projects / apple / libc.git / blame
| Commit | Line | Data |
|---|---|---|
| 5b2abdfb A |
1 | /* |
| 2 | ** This file is in the public domain, so clarified as of | |
| 3 | ** June 5, 1996 by Arthur David Olson (arthur_david_olson@nih.gov). | |
| 4 | ** | |
| 5 | ** $FreeBSD: src/lib/libc/stdtime/difftime.c,v 1.4.8.1 2001/03/05 11:37:21 obrien Exp $ | |
| 6 | */ | |
| 7 | ||
| 8 | #ifndef lint | |
| 9 | #ifndef NOID | |
| 10 | static char elsieid[] = "@(#)difftime.c 7.7"; | |
| 11 | #endif /* !defined NOID */ | |
| 12 | #endif /* !defined lint */ | |
| 13 | ||
| 14 | /*LINTLIBRARY*/ | |
| 15 | ||
| 16 | #include "private.h" | |
| 17 | ||
| 18 | /* | |
| 19 | ** Algorithm courtesy Paul Eggert (eggert@twinsun.com). | |
| 20 | */ | |
| 21 | ||
| 22 | #ifdef HAVE_LONG_DOUBLE | |
| 23 | #define long_double long double | |
| 24 | #endif /* defined HAVE_LONG_DOUBLE */ | |
| 25 | #ifndef HAVE_LONG_DOUBLE | |
| 26 | #define long_double double | |
| 27 | #endif /* !defined HAVE_LONG_DOUBLE */ | |
| 28 | ||
| 29 | double | |
| 30 | difftime(time1, time0) | |
| 31 | const time_t time1; | |
| 32 | const time_t time0; | |
| 33 | { | |
| 34 | time_t delta; | |
| 35 | time_t hibit; | |
| 36 | ||
| 37 | if (sizeof(time_t) < sizeof(double)) | |
| 38 | return (double) time1 - (double) time0; | |
| 39 | if (sizeof(time_t) < sizeof(long_double)) | |
| 40 | return (long_double) time1 - (long_double) time0; | |
| 41 | if (time1 < time0) | |
| 42 | return -difftime(time0, time1); | |
| 43 | /* | |
| 44 | ** As much as possible, avoid loss of precision | |
| 45 | ** by computing the difference before converting to double. | |
| 46 | */ | |
| 47 | delta = time1 - time0; | |
| 48 | if (delta >= 0) | |
| 49 | return delta; | |
| 50 | /* | |
| 51 | ** Repair delta overflow. | |
| 52 | */ | |
| 53 | hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1); | |
| 54 | /* | |
| 55 | ** The following expression rounds twice, which means | |
| 56 | ** the result may not be the closest to the true answer. | |
| 57 | ** For example, suppose time_t is 64-bit signed int, | |
| 58 | ** long_double is IEEE 754 double with default rounding, | |
| 59 | ** time1 = 9223372036854775807 and time0 = -1536. | |
| 60 | ** Then the true difference is 9223372036854777343, | |
| 61 | ** which rounds to 9223372036854777856 | |
| 62 | ** with a total error of 513. | |
| 63 | ** But delta overflows to -9223372036854774273, | |
| 64 | ** which rounds to -9223372036854774784, and correcting | |
| 65 | ** this by subtracting 2 * (long_double) hibit | |
| 66 | ** (i.e. by adding 2**64 = 18446744073709551616) | |
| 67 | ** yields 9223372036854776832, which | |
| 68 | ** rounds to 9223372036854775808 | |
| 69 | ** with a total error of 1535 instead. | |
| 70 | ** This problem occurs only with very large differences. | |
| 71 | ** It's too painful to fix this portably. | |
| 72 | ** We are not alone in this problem; | |
| 73 | ** some C compilers round twice when converting | |
| 74 | ** large unsigned types to small floating types, | |
| 75 | ** so if time_t is unsigned the "return delta" above | |
| 76 | ** has the same double-rounding problem with those compilers. | |
| 77 | */ | |
| 78 | return delta - 2 * (long_double) hibit; | |
| 79 | } |