= The year 2038 problem =

All 32-bit kernels to date use a signed 32-bit time_t type, which can only represent time until January 2038. Since embedded systems running 32-bit Linux are going to survive beyond that date, we have to change all current uses, in a backwards compatible way.

== User space interfaces ==

We will likely keep the 32-bit time_t in all user space interfaces that currently use it, but add new interfaces with a 64-bit timespec or another type that can represent later times. Most importantly that impacts system calls, but also specific ioctl commands and a few other interfaces. User space programs have to be recompiled to use the new interfaces, and the policy whether to use the old or the time time is left to the C library. While that policy is a complex topic itself, we don't cover it here.

=== System calls ===

==== interfaces that uses relative time_t/timespec/timeval ====

These can stay compatible, but we'd have to use a different
type if we change time_t.

   * nanosleep
   * select/pselect/poll/ppoll/epoll
   * getrusage
   * sched_rr_get_interval
   * sigtimedwait
   * alarm

==== interfaces that don't make sense for times in the past ====
Here, we are relatively free to change the start of the
epoch in the kernel but convert to something else on the
user space boundary. One possibility is to scale them to
boot time and use ktime_t in the kernel.

   * getitimer/setitimer
   * timer_settime/timer_gettime
   * gettimeofday/settimeofday
   * adjtimex
   * clock_gettime/clock_settime/clock_adjtime/clock_nanosleep
   * time/stime
   * sysv ipc (msg, sem, shm)
   * sysinfo

==== interfaces that require absolute times ====
These absolutely have to use something better than time_t
both in user space and in the kernel so we can deal with
old files. A lot of file systems need to be fixed as well so
we can actually store the times, regardless of whether we
are running a 32 or 64 bit kernel.

   * stat/lstat/fstatat/
   * utime/utimes/futimesat

=== ioctl ===

There are numerous ioctl commands using a time argument. This list is incomplete

   * audio time stamps
   * v4l time stamps
   * input event time stamps
   * socket time stamps
   * ...

=== memory mapped packet sockets ===

Socket timestamps are exported to user space using a memory mapped
interface defined in include/uapi/linux/if_packet.h. There are currently
three versions of this interface, all use a 32-bit time type. We will likely
need a version 4 to solve this.