It's portably assuming that you can cast an allocation's pointer to a pointer to unsigned char. It's like assuming you can memset(), since that's effectively w

It's portably assuming that you can cast an allocation's pointer to a pointer to unsigned char. It's like assuming you can memset(), since that's effectively what it's doing. In the absence of memory over-commit, this will have no effect beyond wasting resources. In the presence of over-commit -bounded stacks resemble memory over-commitment!- it will have the effect of committing the memory OR failing in whatever way the machine fails when resources are exhausted. However, in the latter case, and when the machine uses stacks for C function calls, the failure will be defined by the machine rather than undefined (due to memory corruption). Also, in the absence of stacks, there's no "direction in which the stack grows", and any attempt to determine that direction will yield a nonsensical result, but doing memset() in one direction or the other will also then be equivalent anyways. EDIT: There is one non-portable thing here: the assumption that machines are more limited than size_t implies. However, that's a distinction without a difference for any 64-bit ABIs or larger because all such machines must be more limited than size_t implies. That there is any failure mode when resources are exhausted is a non-portable assumption is uselessly true. Not being able to assume defined resource exhaustion failure modes would be destructive -- that the C spec does not is irrelevant.