[petsc-dev] Problem with PETSC_NULL and variadic functions

[Jed Brown]

#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?
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