s6-linux-utils
Software
skarnet.org

The s6-uevent-listener program

s6-uevent-listener spawns a long-lived helper program. It then listens to the netlink interface for uevents (also called "hotplug" or "udev" events), and passes those events to the helper program's standard input, using a simple format.

Interface

     s6-uevent-listener [ -v verbosity ] [ -b kbufsz ] prog...

• s6-uevent-listener spawns prog... with a pipe writing to prog' stdin.
• s6-uevent-listener binds to the netlink interface and listens for hotplug events, as the udev program does.
• It transmits event information to prog via the pipe.
• s6-uevent-listener, like prog, is a long-lived program. When it receives a SIGTERM, it stops listening; it will exit as soon as it has flushed its event queue to prog.

Options

• -v verbosity : 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.
• -b kbufsz : try and reserve a kernel buffer of kbufsz 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).

Protocol

• prog should read a series of events on its stdin, and exit on EOF.
• An event is a series of null-terminated strings as they are sent by the kernel to the netlink; s6-uevent-listener adds a final empty string (i.e. an additional null character) to mark the end of the series.
• 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 /proc/sys/kernel/hotplug) would be spawned with.
• Example (newlines added for clarity):

  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 

Notes

• s6-uevent-listener is a daemon; it should be run under a proper supervision system such as s6.
• If you are running s6-uevent-listener, prog... should be the only program handling uevents, which means that /proc/sys/kernel/hotplug should be empty.
• If you want the serialization benefit of the netlink, but still want to spawn a program such as busybox's or toybox's mdev, use s6-uevent-spawner mdev as s6-uevent-listener's prog.