[slang-users] TIme Routines.

[John E. Davis]

jmrobert5 at mchsi.com <jmrobert5 at mchsi.com> wrote:
>A puzzler...

Apparantly, newer versions of glibc return -1 when the tm struct has a
value before the epoch.  This is because various standards require
mktime to return -1 upon error.  As a result, how does one distinguish
one second before the epoch from an error?

The way solaris handles this is via errno, i.e.,

    errno=0;
    if (((time_t)-1 == (secs = mktime(&tm)))
        && errno)
      {
         /* error occurred */
	   .
	   .
      }

I would have preferred glibc to adopt a similar approach.

For now one work-around would be to write a slang routine to "solve"
for the value of t such that

    tm = localtime (t);

where tm is given.  A binary search (bisection) algorithm would
probably work best.  In fact, before mktime was added to slang, I
wrote my own version of mktime using such a method:

define my_mktime (tm)
{
   variable secs1 = _time ();
   variable secs0 = 0;

   while (secs1 != secs0)
     {
	variable tm_1 = localtime (secs1);
	variable tm_0 = localtime (secs0);
	variable secs = (secs0 + secs1)/2;
	variable tm2 = localtime (secs);
	
	if (tm2.tm_year > tm.tm_year)
	  {
	     secs1 = secs;
	     continue;
	  }
	if (tm2.tm_year < tm.tm_year)
	  {
	     secs0 = secs;
	     continue;
	  }
	
	if (tm2.tm_mon > tm.tm_mon)
	  {
	     secs1 = secs;
	     continue;
	  }
	if (tm2.tm_mon < tm.tm_mon)
	  {
	     secs0 = secs;
	     continue;
	  }

	if (tm2.tm_mday > tm.tm_mday)
	  {
	     secs1 = secs;
	     continue;
	  }
	if (tm2.tm_mday < tm.tm_mday)
	  {
	     secs0 = secs;
	     continue;
	  }


	if (tm2.tm_hour > tm.tm_hour)
	  {
	     secs1 = secs;
	     continue;
	  }
	if (tm2.tm_hour < tm.tm_hour)
	  {
	     secs0 = secs;
	     continue;
	  }

	if (tm2.tm_min > tm.tm_min)
	  {
	     secs1 = secs;
	     continue;
	  }
	if (tm2.tm_min < tm.tm_min)
	  {
	     secs0 = secs;
	     continue;
	  }
	
	if (tm2.tm_sec > tm.tm_sec)
	  {
	     secs1 = secs;
	     continue;
	  }
	if (tm2.tm_sec < tm.tm_sec)
	  {
	     secs0 = secs;
	     continue;
	  }
	
	break;
     }
   
   return secs0;
}