lib/timespec-sub.c

   1 /* Subtract two struct timespec values.
   2
   3    Copyright (C) 2011-2017 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert.  */
  19
  20 /* Return the difference between two timespec values A and B.  On
  21    overflow, return an extremal value.  This assumes 0 <= tv_nsec <
  22    TIMESPEC_RESOLUTION.  */
  23
  24 #include <config.h>
  25 #include "timespec.h"
  26
  27 #include "intprops.h"
  28
  29 struct timespec
  30 timespec_sub (struct timespec a, struct timespec b)
  31 {
  32   time_t rs = a.tv_sec;
  33   time_t bs = b.tv_sec;
  34   int ns = a.tv_nsec - b.tv_nsec;
  35   int rns = ns;
  36   time_t tmin = TYPE_MINIMUM (time_t);
  37   time_t tmax = TYPE_MAXIMUM (time_t);
  38
  39   if (ns < 0)
  40     {
  41       rns = ns + TIMESPEC_RESOLUTION;
  42       if (bs < tmax)
  43         bs++;
  44       else if (- TYPE_SIGNED (time_t) < rs)
  45         rs--;
  46       else
  47         goto low_overflow;
  48     }
  49
  50   /* INT_SUBTRACT_WRAPV is not appropriate since time_t might be unsigned.
  51      In theory time_t might be narrower than int, so plain
  52      INT_SUBTRACT_OVERFLOW does not suffice.  */
  53   if (! INT_SUBTRACT_OVERFLOW (rs, bs) && tmin <= rs - bs && rs - bs <= tmax)
  54     rs -= bs;
  55   else
  56     {
  57       if (rs < 0)
  58         {
  59         low_overflow:
  60           rs = tmin;
  61           rns = 0;
  62         }
  63       else
  64         {
  65           rs = tmax;
  66           rns = TIMESPEC_RESOLUTION - 1;
  67         }
  68     }
  69
  70   return make_timespec (rs, rns);
  71 }