Linux polling all key event devices for key presses (minimalist example)

pollkeys.cpp

// Note: uses C++17 fs features; build with
// g++ -std=c++17 event-reader.cpp -lstdc++fs
//
// Licensed under the following terms:
//        DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE 
//                    Version 2, December 2004 
//
// Copyright (C) 2004 Sam Hocevar <[email protected]> 
//
// Everyone is permitted to copy and distribute verbatim or modified 
// copies of this license document, and changing it is allowed as long 
// as the name is changed. 
//
//            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE 
//   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION 
//
//  0. You just DO WHAT THE FUCK YOU WANT TO.

#include <linux/input.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <unistd.h>
#include <poll.h>
#include <signal.h>
#include <iostream>
#include <filesystem>
#include <string_view>
#include <vector>

bool supports_key_events(const int &fd) {
  unsigned long evbit = 0;
  ioctl(fd, EVIOCGBIT(0, sizeof(evbit)), &evbit);
  return (evbit & (1 << EV_KEY));
}

int main() {
  char eventPathStart[] = "/dev/input/event";
  std::vector<pollfd> poll_fds;
  for (auto &i : std::filesystem::directory_iterator("/dev/input")) {
    if(i.is_character_file()) {
      std::string view(i.path());
      if (view.compare(0, sizeof(eventPathStart)-1, eventPathStart) == 0) {
        int evfile = open(view.c_str(), O_RDONLY | O_NONBLOCK);
        if(supports_key_events(evfile)) {
          std::cout << "Adding " << i.path() << std::endl;
          poll_fds.push_back({evfile, POLLIN, 0});
        } else {
          close(evfile);
        }
      }
    }
  }

  input_event ev;
  while(true) {
    poll(poll_fds.data(), poll_fds.size(), -1);
    for (auto &pfd : poll_fds) {
      if (pfd.revents & POLLIN) {
        if(read(pfd.fd, &ev, sizeof(ev)) == sizeof(ev)) {
          if(ev.type == EV_KEY && ev.value == 1 && ev.code < 0x100) {
            std::cout << "Key pressed! Code: " << ev.code << std::endl;
          }
        }
      }
    }
  }

  // cleanup, but it's pointless since the code is unreachable
  // besides, fds will be closed anyway when the program terminates
  for (auto &pfd : poll_fds) {
    close(pfd.fd);
  }
}