[petsc-users] accessing DMDA Vec ghost values

[Sean Dettrick]

Hi,

When discussing DMDAVecGetArrayDOF etc in section 2.4.4,  the PETSc 3.7 manual 
says "The array is accessed using the usual global indexing on the entire grid, 
but the user may only refer to the local and ghost entries of this array as all 
other entries are undefined”.

OK so far.  But how to access the ghost entries?

With a 2D DMDA, I can do this OK:


        PetscInt    xs,xm,ys,ym;

        ierr=DMDAGetCorners(da,&xs,&ys,0,&xm,&ym,0);CHKERRQ(ierr);

        PetscScalar ***es;

        ierr=DMDAVecGetArrayDOF(da,Es,&es);CHKERRQ(ierr);


        for (int j=ys; j < ys+ym; j++) {

            for (int i=xs; i < xs+xm;i++) {

es[j][i][0]=1.;

                es[j][i][1]=1.;

            }

        }

        ierr=DMDAVecRestoreArrayDOF(da,Es,&es);CHKERRQ(ierr);

But if I replace DMDAGetCorners with DMDAGetGhostCorners, then the code crashes 
with a seg fault, presumably due to out of bounds memory access.

Is that supposed to happen?  What’s the remedy?

Thanks very much!

Sean Dettrick