fenv(3) — Linux manual page


fenv(3)                 Library Functions Manual                 fenv(3)

NAME         top

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

LIBRARY         top

       Math library (libm, -lm)

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);

DESCRIPTION         top

       These eleven functions were defined in C99, and describe the
       handling of floating-point rounding and exceptions (overflow,
       zero-divide, etc.).

       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

       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

       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

       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

       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

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()    │               │         │

STANDARDS         top

       C11, POSIX.1-2008, IEC 60559 (IEC 559:1989), ANSI/IEEE 854.

HISTORY         top

       C99, POSIX.1-2001.  glibc 2.1.

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 glibc 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


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