[Mpi-forum] C-MPI DYNAMIC ALLOCATION / DERIVED DATATYPES

[Simone Pellegrini]

On 02/11/2010 01:42 PM, kostas pinokio wrote:
> HALLO,
>
> I would like to send a whole column of a dynamicly allocated array (in 
> C language).
> When i use derived datatypes(for example MPI_Type_vector) i can send a 
> column ONLY if i have static array but when i use dynamic allocation i 
> have problem!
> Please tell me one way to send a column of a dynamc allocated array.
>
> For example :
>       MPI_Datatype columntype;
>       MPI_Type_vector(10, 1, 10, MPI_INT, &columntype);
>       MPI_Type_commit(&columntype);
>  if(rank==0){
>                      dyn_array=(int **) calloc(10,sizeof(int *));
>                      for(i = 0; i < 10; i++){
>                              dyn_array[i] =(int *) calloc(10,sizeof(int));
>                      }
>                      for(i=0;i<10;i++){ for(j=0;j<10;j++) 
> dyn_array[i][j]=9;}
>                   MPI_Send(&dyn_array[0][0], 1, columntype, dest, 
> tag,MPI_COMM_WORLD);
>  }
> if(rank==1){
>     MPI_Recv(&other_dyn_array[0][0], 1, columntype, 0, tag, 
> MPI_COMM_WORLD, &Stat);
> }
>
> the values of the receive are not correct!!
>

The MPI_Type_vector assumes your data is contiguous in memory and this 
is not the case in your code.
As each raw is allocated using a different calloc. You need to arrange 
the 2D array in memory in a more clever way:

if(rank==0){
                      dyn_array=(int **) calloc(10,sizeof(int *));
                      dyn_array[0] = (int*) calloc(10*10, sizeof(int));
                      for(i = 1; i < 10; i++)
                              dyn_array[i] = dyn_array[0] + i*10;
                      /*same code*/
  }

this should work!

cheers, Simone
>
> Thank You!
>
>
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