FENV (3) | C library functions | Unix Manual Pages | :man ▋

feclearexcept , fegetexceptflag , feraiseexcept , fesetexceptflag , fetestexcept , fegetround , fesetround , fegetenv , feholdexcept , fesetenv , feupdateenv , feenableexcept , fedisableexcept , fegetexcept - floating-point environment control
CONTENTS
Library Synopsis Description Exceptions Rounding Modes Environment Control Caveats Examples See Also Standards History Bugs
LIBRARY
.Lb libm
SYNOPSIS
.In fenv.h .Fd "#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 .In fenv.h routines manipulate the floating-point environment, which includes the exception flags and rounding modes defined in -ieee754 .
Exceptions
Exception flags are set as side-effects of floating-point arithmetic operations and math library routines, and they remain set until explicitly cleared. The following macros expand to bit flags of type .Vt int representing 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 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 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 produced, and all exceptions are masked by default. The current mask can be tested with fegetexcept .

-ieee754 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 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 If]. FE_UPWARD Results are rounded towards positive If]. FE_TOWARDZERO Results are rounded towards zero.

The fegetround and fesetround functions query and set the rounding mode.

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-standard mechanisms. The feupdateenv function restores a saved environment similarly to fesetenv , but it also re-raises any floating-point exceptions from the old environment. The macro FE_DFL_ENV expands to a pointer to the default environment.

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.

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);
}

cc (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), fpgetprec (3), fpsetprec (3)
STANDARDS
HISTORY
fpgetround (3).
BUGS
cc (1)