From 7ff4e654c9e55c9f6151dfdfc1e6a92ccc4fd8b1 Mon Sep 17 00:00:00 2001
From: Evan Flynn <evanflynn.msu@gmail.com>
Date: Sat, 20 Jan 2024 10:10:27 -0800
Subject: [PATCH] Use /sys/class/video4linux/ to get list of v4l2 devices

Signed-off-by: Evan Flynn <evanflynn.msu@gmail.com>
---
 CMakeLists.txt            |  4 ++--
 include/usb_cam/utils.hpp | 44 +++++++++++++++++++++++++++------------
 2 files changed, 33 insertions(+), 15 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 791070e9..2387c328 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,9 +1,9 @@
 cmake_minimum_required(VERSION 3.5)
 project(usb_cam)
 
-# Default to C++14
+# Default to C++17
 if(NOT CMAKE_CXX_STANDARD)
-  set(CMAKE_CXX_STANDARD 14)
+  set(CMAKE_CXX_STANDARD 17)
 endif()
 
 if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
diff --git a/include/usb_cam/utils.hpp b/include/usb_cam/utils.hpp
index bf2dbf65..add1ff97 100644
--- a/include/usb_cam/utils.hpp
+++ b/include/usb_cam/utils.hpp
@@ -36,10 +36,14 @@ extern "C" {
 #include <sys/ioctl.h>
 #include <sys/time.h>
 #include <unistd.h>  // for access
+
 #include <cmath>
 #include <ctime>
 #include <cstring>
+#include <filesystem>
+#include <fstream>
 #include <sstream>
+#include <iostream>
 #include <string>
 #include <map>
 
@@ -47,6 +51,8 @@ extern "C" {
 
 #include "usb_cam/constants.hpp"
 
+namespace fs = std::filesystem;
+
 
 namespace usb_cam
 {
@@ -146,31 +152,43 @@ inline std::map<std::string, v4l2_capability> available_devices()
   // Initialize vector of device strings to fill in
   std::map<std::string, v4l2_capability> v4l2_devices;
 
-  // V4L2 spec says there can only be a maximum of 64 devices
-  const uint8_t MAX_DEVICES = 64;
+  // Get a list of all v4l2 devices from /sys/class/video4linux
+  const std::string v4l2_symlinks_dir = "/sys/class/video4linux/";
+  for (const auto & device_symlink : fs::directory_iterator(v4l2_symlinks_dir)) {
+    if (fs::is_symlink(device_symlink)) {
+      // device_str is the full path to the device in /sys/devices/*
+      std::string device_str = fs::canonical(
+        v4l2_symlinks_dir + fs::read_symlink(
+          device_symlink).generic_string());
+      // get the proper device name (e.g. /dev/*)
+      std::ifstream uevent_file(device_str + "/uevent");
+      std::string line;
+      std::string device_name;
+      while (std::getline(uevent_file, line)) {
+        auto dev_name_index = line.find("DEVNAME=");
+        if (dev_name_index != std::string::npos) {
+          device_name = "/dev/" + line.substr(dev_name_index + 8, line.size());
+          break;
+        }
+      }
 
-  for (uint8_t i = 0; i < MAX_DEVICES; i++) {
-    std::string device_str = "/dev/video" + std::to_string(i);
-    // See if device exists
-    if (access(device_str.c_str(), F_OK) != -1) {
       int fd;
       // Try and open device to test access
-      if ((fd = open(device_str.c_str(), O_RDONLY)) == -1) {
-        std::cerr << "Cannot open device: `" << device_str << "`, ";
+      if ((fd = open(device_name.c_str(), O_RDONLY)) == -1) {
+        std::cerr << "Cannot open device: `" << device_name << "`, ";
         std::cerr << "double-check read / write permissions for device" << std::endl;
       } else {
         struct v4l2_capability device_capabilities = {};
-
         if (xioctl(fd, VIDIOC_QUERYCAP, &device_capabilities) == -1) {
-          std::cerr << "Could not retrieve device capabilities: `" << device_str;
+          std::cerr << "Could not retrieve device capabilities: `" << device_name;
           std::cerr << "`" << std::endl;
         } else {
-          v4l2_devices[device_str] = device_capabilities;
+          v4l2_devices[device_name] = device_capabilities;
         }
       }
     } else {
-      // device doesn't exist, break
-      break;
+      // device doesn't exist, continue
+      continue;
     }
   }