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


     feclearexcept,  fegetexceptflag, feraiseexcept, fesetexceptflag, fetes-
     texcept,  fegetenv,  fegetround,  feholdexcept,  fesetround,  fesetenv,
     feupdateenv,  feenableexcept,  fedisableexcept, fegetexcept - floating-
     point rounding and exception handling


     #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.


     These eleven functions were defined in C99, and describe  the  handling
     of  floating-point  rounding  and  exceptions  (overflow,  zero-divide,
     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  (per-
     haps the code address where it occurred).
     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 mode
     The rounding mode determines how the result  of  floating-point  opera-
     tions  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  cur-
     rent 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
  1. 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 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 nonstop  (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 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 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.


     These  functions  return  zero  on  success  and  nonzero  if  an error


     These functions first appeared in glibc in version 2.1.


     For  an  explanation  of  the  terms  used   in   this   section,   see
     allbox; lb lb lb lw35 l l.  Interface Attribute Value T{
     feclearexcept(), fegetexceptflag(), feraiseexcept(), fesetexceptflag(),
     fetestexcept(), fegetround(), fesetround(), fegetenv(), feholdexcept(),
     fesetenv(), feupdateenv(), feenableexcept(), fedisableexcept(),
     fegetexcept() T}   Thread safety  T{ MT-Safe T}


     IEC 60559 (IEC 559:1989), ANSI/IEEE 854, C99, POSIX.1-2001.


 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 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  feen-
     ableexcept()  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 ex-


     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.




     This page is part of release 4.16 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

Linux 2017-09-15 FENV(3)

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