Originální popis anglicky:
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept,
fegetenv, fegetround, feholdexcept, fesetround, fesetenv, feupdateenv - C99
floating point rounding and exception handling
Návod, kniha: Linux Programmer's Manual
#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);
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 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 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.
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.
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.
These functions return zero on success and non-zero if an error occurred.
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 supports the functions
feenableexcept and
fedisableexcept to set individual floating point traps, and
fegetexcept to query the state.
#define _GNU_SOURCE
#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.
Link with
-lm.
IEC 60559 (IEC 559:1989), ANSI/IEEE 854, ISO C99 (ISO/IEC 9899:1999).
