[RFC 1/6] y2038: Introduce _TIME_BITS flag to support 64 bit time on 32 bit systems

[Joseph Myers]

On Sat, 5 Dec 2020, Lukasz Majewski wrote:

> This change introduces new compile time, supported by glibc flag - namely
> _TIME_BITS, which then is responsible for exporting __USE_TIME_BITS64 only

As a major new user-visible feature, this needs to be mentioned in NEWS.

> +/* We need to know the word size in order to check the time size.  */
> +#include <bits/wordsize.h>
> +
> +#if defined _TIME_BITS
> +# if _TIME_BITS == 64
> +#  if ! defined (_FILE_OFFSET_BITS) || _FILE_OFFSET_BITS != 64
> +#   error _TIME_BITS==64 is allowed only when _FILE_OFFSET_BITS==64
> +#  elif __WORDSIZE == 32
> +#   define __USE_TIME_BITS64	1
> +#  endif

__WORDSIZE isn't the right thing to check.  On ARC or RV32 or x32, 
_TIME_BITS == 64 should be a no-op, because time_t has always been 64-bit 
for those glibc ports.  That is, you should be checking __TIMESIZE, not 
__WORDSIZE.

> +# elif _TIME_BITS == 32
> +#  if __WORDSIZE > 32
> +#   error _TIME_BITS=32 is not compatible with __WORDSIZE > 32

Likewise, __TIMESIZE should be checked here, not __WORDSIZE.

> +@defvr Macro _TIME_BITS
> +This macro determines the bit size of @code{time_t} (and therefore the
> +bit size of all @code{time_t} derived types and the prototypes of all
> +related functions). If @code{_TIME_BITS} is undefined, the bit size of
> +time_t equals the bit size of the architecture.

Apart from "bit size of the architecture" not being well-defined, that's 
not right for ARC or RV32, 32-bit architectures with 64-bit time_t.

Note there should be two spaces after "." in Texinfo manuals.

> +If @code{_TIME_BITS} is undefined, or if @code{_TIME_BITS} is defined
> +to the value @code{32} and @code{__WORDSIZE} is defined to the value
> +@code{32}, or or if @code{_TIME_BITS} is defined to the value @code{64}
> +and @code{__WORDSIZE} is defined to the value @code{64}, nothing changes.

__WORDSIZE (and __TIMESIZE after the fix above) is not a user-visible 
interface, it's a glibc header implementation detail.  The manual (other 
than maint.texi) should not mention either such internal macro; you need 
to talk about a concept of word size / default time_t size that's 
explained purely in user-level interfaces, not glibc internal macros.

> +If @code{_TIME_BITS} is defined to the value @code{64} and if
> +@code{__WORDSIZE} is defined to the value @code{32}, then the @w{64 bit}
> +time API and implementation are used even though the architecture word
> +size is @code{32}. Also, if the kernel provides @w{64 bit} time support,
> +it is used; otherwise, the @w{32 bit} kernel time support is used (with
> +no provision to address kernel Y2038 shortcomings).

Use "32-bit" and "64-bit" (with hyphen) as adjectives.

-- 
Joseph S. Myers
joseph@codesourcery.com