Floating-point environment

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Numerics library

Common mathematical functions

Special mathematical functions

Floating-point environment (C++11)

Complex numbers

Numeric arrays

Pseudo-random number generation

Compile-time rational arithmetic (C++11)

Numeric algorithms

gcd (C++17)

lcm (C++17)

Interpolations

midpoint (C++20)

lerp (C++20)

Generic numeric operations

iota (C++11)

accumulate

inner_product

adjacent_difference

partial_sum

reduce (C++17)

transform_reduce (C++17)

inclusive_scan (C++17)

exclusive_scan (C++17)

transform_inclusive_scan (C++17)

transform_exclusive_scan (C++17)

Bit operations

bit_cast (C++20)

ispow2 (C++20)

ceil2 (C++20)

floor2 (C++20)

log2p1 (C++20)

 

Floating-point environment

Functions

feclearexcept (C++11)

fetestexcept (C++11)

feraiseexcept (C++11)

fegetexceptflagfesetexceptflag (C++11)(C++11)

fegetroundfesetround (C++11)(C++11)

fegetenvfesetenv (C++11)

feholdexcept (C++11)

feupdateenv (C++11)

Macro constants

FE_ALL_EXCEPTFE_DIVBYZEROFE_INEXACTFE_INVALIDFE_OVERFLOWFE_UNDERFLOW (C++11)

FE_DOWNWARDFE_TONEARESTFE_TOWARDZEROFE_UPWARD (C++11)

FE_DFL_ENV (C++11)

 

The floating-point environment is the set of floating-point status flags and control modes supported by the implementation. It is thread-local, each thread inherits the initial state of its floating-point environment from the parent thread. Floating-point operations modify the floating-point status flags to indicate abnormal results or auxiliary information. The state of floating-point control modes affects the outcomes of some floating-point operations.

The floating-point environment access and modification is only meaningful when #pragma STDC FENV_ACCESS is supported and is set to ON. Otherwise the implementation is free to assume that floating-point control modes are always the default ones and that floating-point status flags are never tested or modified. In practice, few current compilers, such as HP aCC, Oracle Studio, or IBM XL, support the #pragma explicitly, but most compilers allow meaningful access to the floating-point environment anyway.

Types

Defined in header <cfenv>
fenv_t	The type representing the entire floating-point environment
fexcept_t	The type representing all floating-point status flags collectively

Functions

feclearexcept (C++11)	clears the specified floating-point status flags (function)
fetestexcept (C++11)	determines which of the specified floating-point status flags are set (function)
feraiseexcept (C++11)	raises the specified floating-point exceptions (function)
fegetexceptflagfesetexceptflag (C++11)(C++11)	copies the state of the specified floating-point status flags from or to the floating-point environment (function)
fegetroundfesetround (C++11)(C++11)	gets or sets rounding direction (function)
fegetenvfesetenv (C++11)	saves or restores the current floating point environment (function)
feholdexcept (C++11)	saves the environment, clears all status flags and ignores all future errors (function)
feupdateenv (C++11)	restores the floating-point environment and raises the previously raise exceptions (function)

Macros

FE_ALL_EXCEPTFE_DIVBYZEROFE_INEXACTFE_INVALIDFE_OVERFLOWFE_UNDERFLOW (C++11)	floating-point exceptions (macro constant)
FE_DOWNWARDFE_TONEARESTFE_TOWARDZEROFE_UPWARD (C++11)	floating-point rounding direction (macro constant)
FE_DFL_ENV (C++11)	default floating-point environment (macro constant)

Notes

The floating-point exceptions are not related to the C++ exceptions. When a floating-point operation raises a floating-point exception, the status of the floating-point environment changes, which can be tested with std::fetestexcept, but the execution of a C++ program on most implementations continues uninterrupted.

There are compiler extensions that may be used to generate C++ exceptions automatically whenever a floating-point exception is raised:

• GNU libc function feenableexcept() enables trapping of the floating-point exceptions, which generates the signal SIGFPE. If the compiler option -fnon-call-exceptions was used, the handler for that signal may throw a user-defined C++ exception.
• MSVC function _control87() enables trapping of the floating-point exceptions, which generates a hardware exception, which can be converted to C++ exceptions with _set_se_translator.