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Under GNU General Public License
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FENV(3) Linux Programmer's Manual FENV(3)
NAME
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept,
fegetenv, fegetround, feholdexcept, fesetround, fesetenv, feupdateenv - C99 float-
ing point rounding and exception handling
SYNOPSIS
#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);
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) float-
ing 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 denormal-
ized 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 excep-
tion, and if so then defines the corresponding bit(s), so that one can call excep-
tion 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 excep-
tion 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 repre-
sented 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 previ-
ously set with those in *envp. As before, the object *envp must be known to be
valid.
RETURN VALUE
These functions return zero on success and non-zero if an error occurred.
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 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.
NOTES
Link with -lm.
CONFORMING TO
IEC 60559 (IEC 559:1989), ANSI/IEEE 854, C99.
Linux Manpage 2000-08-12 FENV(3)
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Under GNU General Public License
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