[Bug libstdc++/107008] New: Combine config/os/*/error_constants.h into one file

[Bug libstdc++/107008] New: Combine config/os/*/error_constants.h into one file

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107008

            Bug ID: 107008
           Summary: Combine config/os/*/error_constants.h into one file
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

There's no reason to have multiple files and select one for the target:

config/os/djgpp/error_constants.h
config/os/generic/error_constants.h
config/os/mingw32-w64/error_constants.h
config/os/mingw32/error_constants.h

We can just ensure the generic one has a #ifdef for every errno macro that
isn't present on all targets, and use that everywhere.

Once we only have one error_constants.h file for all targets, we can also make
it work for freestanding like so:

#if _GLIBCXX_HOSTED
#include <cerrno>
namespace std {
  enum class errc
    {
      address_family_not_supported =            EAFNOSUPPORT,
      address_in_use =                          EADDRINUSE,    
      // ...
    };
}
#else
namespace std {
  // For freestanding we have no <errno.h> but also no errors from the OS,
  // so we can just make up our own values for the library's purposes.
  enum class errc
    {
      address_family_not_supported = 1,
      address_in_use, 
      // ...
    };
}
#endif


This will allow a freestanding <charconv> to use std::errc.

[Bug libstdc++/107008] Combine config/os/*/error_constants.h into one file

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107008

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
In theory the existing per-target error_constants.h allows an out-of-tree port
for a new OS to use its own error_constants.h more easily. Meh. They would
still need to edit configure.host to overrride ${error_constants_dir}, so they
can just edit the error_constants.h file instead. Adding a couple of #ifdef
conditions in there seems easier than providing a whole new file anyway.

[Bug libstdc++/107008] Combine config/os/*/error_constants.h into one file

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107008

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---

A related topic is that we simply don't define the errc enumerators that don't
have a corresponding EXXXX macro, which is non-conforming. The C++ standard
says the enumerators should all be there.

We could do something like this for each enumerator:

#ifdef EFOO
   foo = EFOO,
#else
   foo = _GLIBCXX_ERRC_ORIGIN + __LINE__,
#endif


Where _GLIBCXX_ERRC_ORIGIN is some value intended to be higher than the highest
errno value on the OS. We only need fewer than 100 errc enumerators so
INT_MAX/2 might be OK.

We can use #line to set the __LINE__ to a fixed value in the file, so that the
constants don't change if we add/remove lines before the errc definition.

[Bug libstdc++/107008] Combine config/os/*/error_constants.h into one file

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107008

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=104883

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #2)
> A related topic is that we simply don't define the errc enumerators that
> don't have a corresponding EXXXX macro, which is non-conforming. The C++
> standard says the enumerators should all be there.

Oh that's PR 104883