Introduction to Time

• This is chapter 10, you should read it.
• Time is measured as the number of seconds since the Epoch
  • The epoch is Jan 1, 1970 UTC (or GMT) on UNIX
• time
  • time_t was a signed int.
    • Or 2^31-1 seconds
    • That is 2,147,483,647 seconds
    • Or 35,791,394 min
    • OR 596,523 hours
    • Or 24,855 days
    • Or 68 years.
    • 1970+68 = 2038, I'll be 74, so I won't care!
    • You will be 40 or so, and you will.
      • You will have to throw away anything running with less than 64 bit time_t
      • Or at least recompile your code.
      • And you might have a problem on your embedded devices.
    • But perhaps not.
      • On my machine, it is a 64 bit number.
      • That's 1,144,561,273,430,837,494,885,949,696,426
      • Or 36,293,799,893,164,557,803,334 years.
  • #include <time.h>
  • time_t time(time_t *tloc);
  • Returns the current time.
  • If tloc is not null, it is also stored there.
  • If tloc is null no value is stored.
• Time Conversion
  • For now:
  • char *asctime(const struct tm *tm);
  • struct tm *localtime(const time_t *timep);
  • These two routines take a time_t and convert it to a string representing the local time.
  • There is far more to this, but I want to use it in my example program.
• Look at start.C
• We can also measure tprocess time
  • This is not the good way to do this, c++11 has the chrono package.
  • #include <sys/time.h>
  • int gettimeofday(struct timeval *tv, struct timezone *tz);
  • int settimeofday(const struct timeval *tv, const struct timezone *tz);

  • 
               struct timeval {
                   time_t      tv_sec;     /* seconds */
                   suseconds_t tv_usec;    /* microseconds */
               };
              struct timezone {
                   int tz_minuteswest;     /* minutes west of Greenwich */
                   int tz_dsttime;         /* type of DST correction */
               };