boot time, process start time, and NOW time

[Albert Cahalan <albert@users.sf.net>]

Even with the 2.6.7 kernel, I'm still getting reports of process
start times wandering. Here is an example:

   "About 12 hours since reboot to 2.6.7 there was already a
   difference of about 7 seconds between the real start time
   and the start time reported by ps. Now, 24 hours since reboot
   the difference is 10 seconds."

The calculation used is:

   now - uptime + time_from_boot_to_process_start

The code shown below works great on a 2.4.xx or earlier kernel.
It generally relys on USER_HZ, which is supposedly in our ABI.

I have a feeling we'll forever be chasing bugs related to not
using a PLL to drive the clock tick at exactly HZ ticks per second.
Perhaps the DragonflyBSD code could be stolen. Anyway, the code:

///////////////////////////////////////////////////////////////////////////
unsigned long seconds_since_1970 = time(NULL);
unsigned long seconds_since_boot = uptime(0,0);
unsigned long time_of_boot       = seconds_since_1970 - seconds_since_boot;

int pr_stime(char *restrict const outbuf, const proc_t *restrict const pp){
  struct tm *proc_time;
  struct tm *our_time;
  time_t t;
  const char *fmt;
  int tm_year;
  int tm_yday;
  our_time = localtime(&seconds_since_1970);   /* not reentrant */
  tm_year = our_time->tm_year;
  tm_yday = our_time->tm_yday;
  t = time_of_boot + pp->start_time / Hertz;
  proc_time = localtime(&t); /* not reentrant, this corrupts our_time */
  fmt = "%H:%M";                                   /* 03:02 23:59 */
  if(tm_yday != proc_time->tm_yday) fmt = "%b%d";  /* Jun06 Aug27 */
  if(tm_year != proc_time->tm_year) fmt = "%Y";    /* 1991 2001 */
  return strftime(outbuf, 42, fmt, proc_time);
}
///////////////////////////////////////////////////////////////////////////