I am trying to write a custom stack allocator for a game I am working on. To deal with memory alignment, I am using C11’s stdalign.h header; however, since I do not have prior experience with alignment issues, I’d like a expert opinion on whether I am doing it right.

The majority of the alignment stuff happens in alignup and aligndown macros. Specifically, in alignup, I try to go up to the nearest multiple of alignof(max_align_t) (unless we were already a multiple) by shooting past it by adding alignof(max_align_t) - 1 and then coming back using the bitwise AND. In aligndown, I first go below the last multiple of alignof(max_align_t) and then come up using alignup.

NOTE: The stack is in a memory arena and it grows down in memory instead of going up.

typedef size_t    Size; typedef uintptr_t Uptr; typedef char      Byte;  #define alignup(p) ((void*)((((Uptr)(p)) + (alignof(max_align_t) - 1)) & (~ (alignof(max_align_t) - 1))))  #define aligndown(p) ((void*)(alignup((((Uptr)(p)) - (alignof(max_align_t) - 1)))))  struct _Mem_Stack_Header {     Size allocation_size;     Size total_size; };  // This contains the "head" of the stack. The stack grows down, instead of up. void *stack_head;  internal_function void* memPush (Size s) {     void *head = stack_head;      void *mem = (Byte*)head - s;     mem = aligndown(mem);      struct _Mem_Stack_Header *h = (void*)((Byte*)mem - sizeof(*h));     h =  aligndown(h);     h->allocation_size = s;     h->total_size = (Uptr)head - (Uptr)h;      stack_head = h;      return mem; }  internal_function void memPop (void) {     struct _Mem_Stack_Header *h = stack_head;      stack_head = (Byte*)stack_head + h->total_size;      return; } 

Is this the correct way of doing this kind of stuff? Any improvements I can make?