Make time_t size check message more helpful

[mikee47]

Further to #2758 host builds should generally match code run on hardware. As mentioned this is not straightforward for Windows, but for Linux we should expect 64-bits as the default.

GCC 10+ can make use of _TIME_BITS to ensure we get this behaviour. More specifically, this is a feature of glibc shipped with those versions.

For earlier versions, even though the compiler is 64-bit we are building in 32-bit mode and time_t stays stuck in 32-bits. There are various discussions about this but the short answer is that prior to GCC 10 (libc 4) 32-bit applications get 32-bit time_t.

Instead of generating a static_assert (which stops compilation) we get a compiler warning - this still requires building in STRICT mode.

[mikee47]

@slaff Are you OK with making this a hard assert? There may be a way to generate a deprecation message or warning. There's also the minimum GCC version check in the build which is currently set to 8; can't really change that yet though because of Windows and IDF 4.x.

[slaff]

If GCC v10 is ok then wouldn't it be better to change this line

from

#else

to

#elif defined(__GNUC__) && __GNUC__ < 10

[mikee47]

Perhaps... but as this is technically a glibc issue the more correct way would be to check __USE_TIME_BITS64 - see https://lwn.net/Articles/664800/.

[slaff]

Are you OK with making this a hard assert? There may be a way to generate a deprecation message or warning.

@mikee47 I would prefer a warning.

[mikee47]

From Y2038 Proofness Design

API Design

In order to avoid duplicating APIs for 32-bit and 64-bit time, glibc will provide either one but not both for a given application; the application code will have to choose between 32-bit or 64-bit time support, and the same set of symbols (e.g. time_t or clock_gettime) will be provided in both cases.

The following is proposed:

• User code defines _TIME_BITS=64 to get 64-bit time support instead of the legacy 32-bit time.

• If glibc sees _TIME_BITS=64, then it defines __USE_TIME_BITS64 to indicate that time support is 64-bit rather than 32-bit.

This allows user code to check, by verifying whether __USE_TIME_BITS64 is defined once glibc headers are #included, that they are using a glibc which actually supports 64-bit time (or claims to).

• On 64-bit systems, only 64-bit time is supported, __USE_TIME_BITS64 is defined regardless of _TIME_BITS, and the same symbols are used as they were in 32-bit time.

• On 32-bit systems, if __USE_TIME_BITS64 is defined, time support is provided for 64-bit time; otherwise, it is provided for 32-bit time.