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<title>mdevd: the mdevd-netlink program</title>
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<a href="index.html">mdevd</a><br />
<a href="//skarnet.org/software/">Software</a><br />
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<h1> The <tt>mdevd-netlink</tt> program </h1>
<p>
<tt>mdevd-netlink</tt> listens to the netlink interface for uevents
(also called "hotplug" or "udev" events), and writes those uevents to
its standard output, using a simple format.
</p>
<p>
<a href="mdevd.html">mdevd</a> expects uevents in the same
format mdevd-netlink writes them. So the
<tt>mdevd-netlink | mdevd</tt> command line will function as
a daemon reading uevents from the netlink and handling them.
</p>
<h2> Interface </h2>
<pre>
mdevd-netlink [ -d notif ] [ -v <em>verbosity</em> ] [ -b kbufsz ]
</pre>
<ul>
<li> mdevd-netlink binds to the netlink interface and listens for
hotplug events, as the <em>udevd</em> program does. </li>
<li> It writes event information to its stdout. The output contains
null characters, so a terminal will not display them correctly. To
properly use mdevd-netlink, it should be piped into a handler
program such as <a href="mdevd.html">mdevd</a>. </li>
<li> mdevd-netlink is a long-lived program.
When it receives a SIGTERM, it stops listening to hotplug events;
it will exit as soon as it has flushed its event queue to stdout. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-d</tt> <em>notif</em> : when ready (actually
listening to the netlink), write a newline to file descriptor <em>notif</em>
then close it. This allows mdevd-netlink to use the
<a href="//skarnet.org/software/s6/notifywhenup.html">s6 mechanism to
notify readiness</a>. <em>notif</em> cannot be lesser than 3. If this
option is not given, no readiness notification is sent. </li>
<li> <tt>-v</tt> <em>verbosity</em> : be more or less verbose.
Default verbosity is 1. 0 will only print fatal error messages, 3 will
print warnings every time the netlink interface sends something
unexpected. </li>
<li> <tt>-b</tt> <em>kbufsz</em> : try and reserve a kernel buffer of
<em>kbufsz</em> bytes for the netlink queue. Too large a buffer wastes kernel memory;
too small a buffer risks losing events. The default is 65536 (which is on
the large side). </li>
</ul>
<h2> Protocol </h2>
<ul>
<li> An event is a series of null-terminated strings as they are sent by
the kernel to the netlink; mdevd-netlink adds a final empty string
(i.e. an additional null character) to mark the end of the series. </li>
<li> The first string is a short description of the event; it normally
contains the string "@/". Other strings after the first are of the form
"VARIABLE=value", and describe the environment which a hotplug helper
for the event (registered in <tt>/proc/sys/kernel/hotplug</tt>) would be
spawned with. </li>
<li> Example (newlines added for clarity): <pre>
add@/class/input/input9/mouse2\0
ACTION=add\0
DEVPATH=/class/input/input9/mouse2\0
SUBSYSTEM=input\0
SEQNUM=106\0
PHYSDEVPATH=/devices/pci0000:00/0000:00:1d.1/usb2/22/22:1.0
PHYSDEVBUS=usb\0
PHYSDEVDRIVER=usbhid\0
MAJOR=13\0
MINOR=34\0
\0 </pre> </li>
</ul>
<h2> Notes </h2>
<ul>
<li> mdevd-netlink is a daemon; it should be run under a proper supervision system such
as <a href="//skarnet.org/software/s6/">s6</a>. </li>
<li> If you are running mdevd-netlink, the program you pipe its
output into should be the only program handling uevents; that means
that <tt>/proc/sys/kernel/hotplug</tt> should be empty. </li>
<li> The <tt>mdevd-netlink | mdevd</tt> command line is a quick
way of running a uevent manager, but there are ways to improve it:
<ul>
<li> Using the
<a href="//skarnet.org/software/execline/">execline</a> scripting
language, the <tt>pipeline { mdevd-netlink } mdevd</tt> command
line will avoid leaving a shell around. </li>
<li> The best way to use mdevd-netlink and mdevd is with a
supervision system:
under <a href="//skarnet.org/software/s6/">s6</a> or
<a href="//skarnet.org/software/s6-rc/">s6-rc</a>, write a service
pipeline where <tt>mdevd-netlink</tt> is a producer and
<tt>mdevd</tt> is a consumer. This setup has the advantage, among
others, that you can restart the netlink listener and the event handler
separately. The <tt>examples/</tt> subdirectory of the mdevd
package contains example configurations for such a setup. </li>
</ul> </li>
</ul>
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