There's probably an alignment gap between the short and char array, and
possibly an alignment gap between the char array and the double array
(depending on the value of SHORT_INPUT and your architecture).
So for your displacements, you should probably actually measure what the
displacements are instead of using sizeof(short), for example.
tVStruct foo;
aiDispsT5[0] = 0;
aiDispsT5[0] = ((char*) &(foo.sCapacityFile) - (char*) &foo);
(I just typed that in my email client and didn't verify it; pardon typos)
On Jun 29, 2011, at 11:42 AM, jody wrote:
> Hi
>
> I have noticed on my machine that a struct which i have defined as
>
> typedef struct {
> short iSpeciesID;
> char sCapacityFile[SHORT_INPUT];
> double adGParams[NUM_GPARAMS];
> } tVStruct;
>
> (where SHORT_INPUT=64 and NUM_GPARAMS=4)
>
> has size 104 (instead of 98) whereas the corresponding MPI Datatype i created
>
> int aiLengthsT5[3] = {1, SHORT_INPUT, NUM_GPARAMS};
> MPI_Aint aiDispsT5[3] = {0, iShortSize, iShortSize+SHORT_INPUT};
> MPI_Datatype aTypesT5[3] = {MPI_UNSIGNED_SHORT, MPI_CHAR, MPI_DOUBLE};
> MPI_Type_create_struct(3, aiLengthsT5, aiDispsT5, aTypesT5,
> &m_dtVegetationData3);
> MPI_Type_commit(&m_dtVegetationData3);
>
> only has length 98 (as expected). The size differences resulted in an
> error when doing
>
> tVegetationData3 VD;
> MPI_Send(&VD, 1, m_dtVegetationData3, 1, TAG_STEP_CMD, MPI_COMM_WORLD);
>
> and the corresponding
>
> tVegetationData3 VD;
> MPI_Recv(&VD, 1, m_dtVegetationData3, MPI_ANY_SOURCE,
> TAG_STEP_CMD, MPI_COMM_WORLD, &st);
>
> (in fact, the last double in my array was not transmitted correctly)
>
> It seems that on my machine the struct was padded to a multiple of 8.
> By manually adding some padding bytes to my MPI Datatype in order
> to fill it up to the next multiple of 8 i could work around this problem.
> (not very nice, and very probably not portable)
>
>
> My question: is there a way to tell MPI to automatically use the
> required padding?
>
>
> Thank You
> Jody
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> us...@open-mpi.org
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--
Jeff Squyres
jsquy...@cisco.com
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