I'm building a portable arena allocator for embedded systems.
Like for malloc, one of the requirements is to return a pointer aligned with the memory.
The question is: Is there a way to know at compile time what are the memory alignment requirements for a given platform in C?
I've seen this code snippets floating around, but I haven't find any documentation to support it's validity:
#ifndef DEFAULT_ALIGNMENT
#define DEFAULT_ALIGNMENT (2*sizeof(void *))
#endif
malloc is required to return a pointer properly aligned to hold an object of any type. Which means it must be aligned at least as the largest scalar type. Luckily C is defining such a type as max_align_t.
In the C11 standard (draft) section 6.2.8 it says:
A fundamental alignment is represented by an alignment less than or equal to the greatest alignment supported by the implementation in all contexts, which is equal to _Alignof (max_align_t).
Test it out:
#include <stdio.h>
#include <stddef.h> // <- this required for max_align_t
int main(void) {
printf("%zu\n", _Alignof (max_align_t));
return 0;
}
