Yes, the tag UB is correct.  MPICH reserves one of the tag bits to help with internal fault information propagation.

  — Pavan

On Mar 13, 2014, at 1:30 AM, Bibrak Qamar <bibrakc@gmail.com> wrote:

> I am trying to get the value of MPI_TAG_UB using the following code:
>
>   MPI_Aint  * maxTag;;
>   int error, flag;
>
>   error = MPI_Comm_get_attr(MPI_COMM_WORLD, MPI_TAG_UB, &maxTag, &flag);
>
>   if(error != MPI_SUCCESS || !flag ){
>   printf( " ERROR IN native mpitagub\n");
>   }
>
>   printf( "mpitagub = %d\n", *maxTag);
>
>
> For MPICH3 I get
>
> mpitagub = 1073741823
>
> I also did it for OpenMPI and I get
>
> mpitagub = 2147483647
>
> The value from OpenMPI makes sense since its 2^31 -1. But at the end the MPI_TAG_UB is MPI implementation dependent.
>
> So I was wondering the value that I got from MPICH3 (mpitagub = 1073741823) is this OK? or my code is wrong somewhere?
>
> Thanks
> Bibrak
>

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