feclearexcept, fegetexceptflag, feraiseexcept, fesetex
ceptflag, fetestexcept, fegetenv, fegetround, feholdex
cept, fesetround, fesetenv, feupdateenv - C99 floating
point rounding and exception handling
SYNOPSIS
#include <fenv.h>
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 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 rep
resented 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
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 implementa
tion 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 inte
ger 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 argu
ment 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 implementa
tion supports getting and setting the corresponding round
ing direction.
The fegetround function returns the macro corresponding to
the current rounding mode.
The fesetround function sets the rounding mode as speci
fied by its argument and returns zero when it was success
ful.
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
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 excep
tions) mode, if available. It returns zero when success
ful.
The fesetenv function restores the floating point environ
ment 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 envi
ronment represented by the object *envp, except that cur
rently 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 func
tion 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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