FENV(3)                    Linux Programmer's Manual                   FENV(3)

       feclearexcept,  fegetexceptflag, feraiseexcept, fesetexceptflag, fetes-
       texcept,  fegetenv,  fegetround,  feholdexcept,  fesetround,  fesetenv,
       feupdateenv - C99 floating point rounding and exception handling

       #include <fenv.h>

       void feclearexcept(int excepts);
       void fegetexceptflag(fexcept_t *flagp, int excepts);
       void feraiseexcept(int excepts);
       void fesetexceptflag(const fexcept_t *flagp, int excepts);
       int fetestexcept(int excepts);

       int fegetround(void);
       int fesetround(int rounding_mode);

       void fegetenv(fenv_t *envp);
       int feholdexcept(fenv_t *envp);
       void fesetenv(const fenv_t *envp);
       void feupdateenv(const fenv_t *envp);

       These  eleven  functions were defined in C99, and describe the handling
       of floating point rounding and exceptions (overflow, zero-divide etc.).

       The  DivideByZero  exception occurs when an operation on finite numbers
       produces infinity as exact answer.

       The Overflow exception occurs when a result has to be represented as  a
       floating  point  number,  but has (much) larger absolute value than the
       largest (finite) floating point number that is representable.

       The Underflow exception occurs when a result has to be represented as a
       floating point number, but has smaller absolute value than the smallest
       positive normalized floating point number (and would lose much accuracy
       when represented as a denormalized number).

       The Inexact exception occurs when the rounded result of an operation is
       not equal to the infinite precision  result.   It  may  occur  whenever
       Overflow or Underflow occurs.

       The  Invalid  exception occurs when there is no well-defined result for
       an operation, as for 0/0 or infinity - infinity or sqrt(-1).

   Exception handling
       Exceptions are represented in two ways:  as  a  single  bit  (exception
       present/absent),  and  these  bits  correspond  in some implementation-
       defined way with bit positions in an integer, and  also  as  an  opaque
       structure  that  may contain more information about the exception (per-
       haps the code address where it occurred).

       FE_UNDERFLOW  is  defined  when the implementation supports handling of
       the corresponding exception, and if so then defines  the  corresponding
       bit(s),  so  that  one can call exception handling functions e.g. using
       the integer argument FE_OVERFLOW|FE_UNDERFLOW.  Other exceptions may be
       supported. The macro FE_ALL_EXCEPT is the bitwise OR of all bits corre-
       sponding to supported exceptions.

       The feclearexcept function clears the supported exceptions  represented
       by the bits in its argument.

       The  fegetexceptflag  function  stores a representation of the state of
       the exception flags represented by the argument excepts in  the  opaque
       object *flagp.

       The  feraiseexcept function raises the supported exceptions represented
       by the bits in excepts.

       The fesetexceptflag function sets the complete status  for  the  excep-
       tions represented by excepts to the value *flagp.  This value must have
       been obtained by an earlier call of fegetexceptflag with a  last  argu-
       ment that contained all bits in excepts.

       The fetestexcept function returns a word in which the bits are set that
       were set in the argument excepts and for which the corresponding excep-
       tion is currently set.

       is defined when the implementation supports  getting  and  setting  the
       corresponding rounding direction.

       The  fegetround function returns the macro corresponding to the current
       rounding mode.

       The fesetround function sets the rounding  mode  as  specified  by  its
       argument and returns zero when it was successful.

   Floating point environment
       The entire floating point environment, including control modes and sta-
       tus flags, can be handled as one opaque object, of  type  fenv_t.   The
       default  environment is denoted by FE_DFL_ENV (of type const fenv_t *).
       This is the environment setup at program start and it is defined by ISO
       C to have round to nearest, all exceptions cleared and a non-stop (con-
       tinue on exceptions) mode.

       The fegetenv function saves the current floating point  environment  in
       the object *envp.

       The  feholdexcept  function  does  the  same, then clears all exception
       flags, and sets a non-stop (continue on exceptions) mode, if available.
       It returns zero when successful.

       The  fesetenv function restores the floating point environment from the
       object *envp.  This object must be known to be valid, e.g., the  result
       of  a  call  to  fegetenv or feholdexcept or equal to FE_DFL_ENV.  This
       call does not raise exceptions.

       The feupdateenv function installs the floating-point environment repre-
       sented by the object *envp, except that currently raised exceptions are
       not cleared.  After calling this function, the raised  exceptions  will
       be  a  bitwise  OR  of  those  previously  set with those in *envp.  As
       before, the object *envp must be known to be valid.

       If possible, the GNU C Library defines a macro FE_NOMASK_ENV which rep-
       resents  an  environment  where every exception raised causes a trap to
       occur.  You can test for this macro using #ifdef.  It is  only  defined
       if  _GNU_SOURCE  is defined.  The C99 standard does not define a way to
       set individual bits in the floating point mask, e.g. to  trap  on  spe-
       cific  flags.   glibc 2.2 will support the functions feenableexcept and
       fedisableexcept to set individual floating point traps, and fegetexcept
       to query the state.

       int feenableexcept (int excepts);
       int fedisableexcept (int excepts);
       int fegetexcept (void);

       The feenableexcept and fedisableexcept functions enable (disable) traps
       for each of the exceptions represented by excepts and return the previ-
       ous  set  of enabled exceptions when successful, and -1 otherwise.  The
       fegetexcept function returns the set of all  currently  enabled  excep-

       IEC 60559 (IEC 559:1989), ANSI/IEEE 854, ISO C99 (ISO/IEC 9899:1999).

Linux Manpage                     2000-08-12                           FENV(3)