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

NAME
       feclearexcept,  fegetexceptflag,	feraiseexcept, fesetexceptflag,	fetes-
       texcept,	fegetenv, fegetround, feholdexcept, fesetround,	fesetenv,  fe-
       updateenv,  feenableexcept,  fedisableexcept,  fegetexcept  - 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);

       Link with -lm.

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

   Exceptions
       The divide-by-zero 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-de-
       fined  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, for example,
       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 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 mode
       The rounding mode determines how	the result  of	floating-point	opera-
       tions  is  treated when the result cannot be exactly represented	in the
       significand.  Various rounding modes may	be provided: round to  nearest
       (the  default), round up	(toward	positive infinity), round down (toward
       negative	infinity), and round toward zero.

       Each of the macros FE_TONEAREST,	 FE_UPWARD,  FE_DOWNWARD,  and	FE_TO-
       WARDZERO	 is  defined when the implementation supports getting and set-
       ting 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.

       C99 and POSIX.1-2008 specify  an	 identifier,  FLT_ROUNDS,  defined  in
       _float.h_, which	indicates the implementation-defined rounding behavior
       for floating-point addition.  This identifier has one of	the  following
       values:

       -1     The rounding mode	is not determinable.

       0      Rounding is toward 0.

       1      Rounding is toward nearest number.

       2      Rounding is toward positive infinity.

       3      Rounding is toward negative infinity.

       Other values represent machine-dependent, nonstandard rounding modes.

       The value of FLT_ROUNDS should reflect the current rounding mode	as set
       by fesetround() (but see	BUGS).

   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	nonstop	 (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 nonstop (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, for example,
       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 nonzero if an error oc-
       curred.

VERSIONS
       These functions first appeared in glibc in version 2.1.

ATTRIBUTES
   Multithreading (see pthreads(7))
       The feclearexcept(), fegetexceptflag(), fegetexceptflag(), fesetexcept-
       flag(), fetestexcept(), fegetround(), fesetround(), fegetenv(), fehold-
       except(),  fesetenv(),  feupdateenv(),  feenableexcept(),  fedisableex-
       cept(), and fegetexcept() functions are thread-safe.

CONFORMING TO
       IEC 60559 (IEC 559:1989), ANSI/IEEE 854,	C99, POSIX.1-2001.

NOTES
   Glibc notes
       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 defined only
       if _GNU_SOURCE is defined.  The C99 standard does not define a  way  to
       set individual bits in the floating-point mask, for example, to trap on
       specific	flags.	Since version 2.2, glibc supports the functions	 feen-
       ableexcept()  and  fedisableexcept()  to	 set individual	floating-point
       traps, and fegetexcept()	to query the state.

       #define _GNU_SOURCE	   /* See feature_test_macros(7) */
       #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 ex-
       ceptions.

BUGS
       C99 specifies that the value of FLT_ROUNDS should  reflect  changes  to
       the  current  rounding  mode,  as set by	fesetround().  Currently, this
       does not	occur: FLT_ROUNDS always has the value 1.

SEE ALSO
       math_error(7)

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-04-01			       FENV(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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