fenv

FENV(3)                     Linux Programmer's Manual                    FENV(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
       This page is part of release 5.08 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/.



Linux                              2017-09-15                            FENV(3)