to do that, but I just wanted to learn how to do it specifically on Linux. So I'm using the inotify api to wait for the file /dev/input/js0 to show up. But when I detect the file I cannot open it. I have the following c++ code:
#include <iostream>
#include <fstream>
#include <sys/inotify.h>
#include <unistd.h>
#include <linux/joystick.h>
#include <string.h>
constexpr int NAME_MAX = 16;
int main(int argc, char **argv) {
std::string path = std::string(argv[1]);
std::string directory = path.substr(0, path.find_last_of("/"));
std::string file = path.substr(path.find_last_of("/") + 1);
std::cout << "Directory is " << directory << ", file is " << file << std::endl;
int fd = inotify_init();
if (inotify_add_watch(fd, directory.c_str(), IN_CREATE) < 0) {
std::cout << "Could not watch: " << file << std::endl;
return -1;
}
else
std::cout << "Watching: " << file << std::endl;
char buffer[sizeof(struct inotify_event) + NAME_MAX + 1];
while (true) {
if (read(fd, buffer, sizeof(buffer)) < 0) {
std::cout << "Error reading event" << std::endl;
break;
}
struct inotify_event &event = (struct inotify_event &) buffer;
std::cout << event.name << std::endl;
if ((strcmp(event.name, file.c_str()) == 0) && (event.mask & IN_CREATE)) {
std::cout << "File has been created" << std::endl;
close(fd);
break;
}
}
std::fstream file_stream(file, std::fstream::in);
std::cout << file_stream.is_open() << std::endl;
}
If I run it to detect a regular file, it works, it waits for the file creation event, and when trying to open it with a std::fstream, is_open returns true. But if I run it to detect /dev/input/js0, even when the event comes and the file is detected, opening the fstream does not work, as is_open returns false. Is inotify appropriate to detect device files? If not, what would be the right way to do so?

Answer 1

According to inotify(7)
Inotify reports only events that a user-space program triggers
through the filesystem API. As a result, it does not catch
remote events that occur on network filesystems. (Applications
must fall back to polling the filesystem to catch such events.)
Furthermore, various pseudo-filesystems such as /proc, /sys, and
/dev/pts are not monitorable with inotify.
I would say that /dev/input/ also falls into this bucket.
I wonder if udev could be used: you should get info about the device using udevinfo -a -p /dev/input/js0, but also see what events connecting the peripheral generates using udevadm monitor --environment --udev.
Edit: if you successfuly get an inotify event but can't read the file:
Did you try reading the file with another simpler program when the BT device is already connected?
Is there a difference between fstream::open and open from <cstdio>?
Have you checked the permissions on the device? Also what does cat /dev/input/js0 produces?