[petsc-dev] PetscMalloc for size zero

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:

> On Jan 30, 2014, at 3:00 PM, Jed Brown <jed at jedbrown.org> wrote:
>
>     Yes, but in the debugger (or with a printf %p there is no portable
>     way for a user to know that the pointer is invalid) whereas with
>     NULL they would immediately know. A null pointer that leads to a
>     segv conveys something different to someone debugging than some
>     address where the user doesn’t know if that address came from a
>     malloc(0) or some completely different way (like crazy pointer
>     arithmetic).

True, but an array length is needed anyway and any references to that
pointer can still be detected by the memory checker.  Also, if someone
really wants to get NULL (or a special value), we can use a malloc
implementation that does so.  (We can add a runtime option for this if
someone wants it for debugging.)

>    If you are trying to debug some code that you didn’t personally
>    write (that may have been written by an idiot) it is rarely
>    possible for you to know where this out-of-bound information is.

Fair, but you'll have as much trouble doing anything with that code.

>    We would obviously turn off all of this completely when running
>    under valgrind to let valgrind do its thing.
>
>     Side note; PetscMallocn() needs much more error checking: for
>     example PetscMalloc2(-8,&x1,10,&x2) would not be detected as a
>     problem but is problematic. 

Is this a problem actually encountered in practice?

>     Since we have to add all this error checking (likely by making
>     these beasts functions) adding a return NULL on 0 won’t muck up
>     the code much.

We can't make them functions because we use the type sizes and pass
__FILE__, __LINE__.  I'm not motivated to change this unless someone
demonstrates that this is a non-theoretical problem.
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