IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)

Copyright © 2001-2018 IEEE and The Open Group

float.h - floating types

#include <float.h>

^{[CX]}The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.The characteristics of floating types are defined in terms of a model that describes a representation of floating-point numbers and values that provide information about an implementation's floating-point arithmetic.

The following parameters are used to define the model for each floating-point type:

s- Sign (±1).
b- Base or radix of exponent representation (an integer >1).
e- Exponent (an integer between a minimum
e_{min}and a maximume_{max}).p- Precision (the number of base-
bdigits in the significand).f_{k}- Non-negative integers less than
b(the significand digits).A floating-point number

xis defined by the following model:

In addition to normalized floating-point numbers (

f_{1}>0 ifx!=0), floating types may be able to contain other kinds of floating-point numbers, such as subnormal floating-point numbers (x!=0,e=e_{min},f_{1}=0) and unnormalized floating-point numbers (x!=0,e>e_{min},f_{1}=0), and values that are not floating-point numbers, such as infinities and NaNs. ANaNis an encoding signifying Not-a-Number. Aquiet NaNpropagates through almost every arithmetic operation without raising a floating-point exception; asignaling NaNgenerally raises a floating-point exception when occurring as an arithmetic operand.An implementation may give zero and non-numeric values, such as infinities and NaNs, a sign, or may leave them unsigned. Wherever such values are unsigned, any requirement in POSIX.1-2017 to retrieve the sign shall produce an unspecified sign and any requirement to set the sign shall be ignored.

The accuracy of the floating-point operations (

'+','-','*','/') and of the functions in<math.h>and<complex.h>that return floating-point results is implementation-defined, as is the accuracy of the conversion between floating-point internal representations and string representations performed by the functions in<stdio.h>,<stdlib.h>, and<wchar.h>. The implementation may state that the accuracy is unknown.All integer values in the

<float.h>header, except FLT_ROUNDS, shall be constant expressions suitable for use in#ifpreprocessing directives; all floating values shall be constant expressions. All except DECIMAL_DIG, FLT_EVAL_METHOD, FLT_RADIX, and FLT_ROUNDS have separate names for all three floating-point types. The floating-point model representation is provided for all values except FLT_EVAL_METHOD and FLT_ROUNDS.The rounding mode for floating-point addition is characterized by the implementation-defined value of FLT_ROUNDS:

- -1
- Indeterminable.
- 0
- Toward zero.
- 1
- To nearest.
- 2
- Toward positive infinity.
- 3
- Toward negative infinity.
All other values for FLT_ROUNDS characterize implementation-defined rounding behavior.

The values of operations with floating operands and values subject to the usual arithmetic conversions and of floating constants are evaluated to a format whose range and precision may be greater than required by the type. The use of evaluation formats is characterized by the implementation-defined value of FLT_EVAL_METHOD:

- -1
- Indeterminable.
- 0
- Evaluate all operations and constants just to the range and precision of the type.
- 1
- Evaluate operations and constants of type
floatanddoubleto the range and precision of thedoubletype; evaluatelong doubleoperations and constants to the range and precision of thelong doubletype.- 2
- Evaluate all operations and constants to the range and precision of the
long doubletype.All other negative values for FLT_EVAL_METHOD characterize implementation-defined behavior.

The

<float.h>header shall define the following values as constant expressions with implementation-defined values that are greater or equal in magnitude (absolute value) to those shown, with the same sign.

Radix of exponent representation,

b.

- FLT_RADIX
- 2
Number of base-FLT_RADIX digits in the floating-point significand,

p.

- FLT_MANT_DIG
- DBL_MANT_DIG
- LDBL_MANT_DIG
Number of decimal digits,

n, such that any floating-point number in the widest supported floating type withp_{max}radixbdigits can be rounded to a floating-point number withndecimal digits and back again without change to the value.

- DECIMAL_DIG
- 10

Number of decimal digits,

q, such that any floating-point number withqdecimal digits can be rounded into a floating-point number withpradixbdigits and back again without change to theqdecimal digits.

- FLT_DIG
- 6
- DBL_DIG
- 10
- LDBL_DIG
- 10
Minimum negative integer such that FLT_RADIX raised to that power minus 1 is a normalized floating-point number,

e_{min}.

- FLT_MIN_EXP
- DBL_MIN_EXP
- LDBL_MIN_EXP
Minimum negative integer such that 10 raised to that power is in the range of normalized floating-point numbers.

- FLT_MIN_10_EXP
- -37
- DBL_MIN_10_EXP
- -37
- LDBL_MIN_10_EXP
- -37
Maximum integer such that FLT_RADIX raised to that power minus 1 is a representable finite floating-point number,

e_{max}.

- FLT_MAX_EXP
- DBL_MAX_EXP
- LDBL_MAX_EXP

^{[CX]}Additionally, FLT_MAX_EXP shall be at least as large as FLT_MANT_DIG, DBL_MAX_EXP shall be at least as large as DBL_MANT_DIG, and LDBL_MAX_EXP shall be at least as large as LDBL_MANT_DIG; which has the effect that FLT_MAX, DBL_MAX, and LDBL_MAX are integral.Maximum integer such that 10 raised to that power is in the range of representable finite floating-point numbers.

- FLT_MAX_10_EXP
- +37
- DBL_MAX_10_EXP
- +37
- LDBL_MAX_10_EXP
- +37
The

<float.h>header shall define the following values as constant expressions with implementation-defined values that are greater than or equal to those shown:

Maximum representable finite floating-point number.

- FLT_MAX
- 1E+37
- DBL_MAX
- 1E+37
- LDBL_MAX
- 1E+37
The

<float.h>header shall define the following values as constant expressions with implementation-defined (positive) values that are less than or equal to those shown:

The difference between 1 and the least value greater than 1 that is representable in the given floating-point type,

b^{1-p}.

- FLT_EPSILON
- 1E-5
- DBL_EPSILON
- 1E-9
- LDBL_EPSILON
- 1E-9
Minimum normalized positive floating-point number,

b^{emin -1}.

- FLT_MIN
- 1E-37
- DBL_MIN
- 1E-37
- LDBL_MIN
- 1E-37

None.

All known hardware floating-point formats satisfy the property that the exponent range is larger than the number of mantissa digits. The ISO C standard permits a floating-point format where this property is not true, such that the largest finite value would not be integral; however, it is unlikely that there will ever be hardware support for such a floating-point format, and it introduces boundary cases that portable programs should not have to be concerned with (for example, a non-integral DBL_MAX means that

ceil() would have to worry about overflow). Therefore, this standard imposes an additional requirement that the largest representable finite value is integral.

None.

First released in Issue 4. Derived from the ISO C standard.

The description of the operations with floating-point values is updated for alignment with the ISO/IEC 9899:1999 standard.

ISO/IEC 9899:1999 standard, Technical Corrigendum 2 #4 (SD5-XBD-ERN-50) and #5 (SD5-XBD-ERN-51) are applied.

POSIX.1-2008, Technical Corrigendum 1, XBD/TC1-2008/0046 [346] and XBD/TC1-2008/0047 [346] are applied.

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