FENV(3) BSD Library Functions Manual FENV(3)NAME
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag,
fetestexcept, fegetround, fesetround, fegetenv, feholdexcept, fesetenv,
feupdateenv, feenableexcept, fedisableexcept, fegetexcept — floating-
point 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 floating-point environment, which
includes the exception flags and rounding modes defined in IEEE Std
754-1985.
Exceptions
Exception flags are set as side-effects of floating-point arithmetic
operations and math library routines, and they remain set until explic‐
itly cleared. The following macros expand to bit flags of type int rep‐
resenting the five standard floating-point exceptions.
FE_DIVBYZERO A divide-by-zero exception occurs when the program attempts
to divide a finite non-zero 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 calculations for which there is no reasonable
representable answer. For instance, subtraction of infini‐
ties, division of zero by zero, ordered comparison involv‐
ing 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 destina‐
tion type.
FE_UNDERFLOW Underflow occurs when the result of a computation is too
close to zero to be represented as a non-zero value in the
destination type.
Additionally, the FE_ALL_EXCEPT macro expands to the bitwise OR of the
above flags and any architecture-specific 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 floating-point
exception flags, respectively.
Exceptions may be unmasked with feenableexcept() and masked with
fedisableexcept(). Unmasked exceptions cause a trap when they are pro‐
duced, and all exceptions are masked by default. The current mask can be
tested with fegetexcept().
Rounding Modes
IEEE Std 754-1985 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 floating-point variables. The four modes corre‐
spond 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 repre‐
sentable values, the value whose last binary digit is even
(zero) is chosen. This is the default mode.
FE_DOWNWARD Results are rounded towards negative ∞.
FE_UPWARD Results are rounded towards positive ∞.
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 floating-
point environment, which includes exception flags, the current exception
mask, the rounding mode, and possibly other implementation-specific
state. The feholdexcept() function behaves like fegetenv(), but with the
additional effect of clearing the exception flags and installing a
non-stop mode. In non-stop mode, floating-point operations will set
exception flags as usual, but no SIGFPE signals will be generated as a
result. Non-stop mode is the default, but it may be altered by non-stan‐
dard mechanisms. The feupdateenv() function restores a saved environment
similarly to fesetenv(), but it also re-raises any floating-point excep‐
tions 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 ALSOc99(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 lexically-scoped annotation tells the compiler that the
program may access the floating-point environment, so optimizations that
would violate strict IEEE-754 semantics are disabled. If execution
reaches a block of code for which FENV_ACCESS is off, the floating-point
environment will become undefined.
BUGS
The FENV_ACCESS pragma is unimplemented in the system compiler. However,
non-constant expressions generally produce the correct side-effects at
low optimization levels.
BSD March 16, 2005 BSD
