observable-allocations.rst

Observable Allocations

Contents

• Examples
• Other Allocation Properties
  • Nullability
  • Zero Sided Allocations
  • Object Crossing Compares
  • Distinct Heap/Stack/Globals
  • Guessed Pointers

Examples

free(malloc(8));
==>
nop

malloc(0);
==>
nullptr

free(realloc(o, N))
==>
free(o);

o1 = realloc(o, 0)
// (if o1's address is not captured)
==>
free(o);
o1 = nullptr;

free(new int())
==>
unreachable

o = malloc(8);
lifetime_end(o+4, 4)
==>
o = malloc(4)

o = malloc(8);
lifetime_end(o, 4)
==>
o = malloc(4) - 4

if (o != nullptr) free(o)
==>
free(o)

Other Allocation Properties

Nullability

allocate(N) == nullptr?

Zero Sided Allocations

allocate(0) == allocate(0)?

Is the allocator required to return distinct addresses? Or guaranteed to fail via exception or error-val on zero sized allocation?

Object Crossing Compares

allocate(N) + N == allocate(M)?

Is this well defined?

Distinct Heap/Stack/Globals

If the prior example is well defined, are there limits of which objects can be compared?

allocate(N) == &global_var?
allocate(N) == &stack_var?

Which of these are well defined?

Guessed Pointers

allocate(N) == cast<pointer>(0xmyconstant)?

Is this well defined?