FENV(3)

FENV(3) | 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 explicitly cleared. The following macros expand to bit flags of type 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`().

## 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 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 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.

# 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 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.

March 16, 2005 | NetBSD 9.0