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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 <&lt;fenv.h>&gt;

       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  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, 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
       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, 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  non-zero  if  an  error
       occurred.

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

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 <&lt;fenv.h>&gt;

       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.

SEE ALSO
       feature_test_macros(7), math_error(7)

COLOPHON
       This page is part of release 3.05 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                             2000-08-12                           FENV(3)