[mpich-discuss] attribute value of MPI_TAG_UB

Balaji, Pavan balaji at anl.gov
Thu May 2 19:43:20 CDT 2019

Attributes have these weird point-to-integer casting semantics.  Make sure you are using it correctly.

See https://github.com/pmodels/mpich/blob/master/test/mpi/attr/baseattr2.c#L30

Basically, even though the parameter is "void *", it really needs to be treated as "int **" in this case.

  -- Pavan

> On Apr 26, 2019, at 2:18 PM, Wei-keng Liao via discuss <discuss at mpich.org> wrote:
> According to MPI 3.1, Section 8.1.2, the attribute value of
> MPI_TAG_UB attached to MPI_COMM_WORLD can be inquired by function
> MPI_Comm_get_attr(). It has the same value on all processes of
> My first question is should this value be the same across different runs?
> A small test program using MPICH 3.3 shows they are not the same across
> runs and across processes. But when compiled with the latest master branch,
> they are the same. So, can I assume the answer to my question is YES and
> MPICH has fixed this in the master branch?
> My second question is that I notice MPI_TAG_UB is defined as a constant
> of 0x64400001 (=1681915905) in mpi.h. That value is not the one returned
> by MPI_Comm_get_attr(), which is 10345120. Is this intended?
> I also tested OpenMPI 4.0.0. The inconsistency occurs across different
> runs, as well as different processes. I can see OpenMPI defines
> MPI_TAG_UB as an enum type, rather than a constant.
> Wei-keng
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