man pages section 3: Library Interfaces and Headers

fenv.h(3HEAD)

Name

fenv.h, fenv– floating-point environment

Synopsis

#include <fenv.h>

Description

The <fenv.h> header defines the following data types through typedef:

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 the implementation supports the floating-point exception by means of the floating-point functions feclearexcept(), fegetexceptflag(), feraiseexcept(), fesetexceptflag(), and fetestexcept(). Each expands to an integer constant expression with values such that bitwise-inclusive ORs of all combinations of the constants result in distinct values.

FE_DIVBYZERO
FE_INEXACT
FE_INVALID
FE_OVERFLOW
FE_UNDERFLOW

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

FE_ALL_EXCEPT

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

FE_DOWNWARD
FE_TONEAREST
FE_TOWARDZERO
FE_UPWARD

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

FE_DFL_ENV

The FENV_ACCESS pragma provides a means to inform the implementation when an application might access the floating-point environment to test floating-point status flags or run under non-default floating-point control modes. The pragma occurs either outside external declarations or preceding all explicit declarations and statements inside a compound statement. When outside external declarations, the pragma takes effect from its occurrence until another FENV_ACCESS pragma is encountered, or until the end of the translation unit. When inside a compound statement, the pragma takes effect from its occurrence until another FENV_ACCESS pragma is encountered (including within a nested compound statement), or until the end of the compound statement; at the end of a compound statement the state for the pragma is restored to its condition just before the compound statement. If this pragma is used in any other context, the behavior is undefined.

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

Usage

This header is designed to support the floating-point exception status flags and directed-rounding control modes required by the IEC 60559: 1989 standard, and other similar floating-point state information. Also, it is designed to facilitate code portability among all systems. Certain application programming conventions support the intended model of use 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-point environment fall on the application that does so explicitly.

Even though the rounding direction macros might expand to constants corresponding to the values of FLT_ROUNDS, they are not required to do so. For example:

#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 function g() might depend on status flags set as a side effect of the first x+1, or if the second x+1 might depend on control modes set as a side effect of the call to function g(), then the application must contain an appropriately placed invocation as follows:

#pragma STDC FENV_ACCESS ON

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Standard