CODEWITHSUNDEEP
c++linux
ctrlc-root
ctrlc-root

Reputation: 1047

libudev how to use poll with the file descriptor

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?

Upvotes: 2

Views: 5082

Answers (1)

Ambroz Bizjak
Ambroz Bizjak

Reputation: 8095

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.

Upvotes: 4

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