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FENV(3) Linux Programmer's Manual FENV(3)
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept,
fegetenv, fegetround, feholdexcept, fesetround, fesetenv, feupdateenv, feen-
ableexcept, fedisableexcept, fegetexcept - floating-point rounding and excep-
tion handling
#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.
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.
These functions first appeared in glibc in version 2.1.
IEC 60559 (IEC 559:1989), ANSI/IEEE 854, C99, POSIX.1-2001.
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 only defined 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.
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)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-10-31 FENV(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface