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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, it 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 difference between 1 and the smallest floating point number of type

`float`

that is greater than 1. 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`

.