diff options
Diffstat (limited to 'lib/libc/stdtime/difftime.c')
-rw-r--r-- | lib/libc/stdtime/difftime.c | 74 |
1 files changed, 74 insertions, 0 deletions
diff --git a/lib/libc/stdtime/difftime.c b/lib/libc/stdtime/difftime.c new file mode 100644 index 0000000..68f9d7e --- /dev/null +++ b/lib/libc/stdtime/difftime.c @@ -0,0 +1,74 @@ +#ifndef lint +#ifndef NOID +static char elsieid[] = "@(#)difftime.c 7.4"; +#endif /* !defined NOID */ +#endif /* !defined lint */ + +/*LINTLIBRARY*/ + +#include "private.h" + +/* +** Algorithm courtesy Paul Eggert (eggert@twinsun.com). +*/ + +#ifdef HAVE_LONG_DOUBLE +#define long_double long double +#endif /* defined HAVE_LONG_DOUBLE */ +#ifndef HAVE_LONG_DOUBLE +#define long_double double +#endif /* !defined HAVE_LONG_DOUBLE */ + +double +difftime(time1, time0) +const time_t time1; +const time_t time0; +{ + time_t delta; + time_t hibit; + + if (sizeof(time_t) < sizeof(double)) + return (double) time1 - (double) time0; + if (sizeof(time_t) < sizeof(long_double)) + return (long_double) time1 - (long_double) time0; + if (time1 < time0) + return -difftime(time0, time1); + /* + ** As much as possible, avoid loss of precision + ** by computing the difference before converting to double. + */ + delta = time1 - time0; + if (delta >= 0) + return delta; + /* + ** Repair delta overflow. + */ + hibit = 1; + while ((hibit <<= 1) > 0) + continue; + /* + ** The following expression rounds twice, which means + ** the result may not be the closest to the true answer. + ** For example, suppose time_t is 64-bit signed int, + ** long_double is IEEE 754 double with default rounding, + ** time1 = 9223372036854775807 and time0 = -1536. + ** Then the true difference is 9223372036854777343, + ** which rounds to 9223372036854777856 + ** with a total error of 513. + ** But delta overflows to -9223372036854774273, + ** which rounds to -9223372036854774784, and correcting + ** this by subtracting 2 * (long_double) hibit + ** (i.e. by adding 2**64 = 18446744073709551616) + ** yields 9223372036854776832, which + ** rounds to 9223372036854775808 + ** with a total error of 1535 instead. + ** This problem occurs only with very large differences. + ** It's too painful to fix this portably. + ** We are not alone in this problem; + ** many C compilers round twice when converting + ** large unsigned types to small floating types, + ** so if time_t is unsigned the "return delta" above + ** has the same double-rounding problem. + */ + return delta - 2 * (long_double) hibit; +} |