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Floating-point environment

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Revision as of 22:40, 2 November 2012 by P12bot (Talk | contribs)

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 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 support the #pragma explicitly, but most compilers allow meaningful access to the floating-point environment anyway.

Contents

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

Template:cpp/numeric/fenv/dcl list feclearexceptTemplate:cpp/numeric/fenv/dcl list fetestexceptTemplate:cpp/numeric/fenv/dcl list feraiseexceptTemplate:cpp/numeric/fenv/dcl list feexceptflagTemplate:cpp/numeric/fenv/dcl list feroundTemplate:cpp/numeric/fenv/dcl list feenvTemplate:cpp/numeric/fenv/dcl list feholdexceptTemplate:cpp/numeric/fenv/dcl list feupdateenv

Macros

Template:cpp/numeric/fenv/dcl list FE exceptionsTemplate:cpp/numeric/fenv/dcl list FE roundTemplate:cpp/numeric/fenv/dcl list FE DFL ENV

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 converted to C++ exceptions with _set_se_translator.