Manpage of FENV
FENVSection: Linux Programmer's Manual (3)
NAMEfeclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag, fetestexcept, fegetenv, fegetround, feholdexcept, fesetround, fesetenv, feupdateenv, feenableexcept, fedisableexcept, fegetexcept - floating-point rounding and exception handling
#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);
DESCRIPTIONThese eleven functions were defined in C99, and describe the handling of floating-point rounding and exceptions (overflow, zero-divide, etc.).
ExceptionsThe divide-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 whenever overflowor underflowoccurs.
Exception handlingExceptions 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 argument FE_OVERFLOW|FE_UNDERFLOW. Other exceptions may be supported. The macro FE_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 argument exceptsin 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 exceptsto 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.
Rounding modeThe 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_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:
- The rounding mode is not determinable.
- Rounding is toward 0.
- Rounding is toward nearest number.
- Rounding is toward positive infinity.
- Rounding is toward negative infinity.
Other values represent machine-dependent, nonstandard rounding modes.
Floating-point environmentThe 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 thosely set with those in *envp. As before, the object *envpmust be known to be valid.
RETURN VALUEThese functions return zero on success and nonzero if an error occurred.
VERSIONSThese functions first appeared in glibc in version 2.1.
ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7).
|feclearexcept(), fegetexceptflag(), feraiseexcept(), fesetexceptflag(), fetestexcept(), fegetround(), fesetround(), fegetenv(), feholdexcept(), fesetenv(), feupdateenv(), feenableexcept(), fedisableexcept(), fegetexcept()||Thread safety||MT-Safe
CONFORMING TOIEC 60559 (IEC 559:1989), ANSI/IEEE 854, C99, POSIX.1-2001.
Glibc notesIf possible, the GNU C Library defines a macro FE_NOMASK_ENVwhich 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_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 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);
BUGSC99 specifies that the value of FLT_ROUNDSshould reflect changes to the current rounding mode, as set by fesetround(). Currently, this does not occur: FLT_ROUNDSalways has the value 1.
- RETURN VALUE
- CONFORMING TO
- SEE ALSO
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