FENV(3)                 NetBSD 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.

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.

NetBSD 7.0                      March 16, 2005                      NetBSD 7.0

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