SYNOPSIS

DESCRIPTION

RETURN VALUE

VERSIONS

ATTRIBUTES

ATTRIBUTES

CONFORMING TO

NOTES

BUGS

SEE ALSO

COLOPHON

feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept, fegetenv, fegetround, feholdexcept, fesetround, fesetenv, feupdateenv, feenableexcept, fedisableexcept, fegetexcept − floating-point rounding and exception handling

**#include
<fenv.h>**

**int
feclearexcept(int** *excepts***);
int fegetexceptflag(fexcept_t ***

int feraiseexcept(int

int fesetexceptflag(const fexcept_t *

int fetestexcept(int

**int
fegetround(void);
int fesetround(int**

**int
fegetenv(fenv_t ****envp***);
int feholdexcept(fenv_t ***

int fesetenv(const fenv_t *

int feupdateenv(const fenv_t *

Link with
*−lm*.

These eleven functions were defined in C99, and describe the handling of floating-point rounding and exceptions (overflow, zero-divide, etc.).

**Exceptions**

The *divide-by-zero* 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 floating-point number, but has (much)
larger absolute value than the largest (finite)
floating-point number that is representable.

The
*underflow* exception occurs when a result has to be
represented as a floating-point number, but has smaller
absolute value than the smallest positive normalized
floating-point 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
well-defined 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 implementation-defined 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_OVERFLOW**|**FE_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
floating-point 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.1-2008 specify an identifier, **FLT_ROUNDS**,
defined in *<float.h>*, which indicates the
implementation-defined rounding behavior for floating-point
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 machine-dependent, nonstandard rounding modes.

The value of
**FLT_ROUNDS** should reflect the current rounding mode
as set by **fesetround**() (but see BUGS).

**Floating-point
environment**

The entire floating-point 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 floating-point
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 floating-point
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 floating-point
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.

These functions return zero on success and nonzero if an error occurred.

These functions first appeared in glibc in version 2.1.

For an
explanation of the terms used in this section, see
**attributes**(7).

For an
explanation of the terms used in this section, see
**attributes**(7).

IEC 60559 (IEC 559:1989), ANSI/IEEE 854, C99, POSIX.1-2001.

**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 floating-point
mask, for example, to trap on specific flags. Since version
2.2, glibc supports the functions **feenableexcept**()
and **fedisableexcept**() to set individual
floating-point 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**

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.

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.

**math_error**(7)

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