NAME¶
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept,
fegetenv, fegetround, feholdexcept, fesetround, fesetenv, feupdateenv,
feenableexcept, fedisableexcept, fegetexcept - floating-point 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
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 first appeared in glibc in version 2.1.
ATTRIBUTES¶
Multithreading (see pthreads(7))¶
The
feclearexcept(),
fegetexceptflag(),
fegetexceptflag(),
fesetexceptflag(),
fetestexcept(),
fegetround(),
fesetround(),
fegetenv(),
feholdexcept(),
fesetenv(),
feupdateenv(),
feenableexcept(),
fedisableexcept(), and
fegetexcept() functions are thread-safe.
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 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 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 3.74 of the Linux
man-pages project. A
description of the project, information about reporting bugs, and the latest
version of this page, can be found at
http://www.kernel.org/doc/man-pages/.
