Pierre,
There are several issues with the code you provided.
1. You can’t use an MPI_DATATYPE_NULL as the send datatype, not even when count
is zero. At least the root must provide a real datatype. In fact the type
signature of the send message (datatype and count) should match the type
signature of the receiving datatype.
2. I know your count is zero, and no data will be transmitted but your code is
difficult to read and understand.
3. MPI_Iscatter is a collective communication. As such all processes in the
associated communicator (MPI_COMM_WORLD in your case) must participate to the
collective. Thus, calling MPI_Iscatter only where tasked > 0 is incorrect (you
explicitly excluded 0).
4. From the MPI standard perspective your example is not correct, as you are
not allowed to call MPI_Finalize while there are messages pending. Now, Open
MPI tolerate this, but it is clearly not standard behavior.
#include <mpi.h>
int main(int argc, char** argv)
{
int taskid, ntasks;
MPI_Init(&argc, &argv);
MPI_Request rq;
MPI_Comm_rank(MPI_COMM_WORLD,&taskid);
MPI_Comm_size(MPI_COMM_WORLD,&ntasks);
double r;
int l = 0;
MPI_Iscatter(NULL, 0, MPI_DOUBLE, &r, l, MPI_DOUBLE, 0, MPI_COMM_WORLD,
&rq);
MPI_Wait(&rq, MPI_STATUS_IGNORE);
MPI_Finalize();
}
George.
On Nov 21, 2013, at 23:19 , Pierre Jolivet <joli...@ann.jussieu.fr> wrote:
> Hello,
> The following code doesn’t execute properly :
> #include <mpi.h>
>
> int main(int argc, char** argv) {
> int taskid, ntasks;
> MPI_Init(&argc, &argv);
> MPI_Request rq;
>
> MPI_Comm_rank(MPI_COMM_WORLD,&taskid);
> MPI_Comm_size(MPI_COMM_WORLD,&ntasks);
> double* r;
> int l = 0;
> if(taskid > 0)
> MPI_Iscatter(NULL, 0, MPI_DATATYPE_NULL, r, l, MPI_DOUBLE, 0,
> MPI_COMM_WORLD, &rq);
> MPI_Finalize();
> }
>
> Outcome:
> *** An error occurred in MPI_Type_extent
> *** MPI_ERR_TYPE: invalid datatype
>
> Hotfix: change MPI_DATATYPE_NULL to something else.
>
> Thanks for a quick fix.
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