fenv.h(3HEAD) 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 collec‐
tively, including any status the implementation asso‐
ciates 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(), feraiseex‐
cept(), 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 val‐
ues.
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 state‐
ment, 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 condi‐
tion 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 unspec‐
ified and the floating-point control modes have their default set‐
tings.)
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 con‐
trol modes, unless its documentation promises otherwise.
· A function call is assumed to have the potential for raising
floating-point exceptions, unless its documentation promises oth‐
erwise.
With these conventions, an application can safely assume default float‐
ing-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 │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOfeclearexcept(3M), fegetenv(3M), fegetexceptflag(3M), fegetround(3M),
feholdexcept(3M), feraiseexcept(3M), fesetenv(3M), fesetexceptflag(3M),
fesetround(3M), fetestexcept(3M), feupdateenv(3M), attributes(5), stan‐
dards(5)SunOS 5.10 15 Dec 2003 fenv.h(3HEAD)
