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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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fenv(3) Library Functions Manual fenv(3)
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag,
fetestexcept, fegetenv, fegetround, feholdexcept, fesetround,
fesetenv, feupdateenv, feenableexcept, fedisableexcept,
fegetexcept - floating-point rounding and exception handling
Math library (libm, -lm)
#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);
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.
These functions return zero on success and nonzero if an error
occurred.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ feclearexcept(), fegetexceptflag(), │ Thread safety │ MT-Safe │
│ feraiseexcept(), fesetexceptflag(), │ │ │
│ fetestexcept(), fegetround(), │ │ │
│ fesetround(), fegetenv(), │ │ │
│ feholdexcept(), fesetenv(), │ │ │
│ feupdateenv(), feenableexcept(), │ │ │
│ fedisableexcept(), fegetexcept() │ │ │
└──────────────────────────────────────┴───────────────┴─────────┘
C11, POSIX.1-2008, IEC 60559 (IEC 559:1989), ANSI/IEEE 854.
C99, POSIX.1-2001. glibc 2.1.
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 floating-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 (dis‐
able) 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.
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.
math_error(7)
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user-space interface documentation) project. Information about
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Linux man-pages 6.15 2025-05-17 fenv(3)
Pages that refer to this page: execve(2), fenv_t(3type), fma(3), j0(3), lrint(3), lround(3), matherr(3), pthread_create(3), remainder(3), rint(3), round(3), __setfpucw(3), y0(3), math_error(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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