<fenv.h>(P) POSIX Programmer's Manual <fenv.h>(P)
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NAMEfenv.h - floating-point environment
SYNOPSIS
#include <fenv.h>
DESCRIPTION
The <fenv.h> header shall define the following data types through type‐
def:
fenv_t Represents the entire floating-point environment. The floating-
point environment refers collectively to any floating-point sta‐
tus flags and control modes supported by the implementation.
fexcept_t
Represents the floating-point status flags collectively, includ‐
ing 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 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-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 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 round‐
ing direction by means of the fegetround() and fesetround() functions.
Each expands to an integer constant expression whose values are dis‐
tinct 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 declara‐
tions 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.)
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 control
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 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 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 logi‐
cal operations from the exception macros. An implementation might sim‐
ply implement fexcept_t as an int and use the representations reflected
by the exception macros, but is not required to; other representations
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 appli‐
cation. 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(), feraiseex‐
cept(), 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 .
IEEE/The Open Group 2003 <fenv.h>(P)
