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# VC++6:  precision of long double

Posted on 2000-05-09
Medium Priority
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in <float.h> DBL_DIG is defined to be 15 (max digits of precision. While for lone double it's 15 ifndef _M_M68K, it's 18 if _M_M68K is defined. My program need a precision of 18, guess i will have to define _M_M68K in my program? And BTW, what is the meaning of _M_M68K??

Attachment: <float.h>

#ifndef _M_M68K
#define LDBL_DIG        DBL_DIG                 /* # of decimal digits of precision */
#define LDBL_EPSILON    DBL_EPSILON             /* smallest such that 1.0+LDBL_EPSILON != 1.0 */
#define LDBL_MANT_DIG   DBL_MANT_DIG            /* # of bits in mantissa */
#define LDBL_MAX        DBL_MAX                 /* max value */
#define LDBL_MAX_10_EXP DBL_MAX_10_EXP          /* max decimal exponent */
#define LDBL_MAX_EXP    DBL_MAX_EXP             /* max binary exponent */
#define LDBL_MIN        DBL_MIN                 /* min positive value */
#define LDBL_MIN_10_EXP DBL_MIN_10_EXP          /* min decimal exponent */
#define LDBL_MIN_EXP    DBL_MIN_EXP             /* min binary exponent */
#define _LDBL_ROUNDS    DBL_ROUNDS              /* addition rounding: near */
#else
#define LDBL_DIG        18                                      /* # of decimal digits of precision */
#define LDBL_EPSILON    1.08420217248550443412e-019L            /* smallest such that 1.0+LDBL_EPSILON != 1.0 */
#define LDBL_MANT_DIG   64                                      /* # of bits in mantissa */
#define LDBL_MAX        1.189731495357231765e+4932L             /* max value */
#define LDBL_MAX_10_EXP 4932                                    /* max decimal exponent */
#define LDBL_MAX_EXP    16384                                   /* max binary exponent */
#define LDBL_MIN        3.3621031431120935063e-4932L            /* min positive value */
#define LDBL_MIN_10_EXP (-4931)                                 /* min decimal exponent */
#define LDBL_MIN_EXP    (-16381)                                /* min binary exponent */
#define _LDBL_RADIX     2                                       /* exponent radix */
#define _LDBL_ROUNDS    1                                       /* addition rounding: near */
#endif
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Question by:yu_zhang_denver
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LVL 3

Accepted Solution

LucHoltkamp earned 200 total points
ID: 2795870
Defining it yourself won't do any good...
The CPU determins the precision, not the define... the compiler sets the define to reflect the CPU the software will run on.
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LVL 1

Expert Comment

ID: 2795879
Maybe it won't work even you define _M_M68K, M68K means Motorola 68K microprocessor. even u defined _M_M68K, without the suitable processor, cannot u get the precision of 18.
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