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



NAME
       feclearexcept,  fegetexceptflag,  feraiseexcept,  fesetexceptflag, fetestexcept, fegetenv,
       fegetround, feholdexcept, fesetround, fesetenv, feupdateenv, feenableexcept,  fedisableex‐
       cept, 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 num‐
       ber, 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
       (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  func‐
       tions, 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 (toward positive infinity), round
       down (toward negative infinity), and round toward 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  indi‐
       cates  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 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 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.

ATTRIBUTES
   Multithreading (see pthreads(7))
       The  feclearexcept(),  fegetexceptflag(),  fegetexceptflag(), fesetexceptflag(), fetestex‐
       cept(), fegetround(), fesetround(), fegetenv(), feholdexcept(), fesetenv(), feupdateenv(),
       feenableexcept(), fedisableexcept(), 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 represents an environ‐
       ment 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 feenableexcept() 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 cur‐
       rently 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
       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)