feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept,
fegetenv, fegetround, feholdexcept, fesetround, fesetenv, feupdateenv,
feenableexcept, fedisableexcept, fegetexcept - floating-point rounding and
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.).
exception occurs when an operation on finite numbers
produces infinity as exact answer.
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.
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).
exception occurs when the rounded result of an operation is
not equal to the infinite precision result. It may occur whenever
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
Each of the macros FE_DIVBYZERO
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
Other exceptions may be supported. The macro FE_ALL_EXCEPT
bitwise OR of all bits corresponding to supported exceptions.
() function clears the supported exceptions represented
by the bits in its argument.
() function stores a representation of the state of
the exception flags represented by the argument excepts
in the opaque
() function raises the supported exceptions represented
by the bits in excepts
() function sets the complete status for the
exceptions represented by excepts
to the value *flagp
value must have been obtained by an earlier call of fegetexceptflag
with a last argument that contained all bits in excepts
() 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
is defined when the implementation supports getting
and setting the corresponding rounding direction.
() function returns the macro corresponding to the current
() 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
, which indicates the implementation-defined rounding
behavior for floating-point addition. This identifier has one of the following
- The rounding mode is not determinable.
- Rounding is toward 0.
- Rounding is toward nearest number.
- Rounding is toward positive infinity.
- 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
() (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
). 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.
() function saves the current floating-point environment in
the object *envp
() function does the same, then clears all exception
flags, and sets a nonstop (continue on exceptions) mode, if available. It
returns zero when successful.
() function restores the floating-point environment from the
. This object must be known to be valid, for example, the
result of a call to fegetenv
() or feholdexcept
() or equal to
. This call does not raise exceptions.
() 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
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.
Multithreading (see pthreads(7))¶
(), and fegetexcept
() functions are thread-safe.
IEC 60559 (IEC 559:1989), ANSI/IEEE 854, C99, POSIX.1-2001.
If possible, the GNU C Library defines a macro FE_NOMASK_ENV
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
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
() to set individual floating-point traps, and
() to query the state.
#define _GNU_SOURCE /* See feature_test_macros(7) */
int feenableexcept(int excepts);
int fedisableexcept(int excepts);
() and fedisableexcept
() functions enable
(disable) traps for each of the exceptions represented by excepts
return the previous set of enabled exceptions when successful, and -1
otherwise. The fegetexcept
() function returns the set of all currently
C99 specifies that the value of FLT_ROUNDS
should reflect changes to the
current rounding mode, as set by fesetround
(). Currently, this does not
always has the value 1.
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