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linuxlinux-kerneludev

How to register for an "uevent" from a user application in Linux?

I have written battery driver and that driver sends an uevent when a power supply is changed, i.e. from AC to battery

When I run udevadm monitor I see an KERNEL and UDEV CHANGE events from power_supply subsystem upon removing AC supply.

My requirement is to notify the user application when there is an CHANGE event from power_supply subsystem. I do not want to poll for kernel message or netlink socket from user application.

Is it possible for me to register/listen for this particular uevent from user application and gets something like callback function when an event occurs?

Solution

  • It is possible and there is a great reference here. In the reference they use a change rule for a similar application:

    # Rule for when switching to battery
ACTION=="change", SUBSYSTEM=="power_supply", ATTR{type}=="Mains", ATTR{online}=="0", ENV{DISPLAY}=":0", ENV{XAUTHORITY}="/home/USERNAME/.Xauthority" RUN+="/usr/bin/su USERNAME_TO_RUN_SCRIPT_AS -c /usr/local/bin/brightness_notification.sh"

    The ACTION=="change" registers a rule for a change event. The SUBSYSTEM and ATTR parameters specify the rule to only apply to a certain device or devices (you can get these for your device from udevadm). The RUN parameter is how you run a userspace executable when the event is seen. In the case below they also use the ENV parameter to pass a environment variable to the executable and they also use the su command in RUN to run the executable as another user, but this is probably not necessary for most applications.