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

Synopsis

Description

Exceptions

Rounding mode

Versions

Notes

Glibc Notes

Bugs

Colophon

#include <fenv.h>

int feclearexcept(intexcepts);

int fegetexceptflag(fexcept_t *flagp, intexcepts);

int feraiseexcept(intexcepts);

int fesetexceptflag(const fexcept_t *flagp, intexcepts);

int fetestexcept(intexcepts);

int fegetround(void);

int fesetround(introunding_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, etc.).

Thedivide-by-zeroexception occurs when an operation on finite numbers produces infinity as exact answer.The

overflowexception 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

underflowexception 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

inexactexception occurs when the rounded result of an operation is not equal to the infinite precision result. It may occur wheneveroverfloworunderflowoccurs.The

invalidexception occurs when there is no well-defined result for an operation, as for 0/0 or infinity - infinity or sqrt(-1).

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_UNDERFLOWis 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 argumentFE_OVERFLOW|FE_UNDERFLOW. Other exceptions may be supported. The macroFE_ALL_EXCEPTis 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 argumentexceptsin the opaque object*flagp.The

feraiseexcept() function raises the supported exceptions represented by the bits inexcepts.The

fesetexceptflag() function sets the complete status for the exceptions represented byexceptsto the value*flagp. This value must have been obtained by an earlier call offegetexceptflag() with a last argument that contained all bits inexcepts.The

fetestexcept() function returns a word in which the bits are set that were set in the argumentexceptsand for which the corresponding exception is currently set.

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, andFE_TOWARDZEROis 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:Other values represent machine-dependent, nonstandard rounding modes.

-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. The value of

FLT_ROUNDSshould reflect the current rounding mode as set byfesetround() (but see BUGS).

The entire floating-point environment, including control modes and status flags, can be handled as one opaque object, of typefenv_t. The default environment is denoted byFE_DFL_ENV(of typeconst 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 tofegetenv() orfeholdexcept() or equal toFE_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*envpmust be known to be valid.

These functions return zero on success and nonzero if an error occurred.

These functions first appeared in glibc in version 2.1.

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

If possible, the GNU C Library defines a macroFE_NOMASK_ENVwhich represents an environment where every exception raised causes a trap to occur. You can test for this macro using#ifdef. It is only defined if_GNU_SOURCEis 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 functionsfeenableexcept() andfedisableexcept() to set individual floating-point traps, andfegetexcept() to query the state.

#define _GNU_SOURCE/* See feature_test_macros(7) */

#include <fenv.h>

int feenableexcept(intexcepts);

int fedisableexcept(intexcepts);

int fegetexcept(void);The

feenableexcept() andfedisableexcept() functions enable (disable) traps for each of the exceptions represented byexceptsand return the previous set of enabled exceptions when successful, and -1 otherwise. Thefegetexcept() function returns the set of all currently enabled exceptions.

C99 specifies that the value ofFLT_ROUNDSshould reflect changes to the current rounding mode, as set byfesetround(). Currently, this does not occur:FLT_ROUNDSalways has the value 1.

math_error(7)

This page is part of release 3.44 of the Linuxman-pagesproject. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Linux |
FENV (3) | 2010-10-31 |