fenv.h
<fenv.h>(P) <fenv.h>(P)
NAME
fenv.h - floating-point environment
SYNOPSIS
#include <fenv.h>
DESCRIPTION
The <fenv.h> header shall define 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 excep-
tion 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
may be set by the user to affect the subsequent behavior of
floating-point arithmetic.
The <fenv.h> header shall define 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-inclu-
sive ORs of all combinations of the constants result in distinct val-
ues.
FE_DIVBYZERO
FE_INEXACT
FE_INVALID
FE_OVERFLOW
FE_UNDERFLOW
The <fenv.h> header shall define the following constant, which is sim-
ply the bitwise-inclusive OR of all floating-point exception constants
defined above:
FE_ALL_EXCEPT
The <fenv.h> header shall define the following constants if and only
if the implementation supports getting and setting the represented
rounding direction by means of the fegetround() and fesetround() func-
tions. Each expands to an integer constant expression whose values are
distinct non-negative vales.
FE_DOWNWARD
FE_TONEAREST
FE_TOWARDZERO
FE_UPWARD
The <fenv.h> header shall define the following constant, which repre-
sents 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 following shall be declared as functions and may also be defined
as macros. Function prototypes shall be provided.
int feclearexcept(int);
int fegetexceptflag(fexcept_t *, int);
int feraiseexcept(int);
int fesetexceptflag(const fexcept_t *, int);
int fetestexcept(int);
int fegetround(void);
int fesetround(int);
int fegetenv(fenv_t *);
int feholdexcept(fenv_t *);
int fesetenv(const fenv_t *);
int feupdateenv(const fenv_t *);
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 shall occur 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 occur-
rence 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.)
The following sections are informative.
APPLICATION 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 infor-
mation. 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, nor 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 float-
ing-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 responsibil-
ities associated with accessing the floating-point environment fall on
the application that does so explicitly.
Even though the rounding direction macros may expand to constants cor-
responding 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
shall contain an appropriately placed invocation as follows:
#pragma STDC FENV_ACCESS ON
RATIONALE
The fexcept_t Type
fexcept_t does not have to be an integer type. Its values must be
obtained by a call to fegetexceptflag(), and cannot be created by log-
ical operations from the exception macros. An implementation might
simply implement fexcept_t as an int and use the representations
reflected by the exception macros, but is not required to; other rep-
resentations might contain extra information about the exceptions.
fexcept_t might be a struct with a member for each exception (that
might hold the address of the first or last floating-point instruction
that caused that exception). The ISO/IEC 9899:1999 standard makes no
claims about the internals of an fexcept_t, and so the user cannot
inspect it.
Exception and Rounding Macros
Macros corresponding to unsupported modes and rounding directions are
not defined by the implementation and must not be defined by the
application. An application might use #ifdef to test for this.
FUTURE DIRECTIONS
None.
SEE ALSO
The System Interfaces volume of IEEE Std 1003.1-2001, feclearexcept(),
fegetenv(), fegetexceptflag(), fegetround(), feholdexcept(), feraise-
except(), fesetenv(), fesetexceptflag(), fesetround(), fetestexcept(),
feupdateenv()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. In the event of any discrepancy between this version and the
original IEEE and The Open Group Standard, the original IEEE and The
Open Group Standard is the referee document. The original Standard can
be obtained online at http://www.opengroup.org/unix/online.html .
POSIX 2003 <fenv.h>(P)
