[petsc-dev] XXXDestroy() mistaken design in PETSc

[Barry Smith]

On Feb 15, 2011, at 8:03 PM, Matthew Knepley wrote:

> On Tue, Feb 15, 2011 at 7:58 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> 
> On Feb 15, 2011, at 5:26 PM, Matthew Knepley wrote:
> 
> > On Tue, Feb 15, 2011 at 4:47 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> >  In MPI one calls MPI_Comm_free(&comm) to allow the MPI implementation to set the pointer explicitly to 0 after the object is destroyed.
> >
> >  In Petsc XXXDestroy() does not pass the pointer (because it seemed too unnatural to me in 1994) thus not allowing 0ing the pointer.
> >
> >   Was this a bad design decision? Should it be revisited?
> >
> >   Barry
> >
> >  Two use cases
> >
> > 1) error detection when someone tries to reuse a freed object
> >
> > We catch this with other error detection. I do not think we would gain much here.
> 
>  No really. If I do MatDestroy(mat); MatMult(mat,x,y); then it is possible that MatMutl() will crash while looking around inside where mat points. If MatDestroy(&mat); zeroed mat then MatMult(mat,x,y) could do the safe test of if (!mat) nice error message.
> 
> I agree, but the immediate type test at the start of MatMult() has caught most things for me. I do not consider
> double-free a recoverable error, so a SEGV is alright here as well.

  If I am sitting in front of a Matlab or Python scripting session I would much prefer an error that returns to my Matlab or Python prompt so I can keep on doing stuff versus a crash that requires restarting Matlab or Python.

   Barry

> 
>    Matt
>  
> 
>   Barry
> 
> >
> > 2) when removing some objects from a data structure that will be used data one currently needs to do
> >
> >  XXXXDestroy(mystruct->something);CHKERRQ(ierr); mystruct->something = 0;
> >
> > instead of the cleaner XXXDestroy(&mystruct->something);CHKERRQ(ierr);
> >
> > True, but again I do not think the win is large.
> >
> >    Matt
> >
> > --
> > What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> > -- Norbert Wiener
> 
> 
> 
> 
> -- 
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener