petsc-3.3-p7 2013-05-11

VecGhostGetLocalForm

Obtains the local ghosted representation of a parallel vector (obtained with VecCreateGhost(), VecCreateGhostWithArray() or VecCreateSeq()). Returns PETSC_NULL if the Vec is not ghosted.

Synopsis

#include "petscvec.h"   
PetscErrorCode  VecGhostGetLocalForm(Vec g,Vec *l)

Not Collective

Input Parameter

g -the global vector

Output Parameter

l -the local (ghosted) representation, PETSC_NULL if g is not ghosted

Notes

This routine does not actually update the ghost values, but rather it returns a sequential vector that includes the locations for the ghost values and their current values. The returned vector and the original vector passed in share the same array that contains the actual vector data.

To update the ghost values from the locations on the other processes one must call VecGhostUpdateBegin() and VecGhostUpdateEnd() before accessing the ghost values. Thus normal usage is

    VecGhostUpdateBegin(x,INSERT_VALUES,SCATTER_FORWARD);

    VecGhostUpdateEnd(x,INSERT_VALUES,SCATTER_FORWARD);

    VecGhostGetLocalForm(x,&xlocal);

    VecGetArray(xlocal,&xvalues);

       // access the non-ghost values in locations xvalues[0:n-1] and ghost values in locations xvalues[n:n+nghost];

    VecRestoreArray(xlocal,&xvalues);

    VecGhostRestoreLocalForm(x,&xlocal);

One should call VecGhostRestoreLocalForm() or VecDestroy() once one is finished using the object.

Examples