How to invoke aligned new/delete properly? - c++

Question

How do I call new operator with alignment?
auto foo = new(std::align_val_t(32)) Foo; //?
and then, how to delete it properly?
delete(std::align_val_t(32), foo); //?
If this is the right form of using these overloads, why valgring complaining about mismatched free()/delete/delete[]?

Answer 1

exist very basic principle - the memory free routine always must match to allocate routine. if we use mismatch allocate and free - run time behavior can be any: all can be random ok, or crash by run-time, or memory leak, or heap corruption.
if we allocate memory with aligned version of operator new
void* operator new ( std::size_t count, std::align_val_t al);
we must use the corresponding aligned version of operator delete
void operator delete ( void* ptr, std::align_val_t al );
call void operator delete ( void* ptr ); here always must lead to run-time error. let simply test
std::align_val_t al = (std::align_val_t)256;
if (void* pv = operator new(8, al))
{
operator delete(pv, al);
//operator delete(pv); this line crash, or silently corrupt heap
}
why is aligned and not aligned version of operator delete always incompatible ? let think - how is possible allocate align on some value memory ? we initially always allocate some memory block. for return align pointer to use - we need adjust allocated memory pointer to be multiple align. ok. this is possible by allocate more memory than requested and adjust pointer. but now question - how free this block ? in general user got pointer not to the begin of allocated memory - how from this user pointer jump back to begin of allocated block ? without additional info this is impossible. we need store pointer to actual allocated memory before user returned pointer. may be this will be more visible in code typical implementation for aligned new and delete use _aligned_malloc and _aligned_free
void* operator new(size_t size, std::align_val_t al)
{
return _aligned_malloc(size, static_cast<size_t>(al));
}
void operator delete (void * p, std::align_val_t al)
{
_aligned_free(p);
}
when not aligned new and delete use malloc and free
void* operator new(size_t size)
{
return malloc(size);
}
void operator delete (void * p)
{
free(p);
}
now let look for internal implementation of _aligned_malloc and _aligned_free
void* __cdecl _aligned_malloc(size_t size, size_t alignment)
{
if (!alignment || ((alignment - 1) & alignment))
{
// alignment is not a power of 2 or is zero
return 0;
}
union {
void* pv;
void** ppv;
uintptr_t up;
};
if (void* buf = malloc(size + sizeof(void*) + --alignment))
{
pv = buf;
up = (up + sizeof(void*) + alignment) & ~alignment;
ppv[-1] = buf;
return pv;
}
return 0;
}
void __cdecl _aligned_free(void * pv)
{
if (pv)
{
free(((void**)pv)[-1]);
}
}
in general words _aligned_malloc allocate size + sizeof(void*) + alignment - 1 instead requested by caller size. adjust allocated pointer to fit alignment , and store originally allocated memory before pointer returned to caller.
and _aligned_free(pv) call not free(pv) but free(((void**)pv)[-1]); - for always another pointer. because this effect of _aligned_free(pv) always another compare free(pv). and operator delete(pv, al); always not compatible with operator delete(pv); if say delete [] usual have the same effect as delete but align vs not align always run time different.

Answer 2

The below syntax was the only one that worked for me to create and destroy an overaligned array, using clang-cl 13 on Windows 10 x64:
int* arr = new (std::align_val_t(64)) int[555];
::operator delete[] (arr, std::align_val_t(64));
For the same new operation, the below delete expression would not compile ("cannot delete expression of type 'std::align_val_t'):
delete[] (arr, std::align_val_t(64));
The below delete expression will compile, but then throws a runtime error ("
Critical error detected c0000374"):
delete[](std::align_val_t(64), blocks);