
FENV(3) BSD Library Functions Manual FENV(3)
NAME
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept, fegetround,
fesetround, fegetenv, feholdexcept, fesetenv, feupdateenv, feenableexcept, fedisableexcept,
fegetexcept  floatingpoint environment control
LIBRARY
Math Library (libm, lm)
SYNOPSIS
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
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 round);
int
fegetenv(fenv_t *envp);
int
feholdexcept(fenv_t *envp);
int
fesetenv(const fenv_t *envp);
int
feupdateenv(const fenv_t *envp);
int
feenableexcept(int excepts);
int
fedisableexcept(int excepts);
int
fegetexcept(void);
DESCRIPTION
The <fenv.h> routines manipulate the floatingpoint environment, which includes the excep
tion flags and rounding modes defined in IEEE Std 7541985.
Exceptions
Exception flags are set as sideeffects of floatingpoint arithmetic operations and math
library routines, and they remain set until explicitly cleared. The following macros expand
to bit flags of type int representing the five standard floatingpoint exceptions.
FE_DIVBYZERO A dividebyzero exception occurs when the program attempts to divide a finite
nonzero number by zero.
FE_INEXACT An inexact exception is raised whenever there is a loss of precision due to
rounding.
FE_INVALID Invalid operation exceptions occur when a program attempts to perform calcula
tions for which there is no reasonable representable answer. For instance,
subtraction of infinities, division of zero by zero, ordered comparison
involving NaNs, and taking the square root of a negative number are all
invalid operations.
FE_OVERFLOW An overflow exception occurs when the magnitude of the result of a computation
is too large to fit in the destination type.
FE_UNDERFLOW Underflow occurs when the result of a computation is too close to zero to be
represented as a nonzero value in the destination type.
Additionally, the FE_ALL_EXCEPT macro expands to the bitwise OR of the above flags and any
architecturespecific flags. Combinations of these flags are passed to the feclearexcept(),
fegetexceptflag(), feraiseexcept(), fesetexceptflag(), and fetestexcept() functions to
clear, save, raise, restore, and examine the processor's floatingpoint exception flags,
respectively.
Exceptions may be unmasked with feenableexcept() and masked with fedisableexcept().
Unmasked exceptions cause a trap when they are produced, and all exceptions are masked by
default. The current mask can be tested with fegetexcept().
Rounding Modes
IEEE Std 7541985 specifies four rounding modes. These modes control the direction in which
results are rounded from their exact values in order to fit them into binary floatingpoint
variables. The four modes correspond with the following symbolic constants.
FE_TONEAREST Results are rounded to the closest representable value. If the exact result
is exactly half way between two representable values, the value whose last
binary digit is even (zero) is chosen. This is the default mode.
FE_DOWNWARD Results are rounded towards negative infinity.
FE_UPWARD Results are rounded towards positive infinity.
FE_TOWARDZERO Results are rounded towards zero.
The fegetround() and fesetround() functions query and set the rounding mode.
Environment Control
The fegetenv() and fesetenv() functions save and restore the floatingpoint environment,
which includes exception flags, the current exception mask, the rounding mode, and possibly
other implementationspecific state. The feholdexcept() function behaves like fegetenv(),
but with the additional effect of clearing the exception flags and installing a nonstop
mode. In nonstop mode, floatingpoint operations will set exception flags as usual, but no
SIGFPE signals will be generated as a result. Nonstop mode is the default, but it may be
altered by nonstandard mechanisms. The feupdateenv() function restores a saved environment
similarly to fesetenv(), but it also reraises any floatingpoint exceptions from the old
environment.
The macro FE_DFL_ENV expands to a pointer to the default environment.
EXAMPLES
The following routine computes the square root function. It explicitly raises an invalid
exception on appropriate inputs using feraiseexcept(). It also defers inexact exceptions
while it computes intermediate values, and then it allows an inexact exception to be raised
only if the final answer is inexact.
#pragma STDC FENV_ACCESS ON
double sqrt(double n) {
double x = 1.0;
fenv_t env;
if (isnan(n)  n < 0.0) {
feraiseexcept(FE_INVALID);
return (NAN);
}
if (isinf(n)  n == 0.0)
return (n);
feholdexcept(&env);
while (fabs((x * x)  n) > DBL_EPSILON * 2 * x)
x = (x / 2) + (n / (2 * x));
if (x * x == n)
feclearexcept(FE_INEXACT);
feupdateenv(&env);
return (x);
}
SEE ALSO
c99(1), feclearexcept(3), fedisableexcept(3), feenableexcept(3), fegetenv(3),
fegetexcept(3), fegetexceptflag(3), fegetround(3), feholdexcept(3), feraiseexcept(3),
fesetenv(3), fesetexceptflag(3), fesetround(3), fetestexcept(3), feupdateenv(3)
STANDARDS
Except as noted below, <fenv.h> conforms to ISO/IEC 9899:1999 (``ISO C99''). The
feenableexcept(), fedisableexcept(), and fegetexcept() routines are extensions.
HISTORY
The <fenv.h> header first appeared in FreeBSD 5.3 and NetBSD 6.0. It supersedes the non
standard routines defined in <ieeefp.h> and documented in fpgetround(3).
CAVEATS
The FENV_ACCESS pragma can be enabled with
#pragma STDC FENV_ACCESS ON
and disabled with the
#pragma STDC FENV_ACCESS OFF
directive. This lexicallyscoped annotation tells the compiler that the program may access
the floatingpoint environment, so optimizations that would violate strict IEEE754 seman
tics are disabled. If execution reaches a block of code for which FENV_ACCESS is off, the
floatingpoint environment will become undefined.
BUGS
The FENV_ACCESS pragma is unimplemented in the system compiler. However, nonconstant
expressions generally produce the correct sideeffects at low optimization levels.
BSD March 16, 2005 BSD 
