FENV
Section: Linux Programmer's Manual (3)
Updated: 20170915
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NAME
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag,
fetestexcept, fegetenv, fegetround, feholdexcept, fesetround,
fesetenv, feupdateenv, feenableexcept, fedisableexcept,
fegetexcept  floatingpoint rounding and exception handling
SYNOPSIS
#include <fenv.h>
int feclearexcept(int excepts);
int fegetexceptflag(fexcept_t *flagp, int excepts);
int feraiseexcept(int excepts);
int fesetexceptflag(const fexcept_t *flagp, int excepts);
int fetestexcept(int excepts);
int fegetround(void);
int fesetround(int rounding_mode);
int fegetenv(fenv_t *envp);
int feholdexcept(fenv_t *envp);
int fesetenv(const fenv_t *envp);
int feupdateenv(const fenv_t *envp);
Link with lm.
DESCRIPTION
These eleven functions were defined in C99, and describe the handling
of floatingpoint rounding and exceptions (overflow, zerodivide, etc.).
Exceptions
The
dividebyzero
exception occurs when an operation on finite numbers
produces infinity as exact answer.
The
overflow
exception occurs when a result has to be represented as a
floatingpoint number, but has (much) larger absolute value than the
largest (finite) floatingpoint number that is representable.
The
underflow
exception occurs when a result has to be represented as a
floatingpoint number, but has smaller absolute value than the smallest
positive normalized floatingpoint number (and would lose much accuracy
when represented as a denormalized number).
The
inexact
exception occurs when the rounded result of an operation
is not equal to the infinite precision result.
It may occur whenever
overflow
or
underflow
occurs.
The
invalid
exception occurs when there is no welldefined result
for an operation, as for 0/0 or infinity  infinity or sqrt(1).
Exception handling
Exceptions are represented in two ways: as a single bit
(exception present/absent), and these bits correspond in some
implementationdefined way with bit positions in an integer,
and also as an opaque structure that may contain more information
about the exception (perhaps the code address where it occurred).
Each of the macros
FE_DIVBYZERO,
FE_INEXACT,
FE_INVALID,
FE_OVERFLOW,
FE_UNDERFLOW
is defined when the implementation supports handling
of the corresponding exception, and if so then
defines the corresponding bit(s), so that one can call
exception handling functions, for example, using the integer argument
FE_OVERFLOWFE_UNDERFLOW.
Other exceptions may be supported.
The macro
FE_ALL_EXCEPT
is the bitwise OR of all bits corresponding to supported exceptions.
The
feclearexcept()
function clears the supported exceptions represented by the bits
in its argument.
The
fegetexceptflag()
function stores a representation of the state of the exception flags
represented by the argument
excepts
in the opaque object
*flagp.
The
feraiseexcept()
function raises the supported exceptions represented by the bits in
excepts.
The
fesetexceptflag()
function sets the complete status for the exceptions represented by
excepts
to the value
*flagp.
This value must have been obtained by an earlier call of
fegetexceptflag()
with a last argument that contained all bits in
excepts.
The
fetestexcept()
function returns a word in which the bits are set that were
set in the argument
excepts
and for which the corresponding exception is currently set.
Rounding mode
The rounding mode determines how the result of floatingpoint operations
is treated when the result cannot be exactly represented in the significand.
Various rounding modes may be provided:
round to nearest (the default),
round up (toward positive infinity),
round down (toward negative infinity), and
round toward zero.
Each of the macros
FE_TONEAREST,
FE_UPWARD,
FE_DOWNWARD,
and
FE_TOWARDZERO
is defined when the implementation supports getting and setting
the corresponding rounding direction.
The
fegetround()
function returns the macro corresponding to the current
rounding mode.
The
fesetround()
function sets the rounding mode as specified by its argument
and returns zero when it was successful.
C99 and POSIX.12008 specify an identifier,
FLT_ROUNDS,
defined in
<float.h>,
which indicates the implementationdefined rounding
behavior for floatingpoint addition.
This identifier has one of the following values:
 1

The rounding mode is not determinable.
 0

Rounding is toward 0.
 1

Rounding is toward nearest number.
 2

Rounding is toward positive infinity.
 3

Rounding is toward negative infinity.
Other values represent machinedependent, nonstandard rounding modes.
The value of
FLT_ROUNDS
should reflect the current rounding mode as set by
fesetround()
(but see BUGS).
Floatingpoint environment
The entire floatingpoint environment, including
control modes and status flags, can be handled
as one opaque object, of type
fenv_t.
The default environment is denoted by
FE_DFL_ENV
(of type
const fenv_t *).
This is the environment setup at program start and it is defined by
ISO C to have round to nearest, all exceptions cleared and a nonstop
(continue on exceptions) mode.
The
fegetenv()
function saves the current floatingpoint environment in the object
*envp.
The
feholdexcept()
function does the same, then clears all exception flags,
and sets a nonstop (continue on exceptions) mode,
if available.
It returns zero when successful.
The
fesetenv()
function restores the floatingpoint environment from
the object
*envp.
This object must be known to be valid, for example, the result of a call to
fegetenv()
or
feholdexcept()
or equal to
FE_DFL_ENV.
This call does not raise exceptions.
The
feupdateenv()
function installs the floatingpoint environment represented by
the object
*envp,
except that currently raised exceptions are not cleared.
After calling this function, the raised exceptions will be a bitwise OR
of those previously set with those in
*envp.
As before, the object
*envp
must be known to be valid.
RETURN VALUE
These functions return zero on success and nonzero if an error occurred.
VERSIONS
These functions first appeared in glibc in version 2.1.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
Interface  Attribute  Value

feclearexcept(),
fegetexceptflag(),
feraiseexcept(),
fesetexceptflag(),
fetestexcept(),
fegetround(),
fesetround(),
fegetenv(),
feholdexcept(),
fesetenv(),
feupdateenv(),
feenableexcept(),
fedisableexcept(),
fegetexcept()
 Thread safety 
MTSafe

CONFORMING TO
IEC 60559 (IEC 559:1989), ANSI/IEEE 854, C99, POSIX.12001.
NOTES
Glibc notes
If possible, the GNU C Library defines a macro
FE_NOMASK_ENV
which represents an environment where every exception raised causes a
trap to occur.
You can test for this macro using
#ifdef.
It is defined only if
_GNU_SOURCE
is defined.
The C99 standard does not define a way to set individual bits in the
floatingpoint mask, for example, to trap on specific flags.
Since version 2.2, glibc supports the functions
feenableexcept()
and
fedisableexcept()
to set individual floatingpoint traps, and
fegetexcept()
to query the state.
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fenv.h>
int feenableexcept(int excepts);
int fedisableexcept(int excepts);
int fegetexcept(void);
The
feenableexcept()
and
fedisableexcept()
functions enable (disable) traps for each of the exceptions represented by
excepts
and return the previous set of enabled exceptions when successful,
and 1 otherwise.
The
fegetexcept()
function returns the set of all currently enabled exceptions.
BUGS
C99 specifies that the value of
FLT_ROUNDS
should reflect changes to the current rounding mode, as set by
fesetround().
Currently,
this does not occur:
FLT_ROUNDS
always has the value 1.
SEE ALSO
math_error(7)
COLOPHON
This page is part of release 4.13 of the Linux
manpages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/manpages/.
Index
 NAME

 SYNOPSIS

 DESCRIPTION

 Exceptions

 Exception handling

 Rounding mode

 Floatingpoint environment

 RETURN VALUE

 VERSIONS

 ATTRIBUTES

 CONFORMING TO

 NOTES

 Glibc notes

 BUGS

 SEE ALSO

 COLOPHON
