fenv
FENV(3) Linux Programmer’s Manual FENV(3)
NAME
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetes-
texcept, fegetenv, fegetround, feholdexcept, 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 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 (per-
haps 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 excep-
tions 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 avail-
able. 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 rep-
resented 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 supports the functions feenableexcept and
fedisableexcept to set individual floating point traps, and fegetex-
cept 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, ISO C99 (ISO/IEC 9899:1999).
Linux Manpage 2000-08-12 FENV(3)
