[petsc-dev] is VecGetValuesSection correct?

[Lawrence Mitchell]

Hi Vaclav,

> On 21 Jun 2019, at 12:14, Hapla Vaclav via petsc-dev <petsc-dev at mcs.anl.gov> wrote:
> 
> VecGetValuesSection() returns pointer values obtained as follows:
> 
> VecGetArray(v, &baseArray);
> *values = &baseArray[s->atlasOff[p]];
> VecRestoreArray(v, &baseArray);
> 
> It looks to me scary.
> VecGetArray manpage says: If the underlying vector data is not stored in a contiguous array this routine will copy the data to a contiguous array and return a pointer to that.
> VecRestoreArray manpage says: For any special vectors that do not store local vector data in a contiguous array, this routine will copy the data back into the underlying vector data structure from the array obtained with VecGetArray().
> 
> So I guess VecRestoreArray() is free to destroy that contiguous array, right?
> In this case values would point to some nonsensical location in memory.

Yes, for example

VecGetArray_Nest(Vec X, PetscScalar **x)
{
  // allocate space
  ierr = PetscMalloc1(m,x);CHKERRQ(ierr);
  // copy in elided
}

VecRestoreArray_Nest(Vec X, PetscScalar **x)
{
   // copy out elided
   ierr = PetscFree(*x);CHKERRQ(ierr);
}

So VecGetValuesSection is definitely not safe if using VecType == NEST.

I suspect the right thing to do is to make VecGetValuesSection be a method (rather than a single VecSeq implementation). VecNest doesn't support SetValues/GetValues, so arguably it just shouldn't support SetValuesSection/GetValuesSection.

Lawrence