fenv(3) — Linux manual page


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

NAME         top

       feclearexcept,   fegetexceptflag,   feraiseexcept,   fesetexceptflag,
       fetestexcept, fegetenv, fegetround,  feholdexcept,  fesetround,  fes‐
       etenv,  feupdateenv,  feenableexcept,  fedisableexcept, fegetexcept -
       floating-point rounding and exception handling

SYNOPSIS         top

       #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         top

       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
       (perhaps 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
       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         top

       These functions return zero on success and nonzero if an error

VERSIONS         top

       These functions first appeared in glibc in version 2.1.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface                           Attribute     Value   │
       │feclearexcept(), fegetexceptflag(), │ Thread safety │ MT-Safe │
       │feraiseexcept(), fesetexceptflag(), │               │         │
       │fetestexcept(), fegetround(),       │               │         │
       │fesetround(), fegetenv(),           │               │         │
       │feholdexcept(), fesetenv(),         │               │         │
       │feupdateenv(), feenableexcept(),    │               │         │
       │fedisableexcept(), fegetexcept()    │               │         │

CONFORMING TO         top

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

NOTES         top

   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         top

       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         top


COLOPHON         top

       This page is part of release 5.09 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

Linux                            2017-09-15                          FENV(3)

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