fenv(3) Library Functions Manual fenv(3)
NAME
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetes-
texcept, fegetenv, fegetround, feholdexcept, fesetround, fesetenv, fe-
updateenv, feenableexcept, fedisableexcept, fegetexcept - floating-
point rounding and exception handling
LIBRARY
Math library (libm, -lm)
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 divide-by-zero 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-de-
fined 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, for example,
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 repre-
sented 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 repre-
sented 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 argu-
ment 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 mode
The rounding mode determines how the result of floating-point opera-
tions is treated when the result cannot be exactly represented in the
significand. Various rounding modes may be provided: round to nearest
(the default), round up (toward positive infinity), round down (toward
negative infinity), and round toward zero.
Each of the macros FE_TONEAREST, FE_UPWARD, FE_DOWNWARD, and FE_TO-
WARDZERO is defined when the implementation supports getting and set-
ting the corresponding rounding direction.
The fegetround() function returns the macro corresponding to the cur-
rent rounding mode.
The fesetround() function sets the rounding mode as specified by its
argument and returns zero when it was successful.
C99 and POSIX.1-2008 specify an identifier, FLT_ROUNDS, defined in
<float.h>, which indicates the implementation-defined rounding behavior
for floating-point addition. This identifier has one of the following
values:
-1 The rounding mode is not determinable.
0 Rounding is toward 0.
1 Rounding is toward nearest number.
2 Rounding is toward positive infinity.
3 Rounding is toward negative infinity.
Other values represent machine-dependent, nonstandard rounding modes.
The value of FLT_ROUNDS should reflect the current rounding mode as set
by fesetround() (but see BUGS).
Floating-point environment
The entire floating-point environment, including control modes and sta-
tus 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 nonstop (con-
tinue 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 nonstop (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, for example,
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.
RETURN VALUE
These functions return zero on success and nonzero if an error oc-
curred.
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ feclearexcept(), fegetexceptflag(), │ Thread safety │ MT-Safe │
│ feraiseexcept(), fesetexceptflag(), │ │ │
│ fetestexcept(), fegetround(), │ │ │
│ fesetround(), fegetenv(), feholdexcept(), │ │ │
│ fesetenv(), feupdateenv(), │ │ │
│ feenableexcept(), fedisableexcept(), │ │ │
│ fegetexcept() │ │ │
└───────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
C11, POSIX.1-2008, IEC 60559 (IEC 559:1989), ANSI/IEEE 854.
HISTORY
C99, POSIX.1-2001. glibc 2.1.
NOTES
glibc notes
If possible, the GNU C Library defines a macro FE_NOMASK_ENV which rep-
resents an environment where every exception raised causes a trap to
occur. You can test for this macro using #ifdef. It is defined only
if _GNU_SOURCE is defined. The C99 standard does not define a way to
set individual bits in the floating-point mask, for example, to trap on
specific flags. Since glibc 2.2, glibc supports the functions feen-
ableexcept() and fedisableexcept() to set individual floating-point
traps, and fegetexcept() to query the state.
#define _GNU_SOURCE /* See feature_test_macros(7) */
#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 ex-
ceptions.
BUGS
C99 specifies that the value of FLT_ROUNDS should reflect changes to
the current rounding mode, as set by fesetround(). Currently, this
does not occur: FLT_ROUNDS always has the value 1.
SEE ALSO
math_error(7)
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