RFR: JDK-8266536: Provide a variant of os::iso8601_time which works with arbitrary timestamps [v3]

[Xin Liu]

On Thu, 6 May 2021 19:36:49 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> src/hotspot/share/runtime/os.hpp line 231:
>> 
>>> 229:   // E.g., YYYY-MM-DDThh:mm:ss.mmm+zzzz.
>>> 230:   // Returns buffer, or NULL if it failed.
>>> 231:   static char* iso8601_time(jlong milliseconds_since_19700101, char buffer[29],
>> 
>> You are supposed to get a compiler warning on this char buffer[29]. ISO C/C++ can't pass in array type as a function argument.  As a result, the type buffer automatically casts to char* here. 
>> 
>> Further, the function signature is different from its implementation, which use char* buffer instead. 
>> I see that you do enforce length checking in implementation.
>
> Oh, good catch, this is a stray edit I thought I reverted. See below.
> 
>> ISO C/C++ can't pass in array type as a function argument. As a result, the type buffer automatically casts to char* here.
> 
> No, it can, that's totally fine. It gets automatically translated to pointer type, but its a valid expression nonetheless. 
> 
> It is usually done to indicate that a function expects an output array of a particular length instead of letting the caller guess the right length - and letting him to deal with truncation if he guesses wrong.
> 
> I originally planned to define the function as 
> 
> iso8601_time(jlong milliseconds_since_19700101, char buffer[29]);
> 
> to indicate that the only output value length which makes sense is 29, but then I refrained because I wanted to keep the patch simple.

C/C++  allow you to write code like that, but they won't check the length of argument <=29 for you. 
I think you sanity check inside of iso8601_time() is good enough.

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PR: https://git.openjdk.java.net/jdk/pull/3869