Floating-point environment

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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.

Contents 1 Types 2 Functions 3 Macros 4 Notes

[edit] 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

[edit] Functions

feclearexcept	clears the specified floating-point status flags (function)
fegetexceptflag	saves the state of the specified floating-point status flags (function)
feraiseexcept	raises the specified floating-point exceptions (function)
fesetexceptflag	restores the state of the specified floating-point status flags (function)
fetestexcept	determines which of the specified floating-point status flags are set (function)
fegetround	gets the current rounding direction (function)
fesetround	establishes the new rounding direction (function)
fegetenv	saves the current floating point environment (function)
feholdexcept	saves the environment, clears all status flags and ignores all future errors (function)
fesetenv	restores the floating-point environment without raising exceptions (function)
feupdateenv	restores the floating-point environment and raises the previously raise exceptions (function)

[edit] Macros

FE_ALL_EXCEPT FE_DIVBYZERO FE_INEXACT FE_INVALID FE_OVERFLOW FE_UNDERFLOW	floating-point exceptions (macro constant)
FE_DOWNWARD FE_TONEAREST FE_TOWARDZERO FE_UPWARD	floating-point rounding direction (macro constant)
FE_DFL_ENV	default floating-point environment (macro constant)

[edit] 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.
• Microsoft WinAPI function _control87() enables trapping of the floating-point exceptions, which generates a hardware exception, which can be caught with __try/__except.