# Floating-point environment

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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)
fegetexceptflag fesetexceptflag (C++11)(C++11) | copies the state of the specified floating-point status flags from or to the floating-point environment (function)
fegetround fesetround (C++11)(C++11) | gets or sets rounding direction (function)
fegetenv fesetenv (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_EXCEPT FE_DIVBYZERO FE_INEXACT FE_INVALID FE_OVERFLOW FE_UNDERFLOW (C++11) | floating-point exceptions (macro constant)
FE_DOWNWARD FE_TONEAREST FE_TOWARDZERO FE_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`.

### See also

C documentation for Floating-point environment

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