mpi.h: Using a type w/o defining it?

Question

I'm trying to translate the important parts of OpenMPI's mpi.h to the D programming language so I can call it from D. (HTOD didn't work at all.) I can't wrap my head around the following bits of code:

typedef struct ompi_communicator_t *MPI_Comm;
OMPI_DECLSPEC extern struct ompi_communicator_t ompi_mpi_comm_world;
OMPI_DECLSPEC extern struct ompi_communicator_t ompi_mpi_comm_self;
OMPI_DECLSPEC extern struct ompi_communicator_t ompi_mpi_comm_null;

The problem is that ompi_communicator_t is never defined in mpi.h and mpi.h doesn't include any other file besides stddef.h, which clearly doesn't contain a definition. (The comment says it's included for ptrdiff_t.) These are the only four lines in mpi.h that contain the string ompi_communicator_t. Where is the definition of this struct coming from? Are there any tricks I should be aware of where types can appear out of thin air? (There are several other structs like this, but this is the first one I stumbled upon.)

Looks like a typical opaque type. You should be able to safely replace it with anything, as the library only cares about addresses. — Vladimir Panteleev, Aug 04 '11 at 17:43
@dsimcha: nice work! would you share your source and how you compiled the example program? Thank you! — steffen, Feb 05 '13 at 14:52

score 3 · Accepted Answer · answered Aug 04 '11 at 17:44

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This is a pointer to the struct, which internals are not visible outside the OpenMPI. Use any type which can hold a pointer, e.g. (in C) void*.

answered Aug 04 '11 at 17:44

osgx

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Thanks. I never realized that such undeclared-but-used structs are legal in C (they aren't in D). I just made a completely bogus struct to shut the D compiler up about it not being defined since, as you and CyberShadow mention, it's really all just memory addresses anyhow. – dsimcha Aug 04 '11 at 18:09
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I've always used empty structs in these cases: `struct test{}` – jpf Aug 05 '11 at 07:35
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@jpf: That's exactly what I ended up doing. – dsimcha Aug 05 '11 at 21:14

mpi.h: Using a type w/o defining it?

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