How can I create an std::chrono::time_point for 1 Jan 1970 00:00:00?

Question

Is there a portable standard-compliant way to create an std::chrono::time_point for 1 Jan 1970 00:00:00? This has to be thread-safe. Therefore, the solution should avoid functions like std::gmtime() and std::localtime() which do not have thread-safety guarantees.

This is not a duplicate of Convert std::chrono::time_point to unix timestamp since I'm asking for a portable solution that is thread-safe. Neither does the question there ask for that, nor do the answers provide this guarantee.

This sounds like two questions, 1) how to create a specific time point - no thread issues, 2) how to get the current time - potential thread issues. — wally, Nov 04 '16 at 11:53
Possible duplicate of [Convert std::chrono::time\_point to unix timestamp](http://stackoverflow.com/questions/14504870/convert-stdchronotime-point-to-unix-timestamp) — Captain Giraffe, Nov 04 '16 at 12:01
Beware. The Posix standard does not define "the epoch", and then rely on the normal meaning of "seconds" and "since". It defines the complete phrase "seconds since the epoch", and it doesn't mean quite you might have thought it meant. — Martin Bonner supports Monica, Nov 04 '16 at 12:06
@CaptainGiraffe This is not a duplicate, since I'm asking for a portable and thread-safe solution. The question there does not ask for this and the answers do not provide these guarantees. — Ralph Tandetzky, Nov 04 '16 at 12:10
If I can get this: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0355r1.html standardized, then a default constructed `std::chrono::time_point{}` will portably represent 1970-01-01 00:00:00 UTC. It will be thread-safe and lightning fast (requiring storing a 64 bit zero). — Howard Hinnant, Nov 04 '16 at 13:55
@HowardHinnant It would be awesome to have something standardized that can be used safely. Especially printing a `time_point` to an `ostream` without calling thread-unsafe functions from C antiquity would be very helpful. Also extracting the date and time components from a `time_point` would help a lot. — Ralph Tandetzky, Nov 07 '16 at 12:39

score 8 · Accepted Answer · answered Nov 04 '16 at 12:04

8

In practise, std::chrono::system_clock::from_time_t(0) will return the value you want.

In principle, I can't see how to exactly what you want.

As an alternative, calculate this value once (at startup) before multiple threads have started running.

answered Nov 04 '16 at 12:04

Martin Bonner supports Monica

28,528
3
51
88

The phrase *In practice* does not sound very portable. Indeed, the type of `std::time_t` is not explicitly defined in the standard. – Ralph Tandetzky Nov 04 '16 at 12:40
I agree the type of `std::time_t` is not defined. By *in practise*, I meant "I am not aware of any current implementation where this won't work". It might cause you grief on embedded systems which are not Posix (but systems that small tend to be programmed in C not C++), and I don't know about IBM operating system (AIX will be fine). – Martin Bonner supports Monica Nov 04 '16 at 13:00
Note that `std::time_t` is *is* defined to be arithmetic, so a literal zero will convert to it without difficulty. – Martin Bonner supports Monica Nov 04 '16 at 13:01
2

Actually, I found that passing `mktime(&tm)` instead of `0` should work, where `tm` is of type `std::tm` with the appropriate fields set. – Ralph Tandetzky Nov 04 '16 at 13:10
Splendid! I think that would be appropriate as an answer. – Martin Bonner supports Monica Nov 04 '16 at 13:14

How can I create an std::chrono::time_point for 1 Jan 1970 00:00:00?

1 Answers1