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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 (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 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
       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

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