[petsc-dev] meaning of PETSC_USE_EXTERN_CXX?

[Matthew Knepley]

On Wed, Mar 6, 2013 at 7:49 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

>
> On Wed, Mar 6, 2013 at 5:58 AM, Matthew Knepley <knepley at gmail.com> wrote:
>
>> The whole idea of compiler flags is that the compiler understands them. I
>> think its a bad model to have
>> flags that might work on one file and not another. That just seems like a
>> nightmare.
>>
>
> -fvisibility=hidden causes functions to be hidden by default unless (a)
> they are explicitly 'extern' or (b) their visibility is set explicitly.
> This is like the Windows default so it's not exactly esoteric, but most
> packages will not build shared libraries correctly with this flag.
>
> Note that Open MPI sets visibility=hidden by default on systems that
> support it. Now that we have PETSC_INTERN, we have an explicit way of
> stating when symbols are meant to be internal to PETSc. If every function
> in PETSc uses either PETSC_EXTERN, PETSC_INTERN, or static, then
> -fvisibility=hidden will not change anything so we could skip it.
>
>
>> If there really are compiler flags that can break projects, they should
>> be removed from compilerOptions.py, and
>> put directly in PETSc/Configure.py and added into the output in Dump().
>>
>
> There is no way to test it from BuildSystem without affecting the way all
> packages are built?
>

This is not about testing at all, its about using. You explicitly stick it
in CFLAGS.

   Matt


> The visibility macros also should not be used when building user code
> (like examples) because we don't want to enforce the same discipline on
> them. Maybe we should test for visibility so that
> __attribute__((visibility("hidden"))) can be used, but leave off the
> compiler flag (except for developer builds to enforce that all exports are
> explicit---anything that fails will also fail on Windows).
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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