FENV(3) Linux Programmer's Manual FENV(3)
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept, fegetenv,
fegetround, feholdexcept, fesetround, fesetenv, feupdateenv - C99 floating point rounding
and exception handling
void feclearexcept(int excepts);
void fegetexceptflag(fexcept_t *flagp, int excepts);
void feraiseexcept(int excepts);
void fesetexceptflag(const fexcept_t *flagp, int excepts);
int fetestexcept(int excepts);
int fesetround(int rounding_mode);
void fegetenv(fenv_t *envp);
int feholdexcept(fenv_t *envp);
void fesetenv(const fenv_t *envp);
void feupdateenv(const fenv_t *envp);
These eleven functions were defined in C99, and describe the handling of floating point
rounding and exceptions (overflow, zero-divide etc.).
The DivideByZero exception occurs when an operation on finite numbers produces infinity as
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 e.g. 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 sup-
The feclearexcept function clears the supported exceptions represented by the bits in its
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.
Each of the macros FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD 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.
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 non-stop
(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 non-
stop (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, e.g., 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 func-
tion, 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.
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 only defined if _GNU_SOURCE is defined. The C99 standard does not
define a way to set individual bits in the floating point mask, e.g. to trap on specific
flags. glibc 2.2 will support the functions feenableexcept and fedisableexcept to set
individual floating point traps, and fegetexcept to query the state.
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
IEC 60559 (IEC 559:1989), ANSI/IEEE 854, ISO C99 (ISO/IEC 9899:1999).
Linux Manpage 2000-08-12 FENV(3)