Re: ac_cv_sys_largefile_opts undocumented?

[Bruno Haible]

dmitrii.pasechnik wrote:
> By the way, ac_cv_sys_largefile_opts isn't fun to use - as sometimes
> it's a plain text, and sometimes flags which should be added to CFLAGS.
> So one has to write things like
> 
> AS_CASE([$ac_cv_sys_largefile_opts],
>   ["none needed"], [],
>   ["support not detected"], [],
>   dnl otherwise it is a compiler flag
>   [LARGEFILECFLAGS="$ac_cv_sys_largefile_opts"]
>   )
> 
> which are unnecessarily complicated.

All ac_cv_* variables, and in fact all *_cv_* variables, are designed to
contain the answer to a
  checking for <something>...
output line, in a way that is both machine-parseable and human-readable.

As a consequence:
  - Its values look sometimes odd (e.g. 'no (bad O_NOATIME, O_NOFOLLOW)'.
  - Its values are not stable over time and therefore *cannot be documented*.

The entities which can often be documented are AC_SUBSTed variables and
the C/C++ preprocessor macro definitions (AC_DEFINE).

If there is the need for an Autoconf macro to expose more details, in a
documented way, it should be done through a new AC_SUBSTed variable or
a new AC_DEFINE.

Bruno