## table of contents

fenv(3) | Library Functions Manual | fenv(3) |

# NAME¶

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

# LIBRARY¶

Math library (*libm*, *-lm*)

# SYNOPSIS¶

#include <fenv.h>

int feclearexcept(intexcepts);int fegetexceptflag(fexcept_t *flagp, intexcepts);int feraiseexcept(intexcepts);int fesetexceptflag(const fexcept_t *flagp, intexcepts);int fetestexcept(intexcepts);

int fegetround(void);int fesetround(introunding_mode);

int fegetenv(fenv_t *envp);int feholdexcept(fenv_t *envp);int fesetenv(const fenv_t *envp);int feupdateenv(const fenv_t *envp);

# DESCRIPTION¶

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.

# RETURN VALUE¶

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

# VERSIONS¶

These functions were added in glibc 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 | MT-Safe |

# STANDARDS¶

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

# 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 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(intexcepts);int fedisableexcept(intexcepts);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¶

2023-02-05 | Linux man-pages 6.03 |