NAME feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetes‐ texcept, fegetenv, fegetround, feholdexcept, fesetround, fesetenv,
feupdateenv, feenableexcept, fedisableexcept, fegetexcept - floating- point rounding and exception handling SYNOPSIS
#include
int feclearexcept(int excepts); int fegetexceptflag(fexceptt *flagp, int excepts); int feraiseexcept(int excepts); int fesetexceptflag(const fexceptt *flagp, int excepts); int fetestexcept(int excepts); int fegetround(void); int fesetround(int roundingmode); int fegetenv(fenvt *envp); int feholdexcept(fenvt *envp); int fesetenv(const fenvt *envp); int feupdateenv(const fenvt *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 (per‐ haps the code address where it occurred). Each of the macros FEDIVBYZERO, FEINEXACT, FEINVALID, FEOVERFLOW, FEUNDERFLOW 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 FEOVERFLOW|FEUNDERFLOW. Other exceptions may be supported. The macro FEALLEXCEPT is the bitwise OR of all bits corresponding to supported exceptions. The feclearexcept() function clears the supported exceptions repre‐ sented 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 repre‐ sented by the bits in excepts. The fesetexceptflag() function sets the complete status for the excep‐ tions represented by excepts to the value *flagp. This value must have been obtained by an earlier call of fegetexceptflag() with a last argu‐ ment 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 opera‐ tions 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 FETONEAREST, FEUPWARD, FEDOWNWARD, and FETOWARDZERO is defined when the implementation supports getting and setting the corresponding rounding direction. The fegetround() function returns the macro corresponding to the cur‐ rent 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, FLTROUNDS, defined in
, 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 FLTROUNDS 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 sta‐ tus flags, can be handled as one opaque object, of type fenvt. The default environment is denoted by FEDFLENV (of type const fenvt *). 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 (con‐ tinue 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 FEDFLENV. This call does not raise exceptions.
The feupdateenv() function installs the floating-point environment rep‐ resented 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 │ │fegetexceptflag(), fesetexceptflag(), │ │ │ │fetestexcept(), fegetround(), │ │ │ │fesetround(), fegetenv(), │ │ │ │feholdexcept(), fesetenv(), │ │ │ │feupdateenv(), feenableexcept(), │ │ │ │fedisableexcept(), fegetexcept() │ │ │ └───────────────────────────────────────┴───────────────┴─────────┘ ATTRIBUTES For an explanation of the terms used in this section, see at‐ tributes(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 FENOMASKENV which rep‐ resents an environment where every exception raised causes a trap to
occur. You can test for this macro using #ifdef. It is defined only if GNUSOURCE 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 feen‐
ableexcept() and fedisableexcept() to set individual floating-point traps, and fegetexcept() to query the state.
#define GNUSOURCE /* See featuretestmacros(7) */
#include
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 ex‐ ceptions. BUGS C99 specifies that the value of FLTROUNDS should reflect changes to the current rounding mode, as set by fesetround(). Currently, this does not occur: FLTROUNDS always has the value 1. SEE ALSO matherror(7) COLOPHON
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