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FENV
!!!FENV
NAME
SYNOPSIS
DESCRIPTION
GNU DETAILS
CONFORMING TO
----
!!NAME


feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept, fegetenv, fegetround, feholdexcept, fesetround, fesetenv, feupdateenv - C99 floating point rounding and exception handling
!!SYNOPSIS


__#include
__ ''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 fegetround(void);
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''__);
__
!!DESCRIPTION


These eleven functions were defined in C99, and describe the
handling of floating point rounding and exceptions
(overflow, zero-divide etc.).


__Exceptions__


The DivideByZero 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 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 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__


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 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.
!!GNU DETAILS


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, 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 enabled exceptions.
!!CONFORMING TO


IEC 60559 (IEC 559:1989), ANSI/IEEE 854, ISO C99 (ISO/IEC
9899:1999).
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