How to tell what's restarting my phone?

Question

I have Cyanogenmod 11 on a Samsung s3 and sometimes the phone restarts itself. It doesn't do so "stably" enough for me to notice what causes it, but there should be logs of what happened right before restarting (and what caused it).

Where can I find it?

Answer 1

Found this info from other forums, hope it helps. Possibly a bad app design, water damage or a bad battery.

"Restarts are kernel panics. It happens regularly and at random in Android 2.x but is hardly seen in versions 4.x and is caused by many things. One is a full dalvik cache and other times 3G can cause it via a corrupted PRL list update that crashes the modem and takes the kernel with it. (Turning off 3g or if running a custom ROM like CyanogenMod turning force 2G networks on and see if this cures it). I haven't had random restarts on 4.x but deal with them at least twice a week in Gingerbread. I always blamed it on Gingerbread having poor task management built in while 4.x patched it. On one device the 3G auto-PRL GSM mode caused it, while using EDGE and no auto-PRL fixed it but at the expense of speed." From: http://www.helpbuildweb.com/post-2072506-My-Android-phone-get-automaticaly-restart..-whats-the-problem-and-how-to-fix-it

Also this seem to be good http://vinner.hubpages.com/hub/Android-phone-is-restarting-How-to-solve

Answer 2

If CM has implemented it correctly, dmesg should have 2 copies. One current and one from last reboot. You'll have to find where your dmesg copies are located. Try with:

find / -iname "*dmesg*"

Being root when using find is always an advantage.

EDIT: The other option is to look at the kernel messages. They are called "kmsg" and can be found in:

/proc/last_kmsg
/proc/kmsg

But these are special files, so in order not to block your terminal when reading you have to use cat like this:

cat /proc/last_kmsg |less

and you probably need to be root. This is not always present or updated, so you may need to change the contents of this file /sys/module/printk/parameters/always_kmsg_dump from "N" to "Y".