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FENV(3)		       FreeBSD 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 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-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.

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.

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
     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
     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.	 It supersedes the
     non-standard routines defined in <ieeefp.h> and documented	in
     fpgetround(3).

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.

     On	the Alpha platform, cc(1) must be passed the -mieee-with-inexact
     -mfp-rounding-mode=d options in order to generate code that has the stan-
     dard side-effects and uses	the specified rounding modes.

FreeBSD	6.0			March 16, 2005			   FreeBSD 6.0

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | CAVEATS | EXAMPLES | SEE ALSO | STANDARDS | HISTORY | BUGS

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