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(libc.info)Floating Point Parameters
Floating Point Parameters
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These macro definitions can be accessed by including the header file
`float.h' in your program.
Macro names starting with `FLT_' refer to the `float' type, while
names beginning with `DBL_' refer to the `double' type and names
beginning with `LDBL_' refer to the `long double' type. (If GCC does
not support `long double' as a distinct data type on a target machine
then the values for the `LDBL_' constants are equal to the
corresponding constants for the `double' type.)
Of these macros, only `FLT_RADIX' is guaranteed to be a constant
expression. The other macros listed here cannot be reliably used in
places that require constant expressions, such as `#if' preprocessing
directives or in the dimensions of static arrays.
Although the ISO C standard specifies minimum and maximum values for
most of these parameters, the GNU C implementation uses whatever values
describe the floating point representation of the target machine. So in
principle GNU C actually satisfies the ISO C requirements only if the
target machine is suitable. In practice, all the machines currently
supported are suitable.
`FLT_ROUNDS'
This value characterizes the rounding mode for floating point
addition. The following values indicate standard rounding modes:
`-1'
The mode is indeterminable.
`0'
Rounding is towards zero.
`1'
Rounding is to the nearest number.
`2'
Rounding is towards positive infinity.
`3'
Rounding is towards negative infinity.
Any other value represents a machine-dependent nonstandard rounding
mode.
On most machines, the value is `1', in accordance with the IEEE
standard for floating point.
Here is a table showing how certain values round for each possible
value of `FLT_ROUNDS', if the other aspects of the representation
match the IEEE single-precision standard.
0 1 2 3
1.00000003 1.0 1.0 1.00000012 1.0
1.00000007 1.0 1.00000012 1.00000012 1.0
-1.00000003 -1.0 -1.0 -1.0 -1.00000012
-1.00000007 -1.0 -1.00000012 -1.0 -1.00000012
`FLT_RADIX'
This is the value of the base, or radix, of the exponent
representation. This is guaranteed to be a constant expression,
unlike the other macros described in this section. The value is 2
on all machines we know of except the IBM 360 and derivatives.
`FLT_MANT_DIG'
This is the number of base-`FLT_RADIX' digits in the floating point
mantissa for the `float' data type. The following expression
yields `1.0' (even though mathematically it should not) due to the
limited number of mantissa digits:
float radix = FLT_RADIX;
1.0f + 1.0f / radix / radix / ... / radix
where `radix' appears `FLT_MANT_DIG' times.
`DBL_MANT_DIG'
`LDBL_MANT_DIG'
This is the number of base-`FLT_RADIX' digits in the floating point
mantissa for the data types `double' and `long double',
respectively.
`FLT_DIG'
This is the number of decimal digits of precision for the `float'
data type. Technically, if P and B are the precision and base
(respectively) for the representation, then the decimal precision
Q is the maximum number of decimal digits such that any floating
point number with Q base 10 digits can be rounded to a floating
point number with P base B digits and back again, without change
to the Q decimal digits.
The value of this macro is supposed to be at least `6', to satisfy
ISO C.
`DBL_DIG'
`LDBL_DIG'
These are similar to `FLT_DIG', but for the data types `double'
and `long double', respectively. The values of these macros are
supposed to be at least `10'.
`FLT_MIN_EXP'
This is the smallest possible exponent value for type `float'.
More precisely, is the minimum negative integer such that the value
`FLT_RADIX' raised to this power minus 1 can be represented as a
normalized floating point number of type `float'.
`DBL_MIN_EXP'
`LDBL_MIN_EXP'
These are similar to `FLT_MIN_EXP', but for the data types
`double' and `long double', respectively.
`FLT_MIN_10_EXP'
This is the minimum negative integer such that `10' raised to this
power minus 1 can be represented as a normalized floating point
number of type `float'. This is supposed to be `-37' or even less.
`DBL_MIN_10_EXP'
`LDBL_MIN_10_EXP'
These are similar to `FLT_MIN_10_EXP', but for the data types
`double' and `long double', respectively.
`FLT_MAX_EXP'
This is the largest possible exponent value for type `float'. More
precisely, this is the maximum positive integer such that value
`FLT_RADIX' raised to this power minus 1 can be represented as a
floating point number of type `float'.
`DBL_MAX_EXP'
`LDBL_MAX_EXP'
These are similar to `FLT_MAX_EXP', but for the data types
`double' and `long double', respectively.
`FLT_MAX_10_EXP'
This is the maximum positive integer such that `10' raised to this
power minus 1 can be represented as a normalized floating point
number of type `float'. This is supposed to be at least `37'.
`DBL_MAX_10_EXP'
`LDBL_MAX_10_EXP'
These are similar to `FLT_MAX_10_EXP', but for the data types
`double' and `long double', respectively.
`FLT_MAX'
The value of this macro is the maximum number representable in type
`float'. It is supposed to be at least `1E+37'. The value has
type `float'.
The smallest representable number is `- FLT_MAX'.
`DBL_MAX'
`LDBL_MAX'
These are similar to `FLT_MAX', but for the data types `double'
and `long double', respectively. The type of the macro's value is
the same as the type it describes.
`FLT_MIN'
The value of this macro is the minimum normalized positive floating
point number that is representable in type `float'. It is supposed
to be no more than `1E-37'.
`DBL_MIN'
`LDBL_MIN'
These are similar to `FLT_MIN', but for the data types `double'
and `long double', respectively. The type of the macro's value is
the same as the type it describes.
`FLT_EPSILON'
This is the maximum positive floating point number of type `float'
such that `1.0 + FLT_EPSILON != 1.0' is true. It's supposed to be
no greater than `1E-5'.
`DBL_EPSILON'
`LDBL_EPSILON'
These are similar to `FLT_EPSILON', but for the data types
`double' and `long double', respectively. The type of the macro's
value is the same as the type it describes. The values are not
supposed to be greater than `1E-9'.
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