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

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

char *greeting = "Hello again %s";

void fn(void *base) {
  struct foo *a = (struct foo*)base;
  a->a = 12345;
  sprintf(a->data, greeting, getenv("USER"));
  printf("a.a: %d\n", a->a);
  printf("a data: %s\n", a->data);
}

int main(int argc, char *argv[argc+1]) {
  int fd = open(argv[1], O_CREAT | O_RDWR);    
  // 100 MB
  size_t len = 100 << 20;
  if (ftruncate(fd, len) == -1) {
    perror("truncate");
    return EXIT_FAILURE;
  }
  printf("fd: %d\n", fd);
  void *base = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
  if (base == MAP_FAILED) {
    perror("mmap"); 
    return EXIT_FAILURE;
  }
  fn(base);
  printf("Syncing...\n");
  msync(base, len, MS_SYNC);
  printf("Unmapping...\n");
  munmap(base, len);
}