[petsc-dev] Problem with PETSC_NULL and variadic functions

[Lisandro Dalcin]

On 9 November 2011 17:26, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> #define PETSC_NULL           0
> This is an "int" value, though it's special in that it will be implicitly
> converted to a null pointer (not even necessary bitwise 0, according to the
> standard) if its value is assigned to a pointer. If we pass it to a function
> with unspecified arguments (e.g. variadic), then it will be passed as an
> int.
> We usually use PETSC_NULL in place of a pointer value. It is common for an
> entire pointer-length integer register to be used when passing an "int".
> Most architectures pass the first few arguments in registers before passing
> on the stack. We seem to be getting lucky so far in that we haven't used
> functions like DMCompositeGetAccess() on systems where sizeof(void*) !=
> sizeof(int), with more arguments than are passed in registers.
> C99 has stddef.h define NULL to be a null pointer constant ((void*)0). This
> is not compatible with C++ and I don't see a simple resolution.
> It doesn't work in C++ because there is no implicit pointer conversion from
> void*. So instead of having a pointer-valued NULL like in C, they define
> NULL to be 0 or 0L, which is just an integer (unless assigned to a pointer
> in which case 0 is special and is converted implicitly).
> Since it was 2011 and there was still no way to define a null pointer in
> C++, the new standard C++11 introduces a new keyword nullptr. Considering
> that we are still not allowed to use C99 in PETSc, it seems unlikely that we
> would be allowed to rely on C++11 which is less than two months old.
>
> We could pass a "format string" indicating which entries we were actually
> requesting. Other ideas?

Perhaps we could #define PETSC_NULL NULL ?

In my 64 bits Fedora 15, sizeof(NULL) is 8 with g++ and icpc. But I'm
not sure all compilers/platforms will work like that. In those cases,
you could conditionally define to 0 or 0L depending of sizeof(void*)


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Lisandro Dalcin
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