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

int main(void)
 {
size_t  size = 16;
size_t resize = 1024;
size_t align = 1 << 12;
int *ptr;
int *ptr1;

if (posix_memalign((void **)&ptr, align , size) != 0) {
  exit(0EXIT_FAILURE);
}

ptr[0] = 12;
ptr[1] = 25;

printf("memory aligned to %d bytes\n\n",align);
printf("ptr[0]   : %p = %d\n",ptr, ptr[0]);
printf("ptr[1]   : %p = %d\n\n",&ptr[1], ptr[1]);

if ((ptr1 = realloc((int *)ptr, resize)) == NULL) {
  exit(0EXIT_FAILURE);
}

printf("After realloc(): \n");
printf("ptr1[0]   : %p = %d\n",ptr1, ptr1[0]);
printf("ptr1[1]  : %p = %d\n\n",&ptr1[1], ptr1[1]);

free(ptr1);
return 0;
}

memory aligned to 4096 bytes

ptr[0]   : 0x1f8af000 = 12
ptr[1]   : 0x1f8af004 = 250x1621b000

After realloc():
ptr1[0]   : 0x1f8ae010 = 12
ptr1[1]  : 0x1f8ae014 = 250x1621a010