29 Numerics library [numerics]

29.3 The floating-point environment [cfenv]

29.3.1 Header <cfenv> synopsis [cfenv.syn]

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#define FE_ALL_EXCEPT see below #define FE_DIVBYZERO see below // optional #define FE_INEXACT see below // optional #define FE_INVALID see below // optional #define FE_OVERFLOW see below // optional #define FE_UNDERFLOW see below // optional #define FE_DOWNWARD see below // optional #define FE_TONEAREST see below // optional #define FE_TOWARDZERO see below // optional #define FE_UPWARD see below // optional #define FE_DFL_ENV see below namespace std { // types using fenv_t = object type; using fexcept_t = object type; // functions int feclearexcept(int except); int fegetexceptflag(fexcept_t* pflag, int except); int feraiseexcept(int except); int fesetexceptflag(const fexcept_t* pflag, int except); int fetestexcept(int except); int fegetround(); int fesetround(int mode); int fegetenv(fenv_t* penv); int feholdexcept(fenv_t* penv); int fesetenv(const fenv_t* penv); int feupdateenv(const fenv_t* penv); }
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The contents and meaning of the header <cfenv> are a subset of the C standard library header <fenv.h> and only the declarations shown in the synopsis above are present.
[Note 1: 
This document does not require an implementation to support the FENV_ACCESS pragma; it is implementation-defined ([cpp.pragma]) whether the pragma is supported.
As a consequence, it is implementation-defined whether these functions can be used to test floating-point status flags, set floating-point control modes, or run under non-default mode settings.
If the pragma is used to enable control over the floating-point environment, this document does not specify the effect on floating-point evaluation in constant expressions.
— end note]
See also: ISO/IEC 9899:2024, 7.6

29.3.2 Threads [cfenv.thread]

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The floating-point environment has thread storage duration.
The initial state for a thread's floating-point environment is the state of the floating-point environment of the thread that constructs the corresponding thread object ([thread.thread.class]) or jthread object ([thread.jthread.class]) at the time it constructed the object.
[Note 1: 
That is, the child thread gets the floating-point state of the parent thread at the time of the child's creation.
— end note]
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A separate floating-point environment is maintained for each thread.
Each function accesses the environment corresponding to its calling thread.