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NAME
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept, fegetenv, fegetround,
feholdexcept, fesetround, fesetenv, feupdateenv, 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-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, 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 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.
Rounding mode
The rounding mode determines how the result of floating-point operations 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_TOWARDZERO 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.
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 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 nonstop (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 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 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.
RETURN VALUE
These functions return zero on success and nonzero if an error occurred.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌─────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
│ feclearexcept(), fegetexceptflag(), feraiseexcept(), fesetexceptflag(), │ Thread safety │ MT-Safe │
│ 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 represents 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 float‐
ing-point mask, for example, to trap on specific flags. Since glibc 2.2, glibc supports the functions
feenableexcept() 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 other‐
wise. The fegetexcept() function returns the set of all currently enabled exceptions.
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)
Linux man-pages 6.7 2023-10-31 fenv(3)