1 /* Return the difference between two timestamps. */ 2 3 /* 4 ** This file is in the public domain, so clarified as of 5 ** 1996-06-05 by Arthur David Olson. 6 */ 7 8 /*LINTLIBRARY*/ 9 10 #include "namespace.h" 11 #include "private.h" /* for time_t and TYPE_SIGNED */ 12 #include "un-namespace.h" 13 14 /* Return -X as a double. Using this avoids casting to 'double'. */ 15 static double 16 dminus(double x) 17 { 18 return -x; 19 } 20 21 double 22 difftime(time_t time1, time_t time0) 23 { 24 /* 25 ** If double is large enough, simply convert and subtract 26 ** (assuming that the larger type has more precision). 27 */ 28 if (sizeof(time_t) < sizeof(double)) { 29 double t1 = time1, t0 = time0; 30 return t1 - t0; 31 } 32 33 /* 34 ** The difference of two unsigned values can't overflow 35 ** if the minuend is greater than or equal to the subtrahend. 36 */ 37 if (!TYPE_SIGNED(time_t)) 38 return time0 <= time1 ? time1 - time0 : dminus(time0 - time1); 39 40 /* Use uintmax_t if wide enough. */ 41 if (sizeof(time_t) <= sizeof(uintmax_t)) { 42 uintmax_t t1 = time1, t0 = time0; 43 return time0 <= time1 ? t1 - t0 : dminus(t0 - t1); 44 } 45 46 /* 47 ** Handle cases where both time1 and time0 have the same sign 48 ** (meaning that their difference cannot overflow). 49 */ 50 if ((time1 < 0) == (time0 < 0)) 51 return time1 - time0; 52 53 /* 54 ** The values have opposite signs and uintmax_t is too narrow. 55 ** This suffers from double rounding; attempt to lessen that 56 ** by using long double temporaries. 57 */ 58 { 59 long double t1 = time1, t0 = time0; 60 return t1 - t0; 61 } 62 } 63