#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

#define NPAG 200

int main(int argc, char *argv[])
{
  char *src, *dst, *ptr, buf[255];

  if ((src = mmap(0, getpagesize(), PROT_READ | PROT_WRITE,
      MAP_SHARED | MAP_ANONYMOUS, -1, 0)) == MAP_FAILED) {
    perror("mmap()");
    return 1;
  }

  printf("mapped to %p\n", src);
  if ((dst = mremap(src, getpagesize(), NPAG * getpagesize(),
      MREMAP_MAYMOVE)) == MAP_FAILED) {
    perror("mremap()");
    return 1;
  }

  sprintf(buf, "cat /proc/%i/maps", getpid());
  system(buf);
  fflush(stdout);

  ptr = dst + (NPAG - 5) * getpagesize();
  printf("trying %p (%p)...\n", ptr, ptr-dst);
  *ptr = 20;
  printf("%#x, works!\n", *ptr);
  ptr = dst + (NPAG - 3) * getpagesize();
  printf("trying %p (%p)...\n", ptr, ptr-dst);
  *ptr = 20;
  printf("%#x, works!\n", *ptr);
  return 0;
}