#include <stdlib.h>
#include <stdio.h>
#include <sys/mman.h>
#include <unistd.h>

struct foo {
  int a;
  char data[500];
};

void fn(void *base) {
  struct foo *a = (struct foo*)base;
  printf("a.a: %d\n", a->a);
  printf("a data: %s\n", a->data);
  struct foo *b = base + sizeof(struct foo);
  printf("a starts at: %p, foo is %lu big, so b starts at %p\n", a, sizeof(struct foo), b);
}

int main(int argc, char *argv[argc+1]) {
  // 100 MB
  size_t len = 100 << 20;
  void *base = mmap(0, len, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
  fn(base);
  munmap(base, len);
}