The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
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NAME

float.h - floating types

SYNOPSIS

#include <float.h>

DESCRIPTION

[CX] [Option Start] 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 IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

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 emin and a maximum emax).
p
Precision (the number of base-b digits in the significand).
fk
Non-negative integers less than b (the significand digits).

A floating-point number x is defined by the following model:

In addition to normalized floating-point numbers (f1>0 if x!=0), floating types may be able to contain other kinds of floating-point numbers, such as subnormal floating-point numbers ( x!=0, e= emin, f1=0) and unnormalized floating-point numbers ( x!=0, e> emin, f1=0), and values that are not floating-point numbers, such as infinities and NaNs. A NaN is an encoding signifying Not-a-Number. A quiet NaN propagates through almost every arithmetic operation without raising a floating-point exception; a signaling NaN generally raises a floating-point exception when occurring as an arithmetic operand.

The accuracy of the floating-point operations ( '+', '-', '*', '/' ) and of the library functions in <math.h> and <complex.h> that return floating-point results is implementation-defined. 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 #if preprocessing 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 float and double to the range and precision of the double type; evaluate long double operations and constants to the range and precision of the long double type.
 2
Evaluate all operations and constants to the range and precision of the long double type.

All other negative values for FLT_EVAL_METHOD characterize implementation-defined behavior.

The values given in the following list shall be defined as constant expressions with implementation-defined values that are greater or equal in magnitude (absolute value) to those shown, with the same sign.

The values given in the following list shall be defined as constant expressions with implementation-defined values that are greater than or equal to those shown:

The values given in the following list shall be defined as constant expressions with implementation-defined (positive) values that are less than or equal to those shown:


The following sections are informative.

APPLICATION USAGE

None.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

<complex.h>, <math.h>

CHANGE HISTORY

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

Issue 6

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

End of informative text.

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