fesetround(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);

```

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

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.

```

RETURNVALUE

```   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()    │               │         │
└────────────────────────────────────┴───────────────┴─────────┘
```

CONFORMINGTO

```   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.

```

SEEALSO

```   math_error(7)

```

COLOPHON

```   This page is part of release 4.09 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
https://www.kernel.org/doc/man-pages/.
```

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