FENV(3) Linux Programmer's Manual FENV(3)
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept, fegetenv,
fegetround, feholdexcept, fesetround, fesetenv, feupdateenv, feenableexcept, fedisableex-
cept, fegetexcept - floating-point rounding and exception handling
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 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 num-
ber, 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 func-
tions, 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
The feclearexcept() function clears the supported exceptions represented by the bits in
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
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 indi-
cates 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 environ-
ment 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
#define _GNU_SOURCE /* See feature_test_macros(7) */
int feenableexcept(int excepts);
int fedisableexcept(int excepts);
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 cur-
rently 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
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Linux 2010-10-31 FENV(3)