dot dot is dead

// Copyright 2007, Josh MacDonald
// 
// I run Gentoo on a small computer with a bad board.  I had run
// Windows on it "successfully", but the electronics were too noisy
// for audio, so I could run Kazaa but couldn't play the music.  It
// wasn't until installing Linux that I found it had bad RAM.  I
// replaced the memory, and now it's working with what you might call a
// "variable speed" clock.  The clock skews several seconds in
// several days.
//
// Laziness made me do it.

#include <sys/time.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

int change_time(double arg) {

  struct timeval tv;
  struct timezone tz;

  if (gettimeofday(&tv, &tz)) {
    perror("gettimeofday");
    return -1;
  }

  // in version 2, finer resolution!
  tv.tv_sec += (int)arg;

  printf("setting time %+f\n", arg);

  if (settimeofday(&tv, &tz)) {
    perror("settimeofday");
    return -2;
  }
  return 0;
}

int how_far_off(char *dir, double *offset) {
  struct timezone tz;
  struct timeval tv;
  struct stat buf;
  char *fn = (char*)malloc(1024);

  strcpy(fn, dir);
  strcat(fn, "/time-stamp");
  unlink(fn);
  FILE *f = fopen(fn, "w");
  fclose(f);
  if (stat(fn, &buf)) {
      perror("stat");
      return -1;
  }
  double ft = buf.st_mtime /* version 2: buf.st_mtim.tv_nsec * 10e-9 */;

  if (gettimeofday(&tv, &tz)) {
    perror("gettimeofday");
    return -1;
  }

  double tt = tv.tv_sec /* version 2: tv.tv_usec * 10e-6 */;

  *offset = (ft - tt);

  printf("difference is %.05f secs\n", *offset);
  return 0;
}

int main(int argc, char **argv) {
  double arg;
  int ret;

  if (argc != 2) {
    printf("usage: program directory\n");
    return 2;
  }

  if ((ret = how_far_off(argv[1], &arg)) ||
      (arg == 0) ||
      (ret = change_time(arg))) {
    return -ret;
  }

  return 0;
}

(amaranta here's a kiss, I chose you to end this list, the unlucky márquez daughter, the nails lyrics #44, 88 line UNIX/C source code to set the local time based on a network filer)