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fenv_t: Represents the entire floating-point environment. The floating-point environment refers collectively to any floating-point status flags and control modes supported by the implementation.
fexcept_t: Represents the floating-point status flags collectively, including any status the implementation associates with the flags. A floating-point status flag is a system variable whose value is set (but never cleared) when a floating-point exception is raised, which occurs as a side effect of exceptional floating-point arithmetic to provide auxiliary information. A floating-point control mode is a system variable whose value can be set by the user to affect the subsequent behavior of floating-point arithmetic.

The <fenv.h> header defines the following constants if and only if theimplementation supports the floating-point exception by means of the floating-point functionsfeclearexcept(),fegetexceptflag(),feraiseexcept(),fesetexceptflag(), andfetestexcept(). Each expands to an integer constantexpression with values such that bitwise-inclusive ORs of all combinations of theconstants result in distinct values.

FE_DIVBYZEROFE_INEXACTFE_INVALIDFE_OVERFLOWFE_UNDERFLOW

The <fenv.h> header defines the following constant, which is simply the bitwise-inclusiveOR of all floating-point exception constants defined above:

FE_ALL_EXCEPT

The <fenv.h> header defines the following constants. Each expands to an integerconstant expression whose values are distinct non-negative values.

FE_DOWNWARDFE_TONEARESTFE_TOWARDZEROFE_UPWARD

The <fenv.h> header defines the following constant, which represents the default floating-pointenvironment (that is, the one installed at program startup) and has typepointer to const-qualifiedfenv_t. It can be used as an argument tothe functions within the <fenv.h> header that manage the floating-point environment.

FE_DFL_ENV

TheFENV_ACCESS pragma provides a means to inform the implementation when anapplication might access the floating-point environment to test floating-point status flags orrun under non-default floating-point control modes. The pragma occurs either outside externaldeclarations or preceding all explicit declarations and statements inside a compound statement. Whenoutside external declarations, the pragma takes effect from its occurrence until anotherFENV_ACCESS pragma is encountered, or until the end of the translation unit.When inside a compound statement, the pragma takes effect from its occurrence untilanotherFENV_ACCESS pragma is encountered (including within a nested compound statement), oruntil the end of the compound statement; at the end of acompound statement the state for the pragma is restored to its conditionjust before the compound statement. If this pragma is used in any othercontext, the behavior is undefined.

If part of an application tests floating-point status flags, sets floating-point controlmodes, or runs under non-default mode settings, but was translated with thestate for theFENV_ ACCESS pragma off, the behavior is undefined. The defaultstate (on or off) for the pragma is implementation-defined. (When execution passes froma part of the application translated withFENV_ACCESS off to a parttranslated withFENV_ACCESS on, the state of the floating-point status flags isunspecified and the floating-point control modes have their default settings.)

Usage

This header is designed to support the floating-point exception status flags anddirected-rounding control modes required by the IEC 60559: 1989 standard, and othersimilar floating-point state information. Also, it is designed to facilitate code portabilityamong all systems. Certain application programming conventions support the intended model ofuse for the floating-point environment:

A function call does not alter its caller's floating-point control modes, clear its caller's floating-point status flags, or depend on the state of its caller's floating-point status flags unless the function is so documented.
A function call is assumed to require default floating-point control modes, unless its documentation promises otherwise.
A function call is assumed to have the potential for raising floating-point exceptions, unless its documentation promises otherwise.

With these conventions, an application can safely assume default floating-point control modes(or be unaware of them). The responsibilities associated with accessing the floating-pointenvironment fall on the application that does so explicitly.

Even though the rounding direction macros might expand to constants corresponding tothe values ofFLT_ROUNDS, they are not required to do so. Forexample:

#include <fenv.h>void f(double x){    #pragma STDC FENV_ACCESS ON    void g(double);    void h(double);    /* ... */    g(x + 1);    h(x + 1);    /* ... */}

If the functiong() might depend on status flags set as aside effect of the firstx+1, or if the secondx+1 mightdepend on control modes set as a side effect of the callto functiong(), then the application must contain an appropriately placed invocation asfollows:

#pragma STDC FENV_ACCESS ON

Attributes

Seeattributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Committed
Standard	See`standards`(5).

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	man pages section 3: Library Interfaces and Headers Oracle Solaris 11 Information Library

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fenv.h

, fenv

Synopsis

Description

Usage

Attributes

See Also