Re: memory allocators & proper alignment...
On May 22, 8:14 pm, "Chris Thomasson" <cris...@comcast.net> wrote:
One more quick question...
wrt overloading global new and delete operators...
struct alignchar {
char m_buf;
};
Not sure what you're trying to achieve here, but on all of the
platforms I have access to, sizeof( alignchar ) is 1. About the
only cases where I would expect something different would be on
a word addressed machine.
void* ::operator new(size_t sz) {
// alloc and align buf on sizeof(alignchar) boundary
char* const buf = ::mymalloc_aligned(sz, sizeof(alignchar));
if (! buf) { throw std::bad_alloc(); }
return buf ;
}
void ::operator delete(void *buf) {
::myfree(buf);
}
Is that sufficient alignment? AFAICT, void* ::operator new(size_t) does n=
ot
have to align for the specific C++ type... It just has to mimic malloc
alignment requirements right?
The returned address should be sufficiently aligned for any data
type. Just like malloc, yes.
And, the size variable (e.g., size_t sz) passed to the void* ::operator
new(size_t) function when new is called with any type should be large eno=
ugh
to accommodate the alignment for those types?
Run that by me again. There's no relationship between the size
argument and the alignment requirements for operator new.
Although logically... my machine requires an alignment of 8 for
double, but it doesn't make much sense to require operator
new(4) to return memory aligned on an 8 byte boundary, because
there's no way you can write anything larger than a float or an
int into the returned memory, and they only require an alignment
of 4. But the requirement in the standard (=A73.7.3.1) is clear:
"The pointer returned shall be suitably aligned so that it can
be converted to a pointer of any complete object type and then
used to access the object or array in the storage allocated."
"Any complete object type", and not "Any complete object type
which will fit in the allocated memory".
The usual way to determine alignment of a type T, I believe, is
something like:
struct AlignOfT { char a ; T b ; } ;
size_t alignOfT() { return offsetof( AlignOfT, b ) ; }
(This only works for POD types, of course.)
If you want to ensure alignment for any possible type, use a
union of all the basic types, plus a few pointers (to char, to a
struct, to a function, etc.) for T in the above. It's not
formally guaranteed by the standard, but it should work on any
real platform.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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