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POlibudev how to use poll with the file descriptor
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  1. POlibudev how to use poll with the file descriptor
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    copied!<p>So, I have an application that I want to be notified of hotplug events on linux. Naturally, I looked at libudev and its <a href="https://www.kernel.org/pub/linux/utils/kernel/hotplug/libudev/api-index-full.html" rel="nofollow">API</a>. I also found a useful <a href="http://www.signal11.us/oss/udev/" rel="nofollow">tutorial</a> 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.</p> <pre><code>#include &lt;poll.h&gt; #include &lt;libudev.h&gt; #include &lt;stdexcept&gt; #include &lt;iostream&gt; udev* hotplug; udev_monitor* hotplug_monitor; void init() { // create the udev object hotplug = udev_new(); if(!this-&gt;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) &gt; 0) { // XXX std::cout &lt;&lt; "hotplug[ " &lt;&lt; items[0].revents &lt;&lt; " ]" &lt;&lt; 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 &lt;&lt; "hotplug[" &lt;&lt; udev_device_get_action(dev) &lt;&lt; "] "; std::cout &lt;&lt; udev_device_get_devnode(dev) &lt;&lt; ","; std::cout &lt;&lt; udev_device_get_subsystem(dev) &lt;&lt; ","; std::cout &lt;&lt; udev_device_get_devtype(dev) &lt;&lt; std::endl; // destroy the relevant device udev_device_unref(dev); // XXX std::cout &lt;&lt; "done" &lt;&lt; std::endl; // clear the revents items[0].revents = 0; } } int main(int args, char* argv[]) { init(); while(true) { run(); } deinit(); } </code></pre> <p>Well, it doesn't work. Here's the output I get when I plug in a usb mouse.</p> <pre><code>hotplug[ 1 ] hotplug[add] /dev/bus/usb/008/002,usb,usb_device done hotplug[ 1 ] hotplug[add] </code></pre> <p>At that point the program freezes and I have to stop it with Ctrl-C. What am I doing wrong?</p>
 

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