[petsc-dev] multiple VecGetArray

[Barry Smith]

On Aug 31, 2010, at 1:07 PM, Jed Brown wrote:

> On Tue, 31 Aug 2010 11:21:02 -0300, Lisandro Dalcin <dalcinl at gmail.com> wrote:
>> Calling VecGetArray multiple times on the same vec seems to work (at
>> least for native vectors). However, in VecSeq impl there is code to
>> prevent such usage. What's the correct usage?
> 
> I think that correct usage should not allow multiple access to the same
> vector, so that VecGetArray implies exclusive access.  

   This was the intention. Only one writer could access at a time (several can use VecGetArrayRead() at the same time but there cannot be a writer at the same time). The code that generated an error with multiple access dissolved over time leaving that one useless fragment in the VecGetArray_Seq() which is not used. It would be nice if it was put back in (note by putting the flag into the _p_Vec structure you don't need a function call to do the error checking).

   Barry


> I think PETSc is
> consistent about this internally and it just doesn't check for native
> vectors to avoid a function call.
> 
>  VecGetArray(X,&x);
>  VecGetArray(X,&y);
>  FunctionAssumingNoAliasing(x,y);
>  VecRestoreArray(X,&x);  /* Is it safe to free buffer/move to GPU? */
>  y[0] = 2.;              /* If so, then this line is invalid. */
> 
> Jed