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

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

SYNOPSIS
       #include <fenv.h>

       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 rounding_mode);

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

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

   Exceptions
       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).

       Each  of	 the macros FE_DIVBYZERO, FE_INEXACT, FE_INVALID, FE_OVERFLOW,
       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 repre‐
       sented 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	repre‐
       sented 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
       exception is currently set.

   Rounding
       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  cur‐
       rent 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 rep‐
       resented 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.

RETURN VALUE
       These  functions	 return	 zero  on  success  and	 non-zero  if an error
       occurred.

GNU DETAILS
       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  supports the functions feenableexcept() and
       fedisableexcept() to set individual floating point traps, and  fegetex‐
       cept() to query the state.

       #define _GNU_SOURCE
       #include <fenv.h>

       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.

NOTES
       Link with -lm.

CONFORMING TO
       IEC 60559 (IEC 559:1989), ANSI/IEEE 854, C99.

Linux Manpage			  2000-08-12			       FENV(3)
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