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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
     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.

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
     standard side-effects and uses the specified rounding modes.

FreeBSD 11.0-PRERELEASE         March 16, 2005         FreeBSD 11.0-PRERELEASE

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

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