The alignment of dynamically allocating memory

Check Notes from max_align_t:

Pointers returned by allocation functions such as malloc are suitably aligned for any object, which means they are aligned at least as strictly as max_align_t.

It means the memory allocated dynamically is guaranteed to alignof(max_align_t) bytes aligned.

Check Notes from aligned_alloc:

Passing a size which is not an integral multiple of alignment or a alignment which is not valid or not supported by the implementation causes the function to fail and return a null pointer (C11, as published, specified undefined behavior in this case, this was corrected by DR 460).

It means the alignment for aligned_alloc is implementation dependent.

Write a simple program to test aligned_alloc behavior in macOS and Linux (X86_64):

$ cat align.c
#include <stdalign.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>

int main()
{
    printf("alignof(max_align_t)=%zu\n\n", alignof(max_align_t));

    size_t size = 1024;
    size_t align[] = {1, 2, 4, 8, 16, 32, 64};
    for (size_t i = 0; i < sizeof(align) / sizeof(align[0]); i++)
    {
        void *p = aligned_alloc(align[i], size);
        printf("align=%zu, pointer is %p\n", align[i], p);
        free(p);
    }
}

Build and run it in macOS:

$ cc align.c -o align
$ ./align
alignof(max_align_t)=16

align=1, pointer is 0x0
align=2, pointer is 0x0
align=4, pointer is 0x0
align=8, pointer is 0x7fbd48801600
align=16, pointer is 0x7fbd48801600
align=32, pointer is 0x7fbd48801600
align=64, pointer is 0x7fbd48801600

In Linux (X86_64):

$ cc align.c -o align
$ ./align
alignof(max_align_t)=16

align=1, pointer is 0x5645aec676b0
align=2, pointer is 0x5645aec676b0
align=4, pointer is 0x5645aec676b0
align=8, pointer is 0x5645aec676b0
align=16, pointer is 0x5645aec676b0
align=32, pointer is 0x5645aec67ac0
align=64, pointer is 0x5645aec67f40

Both macOS and Linux (X86_64) have the same alignment of allocating memory from free storage: 16 bytes. macOS requires the alignment of aligned_alloc is at least 8 bytes; whilst Linux (X86_64) doesn’t have this requirement.

P.S., the code can be downloaded here.