Reliable unique identifier per boot?

[Ricardo F]

Hello,

I’m looking for a reliable way to identify a system startup, something
guaranteed to be unique on every boot.
kern.boottime reflects the system time at boot and is accurate 
initially,
but the man page notes: “The boottime variable holds the system boot 
time. It
is set from time at system boot, and is updated when the system time is
adjusted with settimeofday(2).”, so it’s not fully immutable.

Is there a better method to obtain a boot reference that is stable and 
unique
for each startup? As example, something like FreeBSD kern.boot_id 
sysctl.

Thanks,
Ricardo F.