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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,  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-
       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, 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  (towards  positive  infinity),  round  down
       (towards negative infinity), and round towards zero.

       Each  of  the  macros   FE_TONEAREST,   FE_UPWARD,   FE_DOWNWARD,   and
       FE_TOWARDZERO  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.

       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 towards 0.

       1      Rounding is towards nearest number.

       2      Rounding is towards positive infinity.

       3      Rounding is towards 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
       occurred.

VERSIONS
       These functions first appeared in glibc in version 2.1.

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  only  defined
       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.  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.

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
       feature_test_macros(7), math_error(7)

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2008-08-11                           FENV(3)
 

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