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

Math library (`libm`

, `-lm`

)

```
#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);
```

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

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).

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.

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).

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.

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 | MT-Safe |

C11, POSIX.1-2008, IEC 60559 (IEC 559:1989), ANSI/IEEE 854.

C99, POSIX.1-2001. glibc 2.1.

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 glibc 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 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.

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)