fegetexcept(3)

NAME

   feclearexcept,    fegetexceptflag,    feraiseexcept,   fesetexceptflag,
   fetestexcept, 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
   (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 (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 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
   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

   For  an  explanation  of  the  terms  used   in   this   section,   see
   attributes(7).

   ┌────────────────────────────────────┬───────────────┬─────────┐
   │Interface                           │ Attribute     │ Value   │
   ├────────────────────────────────────┼───────────────┼─────────┤
   │feclearexcept(), fegetexceptflag(), │ Thread safety │ MT-Safe │
   │feraiseexcept(), fesetexceptflag(), │               │         │
   │fetestexcept(), fegetround(),       │               │         │
   │fesetround(), fegetenv(),           │               │         │
   │feholdexcept(), fesetenv(),         │               │         │
   │feupdateenv(), feenableexcept(),    │               │         │
   │fedisableexcept(), fegetexcept()    │               │         │
   └────────────────────────────────────┴───────────────┴─────────┘

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 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 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

   math_error(7)

COLOPHON

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   description  of  the project, information about reporting bugs, and the
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