Re: boot time on Linux

[Po Lu <luangruo@yahoo.com>]

Bruno Haible <bruno@clisp.org> writes:

> Po Lu wrote:
>> > Also, I don't know how Android records boot time so I'll cc this to Po
>> > Lu, the main developer for Emacs on Android.
>> 
>> The boot time is off limits to user programs on Android, for security
>> reasons.
>
> No, it isn't. The attached file, when compiled and run under Termux (which
> doesn't have particular permissions), prints e.g.:
>
> from clock  : 1691616762.476870660 = 2023-08-09 21:32:42.476870660
> from sysinfo: 1691616762.329261637 = 2023-08-09 21:32:42.329261637
>
> Note that this uses the kernel's uptime counter, so it will not work well
> when the user changes the current time manually. But this is rare on Android.

This uses the uptime counter (which also results in an SELinux denial
for me, but different Android distributions have SELinux policies of
varying strictness), which cannot establish the precise time the system
started, since time elapses between the read from the uptime counter and
the read from the RTC.