#include "petscmat.h" PetscErrorCode MatMPIAIJSetPreallocationCSR(Mat B,const PetscInt i[],const PetscInt j[], const PetscScalar v[])Collective
| B | - the matrix | |
| i | - the indices into j for the start of each local row (starts with zero) | |
| j | - the column indices for each local row (starts with zero) | |
| v | - optional values in the matrix | 
The i and j indices are 0 based, and i indices are indices corresponding to the local j array.
The format which is used for the sparse matrix input, is equivalent to a row-major ordering.. i.e for the following matrix, the input data expected is as shown
       1 0 0
       2 0 3     P0
      -------
       4 5 6     P1
    Process0 [P0]: rows_owned=[0,1]
       i =  {0,1,3}  [size = nrow+1  = 2+1]
       j =  {0,0,2}  [size = 3]
       v =  {1,2,3}  [size = 3]
    Process1 [P1]: rows_owned=[2]
       i =  {0,3}    [size = nrow+1  = 1+1]
       j =  {0,1,2}  [size = 3]
       v =  {4,5,6}  [size = 3]