Floating-point environment

< cpp‎ | numeric

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.


[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

clears the specified floating-point status flags
(function) [edit]
determines which of the specified floating-point status flags are set
(function) [edit]
raises the specified floating-point exceptions
(function) [edit]
copies the state of the specified floating-point status flags from or to the floating-point environment
(function) [edit]
gets or sets rounding direction
(function) [edit]
saves or restores the current floating point environment
(function) [edit]
saves the environment, clears all status flags and ignores all future errors
(function) [edit]
restores the floating-point environment and raises the previously raise exceptions
(function) [edit]

[edit] Macros

floating-point exceptions
(macro constant) [edit]
floating-point rounding direction
(macro constant) [edit]
default floating-point environment
(macro constant) [edit]

[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 converted to C++ exceptions with _set_se_translator.