c89_align_ptr.c

#if defined(__STDC__)
#define IS_MAYBE_C89
  #if defined(__STDC_VERSION__)
    #if (__STDC_VERSION__ >= 199409L)
      #define IS_NOT_C89
    #endif
  #endif
#endif

#ifndef IS_MAYBE_C89
  #error "not C89 compatible"
#endif
#ifdef IS_NOT_C89
  #error "not C89 compatible"
#endif

/* only for x86_64 */
typedef short unsigned int uint16_t;
typedef unsigned int uint32_t;
typedef unsigned long uint64_t;
typedef unsigned long size_t;
typedef unsigned long uintptr_t;

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void ptrtointtoptr(void);

static void memset_16aligned(void * ptr, char byte, size_t size_bytes, uint16_t alignment) {
    assert((size_bytes & (alignment-1)) == 0);
    assert(((uintptr_t)ptr & (alignment-1)) == 0);
    memset(ptr, byte, size_bytes);
}
void ptrtointtoptr(void) {
  const uint16_t alignment = 16;
  const uint16_t align_min_1 = alignment - 1;
  void * mem = malloc(1024+align_min_1);
  void *ptr = (void *)((((uint64_t)mem)+align_min_1) & ~(uint64_t)align_min_1);
  printf("0x%lu, 0x%lu\n", (unsigned long)mem, (unsigned long)ptr);
  memset_16aligned(ptr, 0, 1024, alignment);
  free(mem);
}

int main(void) { ptrtointtoptr(); }

usage

clang -std=c89 -Weverything ptrtoint_inttoptr.c && ./a.out