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Showing content from https://cplusplus.com/reference/cfenv/feupdateenv/ below:

function

<cfenv>

feupdateenv 
int feupdateenv (const fenv_t* envp);

Update floating-point environment

Attempts to establish the state of the floating-point environment as represented by the object pointed by envp. It then attempts to raise the exceptions that were set in the floating-point environment before the call.

Programs calling this function shall ensure that pragma FENV_ACCESS is enabled for the call.



Parameters
envp
Either a pointer to a fenv_t value (filled by a previous call to fegetenv or feholdexcept), or one of the floating-point environment macro values:
value description FE_DFL_ENV Default floating-point environment (the same as at program startup). Certain library implementations may support additional floating-point environment state values (with their corresponding macros also beginning with FE_).

Return Value Zero, if successful.
A non-zero value otherwise.

Example 
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/* feholdexcept/feupdateenv example */
#include <stdio.h>      /* printf, puts */
#include <fenv.h>       /* feholdexcept, feclearexcept, fetestexcept, feupdateenv, FE_* */
#include <math.h>       /* log */
#pragma STDC FENV_ACCESS on

double log_zerook (double x) {
  fenv_t fe;
  feholdexcept(&fe);
  x=log(x);
  feclearexcept (FE_OVERFLOW|FE_DIVBYZERO);
  feupdateenv(&fe);
  return x;
}

int main ()
{
  feclearexcept (FE_ALL_EXCEPT);
  printf ("log(0.0): %f\n", log_zerook(0.0));
  if (!fetestexcept(FE_ALL_EXCEPT))
    puts ("no exceptions raised");
  return 0;
}

Possible output: 
log(0.0): -inf
no exceptions raised


Data races Each thread maintains a separate floating-point environment with its own state. Spawning a new thread copies the current state. [This applies to C11 and C++11 implementations]

ExceptionsNo-throw guarantee: this function never throws exceptions.
Note that C floating-point exceptions are not C++ exceptions, and thus are not caught by try/catch blocks.

See also
feupdateenv
Update floating-point environment (function)
fegetenv
Get floating-point environment (function)
fesetenv
Set floating-point environment (function)

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