warning: use of old-style cast to ‘void*’ [-Wold-style-cast]

[visiblehawk]

Thank you for taking the time to submit an issue!

Background information

What version of Open MPI are you using? (e.g., v1.10.3, v2.1.0, git branch name and hash, etc.)

openmpi-4.0.0

Describe how Open MPI was installed (e.g., from a source/distribution tarball, from a git clone, from an operating system distribution package, etc.)

Compiled from a source/distribution tarball.

Please describe the system on which you are running

• Operating system/version: Ubuntu 18.10
• Computer hardware: x86_64 arch
• Network type:

---

Details of the problem

This macro in mpi.h produces so many warnings about old-style-cast in any code using OpenMPI, e.g. OpenFOAM opensource package.

#if !OMPI_BUILDING
#define OMPI_PREDEFINED_GLOBAL(type, global) ((type) ((void *) &(global)))
#else
#define OMPI_PREDEFINED_GLOBAL(type, global) ((type) &(global))
#endif

the warnings:

In file included from db/IOstreams/Pstreams/OPwrite.C:29:
db/IOstreams/Pstreams/OPwrite.C: In static member function ‘static bool Foam::OPstream::write(Foam::Pstream::commsTypes, int, const char*, std::streamsize, int, Foam::label)’:
/home/shadowfax/foam/foam-extend-4.1/ThirdParty/packages/openmpi-4.0.0/platforms/linux64GccDPOpt/include/mpi.h:329:72: warning: use of old-style cast to ‘void*’ [-Wold-style-cast]
#define OMPI_PREDEFINED_GLOBAL(type, global) ((type) ((void *) &(global)))
                                                                    ^
/home/shadowfax/foam/foam-extend-4.1/ThirdParty/packages/openmpi-4.0.0/platforms/linux64GccDPOpt/include/mpi.h:1049:18: note: in expansion of macro ‘OMPI_PREDEFINED_GLOBAL’
#define MPI_BYTE OMPI_PREDEFINED_GLOBAL(MPI_Datatype, ompi_mpi_byte)
              ^~~~~~~~~~~~~~~~~~~~~~
db/IOstreams/Pstreams/OPwrite.C:101:13: note: in expansion of macro ‘MPI_BYTE’
         MPI_BYTE,

[jsquyres]

@Shadow-fax What compiler and version are you using?

[visiblehawk]

Dear jsquyres,

I'm using GCC version 8.2 on Ubuntu 18.10

[jsquyres]

So I think you're saying that you're compiling mpi.h with a C++ compiler, and it's warning about (void*)-style casts instead of static_cast<void*>, right?

If that's the case, I am not a C++ expert: do all C++ compilers support static_cast<void*>? If not, is there a compile-time way to detect of C++ compiler support static_cast<void*> or not?

[visiblehawk]

The OpenFOAM package use openmpi to manage parallel operations, and it includes mpi.h from openmpi package anywhere it needs. The source code of OpenFOAM needs to be compiled with C++ compiler and that is why anywhere it includes and uses mpi.h function, the c++ compiler warns about the usage of old-style-cast.

yes, it should be static_cast<void> instead of (void). I think every modern c++ compiler should support that because it is not new feature...

I have already tried to recompile openmpi-4.0.0 with the following modification, however it does not compile with the "mpi.h:329:55: error: 'static_cast' undeclared (first use in this function)" error.

#define OMPI_PREDEFINED_GLOBAL(type, global) ((type) (static_cast<void *>( &(global))))

[jsquyres]

Open MPI probably fails to compile because it is compiled with a C compiler, not a C++ compiler. At a minimum, I think it would need to be something like this:

#if !OMPI_BUILDING
#if defined(c_plusplus) || defined(__cplusplus)
#define OMPI_PREDEFINED_GLOBAL(type, global) ((static_cast<type>) ((static_cast<void *>) &(global)))
#else
#define OMPI_PREDEFINED_GLOBAL(type, global) ((type) ((void *) &(global)))
#endif
#else
#define OMPI_PREDEFINED_GLOBAL(type, global) ((type) &(global))
#endif

But Open MPI supports fairly old compilers, too. I'm concerned that we'll need something a bit more stringent than just #if defined(c_plusplus) || defined(__cplusplus) to protect the static_cast<> usage.

...that being said, I can easily test as far back as gcc/g++ 4.4.7 (i.e., RHEL 6), and it seems to work. I'll make up a PR and let's see if others can try with ancient c++ compilers and see what happens.