fenv(3) Library Functions Manual fenv(3)
NAME
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept, fegetenv, fegetround, feholdexcept, fes‐
etround, fesetenv, feupdateenv, feenableexcept, fedisableexcept, fegetexcept - floating-point rounding and exception han‐
dling
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 denor‐
malized 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 imple‐
mentation-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 sup‐
ports handling of the corresponding exception, and if so then defines the corresponding bit(s), so that one can call excep‐
tion handling functions, for example, using the integer argument FE_OVERFLOW|FE_UNDERFLOW. Other exceptions may be sup‐
ported. 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 ex‐
cepts 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 ex‐
cepts.
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 repre‐
sented in the significand. Various rounding modes may be provided: round to nearest (the default), round up (toward posi‐
tive 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 get‐
ting 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 excep‐
tions) 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 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 nonzero if an error occurred.
VERSIONS
These functions were added in glibc 2.1.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌─────────────────────────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├─────────────────────────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
│feclearexcept(), fegetexceptflag(), feraiseexcept(), fesetexceptflag(), fetestexcept(), │ Thread safety │ MT-Safe │
│fegetround(), fesetround(), fegetenv(), feholdexcept(), fesetenv(), feupdateenv(), │ │ │
│feenableexcept(), fedisableexcept(), fegetexcept() │ │ │
└─────────────────────────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
IEC 60559 (IEC 559:1989), ANSI/IEEE 854, C99, POSIX.1-2001.
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 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 (disable) traps for each of the exceptions represented by ex‐
cepts and return the previous set of enabled exceptions when successful, and -1 otherwise. The fegetexcept() function re‐
turns 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 6.03 2023-02-05 fenv(3)