libudev how to use poll with the file descriptor - c++

Question

So, I have an application that I want to be notified of hotplug events on linux. Naturally, I looked at libudev and its API. I also found a useful tutorial on how to use select() with libudev. Following the tutorial and glancing at the API, I came up with this example program that waits for hotplug events and then outputs some basic information about the device that was just added or removed.
#include <poll.h>
#include <libudev.h>
#include <stdexcept>
#include <iostream>
udev* hotplug;
udev_monitor* hotplug_monitor;
void init()
{
// create the udev object
hotplug = udev_new();
if(!this->hotplug)
{
throw std::runtime_error("cannot create udev object");
}
// create the udev monitor
hotplug_monitor = udev_monitor_new_from_netlink(hotplug, "udev");
// start receiving hotplug events
udev_monitor_enable_receiving(hotplug_monitor);
}
void deinit()
{
// destroy the udev monitor
udev_monitor_unref(hotplug_monitor);
// destroy the udev object
udev_unref(hotplug);
}
void run()
{
// create the poll item
pollfd items[1];
items[0].fd = udev_monitor_get_fd(hotplug_monitor);
items[0].events = POLLIN;
items[0].revents = 0;
// while there are hotplug events to process
while(poll(items, 1, 50) > 0)
{
// XXX
std::cout << "hotplug[ " << items[0].revents << " ]" << std::endl;
// receive the relevant device
udev_device* dev = udev_monitor_receive_device(hotplug_monitor);
if(!dev)
{
// error receiving device, skip it
continue;
}
// XXX
std::cout << "hotplug[" << udev_device_get_action(dev) << "] ";
std::cout << udev_device_get_devnode(dev) << ",";
std::cout << udev_device_get_subsystem(dev) << ",";
std::cout << udev_device_get_devtype(dev) << std::endl;
// destroy the relevant device
udev_device_unref(dev);
// XXX
std::cout << "done" << std::endl;
// clear the revents
items[0].revents = 0;
}
}
int main(int args, char* argv[])
{
init();
while(true)
{
run();
}
deinit();
}
Well, it doesn't work. Here's the output I get when I plug in a usb mouse.
hotplug[ 1 ]
hotplug[add] /dev/bus/usb/008/002,usb,usb_device
done
hotplug[ 1 ]
hotplug[add]
At that point the program freezes and I have to stop it with Ctrl-C. What am I doing wrong?

Answer 1

The program doesn't actually stop; it continues running, but std::cout gets messed up when you try to print a NULL string (not all events have all properties). A fix is to make the three prints (devnode, subsystem, devtype) conditional.