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

       feclearexcept,  fegetexceptflag,  feraiseexcept,  fesetexceptflag, fetestexcept, 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 num-
       ber, 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
       (perhaps the code address where it occurred).

       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  func-
       tions  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  corresponding  to  sup-
       ported exceptions.

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

       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

       The fesetexceptflag function sets the complete status for the  exceptions  represented  by
       excepts	to  the  value	*flagp.  This value must have been obtained by an earlier call of
       fegetexceptflag with a last argument 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 exception is currently set.

       Each of the macros FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD 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 status 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
       (continue 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 represented by the object
       *envp,  except that currently raised exceptions are not cleared.  After calling this func-
       tion, 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 represents an environ-
       ment 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  specific
       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 previous set of enabled exceptions when
       successful, and -1 otherwise.  The fegetexcept function returns the set of  all	currently
       enabled exceptions.

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

Linux Manpage				    2000-08-12					  FENV(3)
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