/* Demonstrate how mmapping a file shared can make your program segfault
 * if someone truncates it while you’re accessing it
 */
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <err.h>
#include <errno.h>
#include <stdio.h>

enum { size = 8192 };

int main()
{
  char *n = "tmp.mmapcrash";
  int f = open(n, O_RDWR | O_CREAT, 0666);
  if (f < 0) err(errno, "open: %s", n);
  for (size_t i = 0; i < size; i++) {
    if (write(f, "x", 1) < 0) err(errno, "write");
  }

  close(f);

  int g = open(n, O_RDONLY);
  if (g < 0) err(errno, "open: %s", n);
  int h = open(n, O_RDWR);
  if (h < 0) err(errno, "open: %s", n);

  unsigned char *m = mmap(NULL, size, PROT_READ, MAP_SHARED, g, 0);
  if (!m) err(errno, "mmap");

  size_t checksum = 0;
  for (size_t i = 0; i < size; i++) checksum += m[i];
  printf("checksum is %lu\n", (unsigned long)checksum);

  if (ftruncate(h, size - 4096) < 0) err(errno, "ftruncate");
  checksum = 0;
  for (size_t i = 0; i < size; i++) {
    printf("[%d] ", (int)i);
    fflush(stdout);
    checksum += m[i];
  }

  printf("This is a triumph!  It's still %lu\n", (unsigned long)checksum);

  return 0;
}