Tribblix: manual page: fenv.h.3head

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 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:

o 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.

o A function call is assumed to require default floating-
point control modes, unless its documentation promises
otherwise.

o 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

NAME

SYNOPSIS

DESCRIPTION

USAGE

ATTRIBUTES

SEE ALSO