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



NAME
       feclearexcept,  fegetexceptflag,  feraiseexcept,  fesetexceptflag, fetestexcept, fegetenv, fegetround, feholdexcept, fes-
       etround, fesetenv, feupdateenv, feenableexcept, fedisableexcept, fegetexcept - floating-point rounding and exception han-
       dling

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 (over-
       flow, 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 abso-
       lute 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 infor-
       mation about the exception (perhaps 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 represented 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 represented by the bits in excepts.

       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.

   Rounding mode
       The rounding mode determines how the result of floating-point operations 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 current 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 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 nonstop  (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 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 represented 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 pre-
       viously 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 represents 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 fegetexcept() 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)