Accurately serializing and deserializing a SysTime in binary format

[Ecstatic Coder]

On Tuesday, 21 July 2020 at 12:21:16 UTC, Steven Schveighoffer 
wrote:
> On 7/21/20 7:44 AM, Ecstatic Coder wrote:
>> On Tuesday, 21 July 2020 at 11:01:20 UTC, drug wrote:
>>> On 7/20/20 10:04 PM, Ecstatic Coder wrote:
>>>> I'm currently implementing a small open source backup tool 
>>>> (dub), and therefore I need to accurately store the file 
>>>> modification SysTime in binary format, so that I can later 
>>>> load this SysTime from the snapshot file to compare it with 
>>>> the current file modification SysTime.
>>>>
>>>> Having unfortunately not understood how to do this from the 
>>>> SysTime documentation, in despair, I've tried to directly 
>>>> serialize the 16 bytes of the SysTime value. This worked 
>>>> fine until I call the ".toISOString()" on the deserialized 
>>>> SysTime, which inevitably crashes the executable ;)
>>>
>>> That is probably a bug. I serialize SysTime as long by means 
>>> msgpack for exchanging between C++ client and D server and it 
>>> works pretty nice.
>>>
>> 
>> Ah thanks for telling me :)
>> 
>> The loaded byte array in the union type was indeed the same as 
>> the saved one, so I immediately thought it was crashing 
>> because of some hidden pointer for timezone or something which 
>> was then pointing to garbage at reloading, causing the crash 
>> of the ".toISOString" call.
>
> Not a bug.
>
> 8 of those 16 bytes is a pointer to the timezone, which is 
> going to be different on different processes.
>
> What you should do I think is serialize the stdTime [1], and 
> set the time zone to whatever you want:
>
> long serialize(SysTime st) { return st.stdTime; }
> SysTime deserialize(long st) { return SysTime(st, UTC()); }
>
> The stdTime is always stored as UTC to make math a lot easier. 
> The time zone is only used for display.
>
> -Steve
>
> [1] 
> https://dlang.org/phobos/std_datetime_systime.html#.SysTime.stdTime

Smart :)

Now I understand my mistake was to try to directly serialize a 
SysTime as provided by the "getTimes" function, instead of 
converting it to a StdTime, which is more versatile...