1  /* Copyright (C) 1991-2023 Free Software Foundation, Inc.
       2     This file is part of the GNU C Library.
       3  
       4     The GNU C Library is free software; you can redistribute it and/or
       5     modify it under the terms of the GNU Lesser General Public
       6     License as published by the Free Software Foundation; either
       7     version 2.1 of the License, or (at your option) any later version.
       8  
       9     The GNU C Library is distributed in the hope that it will be useful,
      10     but WITHOUT ANY WARRANTY; without even the implied warranty of
      11     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
      12     Lesser General Public License for more details.
      13  
      14     You should have received a copy of the GNU Lesser General Public
      15     License along with the GNU C Library; if not, see
      16     <https://www.gnu.org/licenses/>.  */
      17  
      18  
      19  #include <time.h>
      20  
      21  #include <limits.h>
      22  #include <float.h>
      23  #include <stdint.h>
      24  
      25  #define TYPE_BITS(type) (sizeof (type) * CHAR_BIT)
      26  #define TYPE_FLOATING(type) ((type) 0.5 == 0.5)
      27  #define TYPE_SIGNED(type) ((type) -1 < 0)
      28  
      29  /* Return the difference between TIME1 and TIME0, where TIME0 <= TIME1.
      30     time_t is known to be an integer type.  */
      31  
      32  static double
      33  subtract (__time64_t time1, __time64_t time0)
      34  {
      35    if (! TYPE_SIGNED (__time64_t))
      36      return time1 - time0;
      37    else
      38      {
      39        /* Optimize the common special cases where time_t
      40  	 can be converted to uintmax_t without losing information.  */
      41        uintmax_t dt = (uintmax_t) time1 - (uintmax_t) time0;
      42        double delta = dt;
      43  
      44        if (UINTMAX_MAX / 2 < INTMAX_MAX)
      45  	{
      46  	  /* This is a rare host where uintmax_t has padding bits, and possibly
      47  	     information was lost when converting time_t to uintmax_t.
      48  	     Check for overflow by comparing dt/2 to (time1/2 - time0/2).
      49  	     Overflow occurred if they differ by more than a small slop.
      50  	     Thanks to Clive D.W. Feather for detailed technical advice about
      51  	     hosts with padding bits.
      52  
      53  	     In the following code the "h" prefix means half.  By range
      54  	     analysis, we have:
      55  
      56                    -0.5 <= ht1 - 0.5*time1 <= 0.5
      57                    -0.5 <= ht0 - 0.5*time0 <= 0.5
      58                    -1.0 <= dht - 0.5*(time1 - time0) <= 1.0
      59  
      60               If overflow has not occurred, we also have:
      61  
      62                    -0.5 <= hdt - 0.5*(time1 - time0) <= 0
      63                    -1.0 <= dht - hdt <= 1.5
      64  
      65               and since dht - hdt is an integer, we also have:
      66  
      67                    -1 <= dht - hdt <= 1
      68  
      69               or equivalently:
      70  
      71                    0 <= dht - hdt + 1 <= 2
      72  
      73               In the above analysis, all the operators have their exact
      74               mathematical semantics, not C semantics.  However, dht - hdt +
      75               1 is unsigned in C, so it need not be compared to zero.  */
      76  
      77  	  uintmax_t hdt = dt / 2;
      78  	  __time64_t ht1 = time1 / 2;
      79  	  __time64_t ht0 = time0 / 2;
      80  	  __time64_t dht = ht1 - ht0;
      81  
      82  	  if (2 < dht - hdt + 1)
      83  	    {
      84  	      /* Repair delta overflow.
      85  
      86  		 The following expression contains a second rounding,
      87  		 so the result may not be the closest to the true answer.
      88  		 This problem occurs only with very large differences.
      89  		 It's too painful to fix this portably.  */
      90  
      91  	      delta = dt + 2.0L * (UINTMAX_MAX - UINTMAX_MAX / 2);
      92  	    }
      93  	}
      94  
      95        return delta;
      96      }
      97  }
      98  
      99  /* Return the difference between TIME1 and TIME0.  */
     100  double
     101  __difftime64 (__time64_t time1, __time64_t time0)
     102  {
     103    /* Convert to double and then subtract if no double-rounding error could
     104       result.  */
     105  
     106    if (TYPE_BITS (__time64_t) <= DBL_MANT_DIG
     107        || (TYPE_FLOATING (__time64_t) && sizeof (__time64_t) < sizeof (long double)))
     108      return (double) time1 - (double) time0;
     109  
     110    /* Likewise for long double.  */
     111  
     112    if (TYPE_BITS (__time64_t) <= LDBL_MANT_DIG || TYPE_FLOATING (__time64_t))
     113      return (long double) time1 - (long double) time0;
     114  
     115    /* Subtract the smaller integer from the larger, convert the difference to
     116       double, and then negate if needed.  */
     117  
     118    return time1 < time0 ? - subtract (time0, time1) : subtract (time1, time0);
     119  }
     120  
     121  /* Provide a 32-bit wrapper if needed.  */
     122  
     123  #if __TIMESIZE != 64
     124  
     125  libc_hidden_def (__difftime64)
     126  
     127  double
     128  __difftime (time_t time1, time_t time0)
     129  {
     130    return __difftime64 (time1, time0);
     131  }
     132  
     133  #endif
     134  
     135  strong_alias (__difftime, difftime)