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[MirageOS-devel] Timestamp representation and CLOCK


While working on ptime David Sheets suggested me that moving away from OCaml's 
usual representation for POSIX timestamps, namely an OCaml float representing 
seconds from the epoch, might be a good idea as it is nowadays possible to get 
nanosecond precision timestamps and those cannot be represented exactly in this 
representation (he also mentioned hardware not having FP support as an 

I'm now quite convinced that this should be done and would like to support this 
both at the API level and internally in ptime (which currently only has 
millisecond precision). I propose that the canonical representation should be 
the following one which is basically what a POSIX timespec definition [1] gives 
us and is OCaml 32-bit clean: 

  type posix_t = int64 * int

Understood as a signed, second precision, time span paired with a non-negative 
number of nano seconds ranging from 0 to 999'999'999. Formally a tuple `(s, 
ns)` represents the time that happens at `s + 1e-9 * ns` from the epoch.  

It also seems that there is consensus that the current CLOCK interface [2] is 
absurd, forcing any clock source implementer to come up with calendar field 
decomposition functions (gmtime). I think that the following interface would be 

module type CLOCK = sig
  val now_s : unit -> int64 * int
  (** [now_s ()] is [(s, ns)] the operating system's POSIX timestamp
      for the current time understood as the number of seconds [s] +
      1e-9 * [ns] from the epoch 1970-01-01 00:00:00 UTC. [ns] ranges
      from [0] to [999_999_999]. *)

  val current_tz_offset_s : unit -> int
  (** [current_tz_offset_s ()] is the operating system's current local
      time zone offset to UTC in seconds. *)

Would people agree on such an API and change for the next major release of 
mirage ? If so, I'd be willing to actually delve into the various repos to make 
that happen. 



[1] http://pubs.opengroup.org/onlinepubs/009695399/basedefs/time.h.html

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