fegetenv, fesetenv

From cppreference.com
< clrm; | numericlrm; | fenv
Defined in header <fenv.h>
int fegetenv( fenv_t* envp );
(1) (since C99)
int fesetenv( const fenv_t* envp );
(2) (since C99)

1) Attempts to store the status of the floating-point environment in the object pointed to by envp.

2) Attempts to establish the floating-point environment from the object pointed to by envp. The value of that object must be previously obtained by a call to feholdexcept or fegetenv or be a floating-point macro constant. If any of the floating-point status flags are set in envp, they become set in the environment (and are then testable with fetestexcept), but the corresponding floating-point exceptions are not raised (execution continues uninterrupted)

Parameters

envp - pointer to the object of type fenv_t which holds the status of the floating-point environment

Return value

0 on success, non-zero otherwise.

Example

#include <stdio.h>
#include <math.h>
#include <fenv.h>

#pragma STDC FENV_ACCESS ON

void show_fe_exceptions(void)
{
    printf("current exceptions raised: ");
    if(fetestexcept(FE_DIVBYZERO))     printf(" FE_DIVBYZERO");
    if(fetestexcept(FE_INEXACT))       printf(" FE_INEXACT");
    if(fetestexcept(FE_INVALID))       printf(" FE_INVALID");
    if(fetestexcept(FE_OVERFLOW))      printf(" FE_OVERFLOW");
    if(fetestexcept(FE_UNDERFLOW))     printf(" FE_UNDERFLOW");
    if(fetestexcept(FE_ALL_EXCEPT)==0) printf(" none");
    printf("\n");
}

void show_fe_rounding_method(void)
{
    printf("current rounding method:    ");
    switch (fegetround()) {
           case FE_TONEAREST:  printf ("FE_TONEAREST");  break;
           case FE_DOWNWARD:   printf ("FE_DOWNWARD");   break;
           case FE_UPWARD:     printf ("FE_UPWARD");     break;
           case FE_TOWARDZERO: printf ("FE_TOWARDZERO"); break;
           default:            printf ("unknown");
    };
    printf("\n");
}

void show_fe_environment(void)
{
    show_fe_exceptions();
    show_fe_rounding_method();
}    

int main(void)
{
    fenv_t curr_env;
    int rtn;

    /* Show default environment. */
    show_fe_environment();
    printf("\n");

    /* Perform some computation under default environment. */
    printf("+11.5 ->%+4.1f\n", rint(+11.5)); /* midway between two integers */
    printf("+12.5 ->%+4.1f\n", rint(+12.5)); /* midway between two integers */
    show_fe_environment();
    printf("\n");

    /* Save current environment. */
    rtn = fegetenv(&curr_env);

    /* Perform some computation with new rounding method. */
    feclearexcept(FE_ALL_EXCEPT);
    fesetround(FE_DOWNWARD);
    printf("1.0/0.0 =%f\n", 1.0/0.0);
    printf("+11.5 ->%+4.1f\n", rint(+11.5));
    printf("+12.5 ->%+4.1f\n", rint(+12.5));
    show_fe_environment();
    printf("\n");

    /* Restore previous environment. */
    rtn = fesetenv(&curr_env);
    show_fe_environment();

    return 0;
}

Output:

current exceptions raised: none
current rounding method:   FE_TONEAREST

+11.5 -> +12.0
+12.5 -> +12.0
current exceptions raised: FE_INEXACT
current rounding method:   FE_TONEAREST

1.0/0.0 = inf
+11.5 -> +11.0
+12.5 -> +12.0
current exceptions raised: FE_DIVBYZERO FE_INEXACT
current rounding method:   FE_DOWNWARD

current exceptions raised: FE_INEXACT
current rounding method:   FE_TONEAREST

References

  • C11 standard (ISO/IEC 9899:2011):
  • 7.6.4.1 The fegetenv function (p: 213)
  • 7.6.4.3 The fesetenv function (p: 214)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.6.4.1 The fegetenv function (p: 194)
  • 7.6.4.3 The fesetenv function (p: 195)

See also

saves the environment, clears all status flags and ignores all future errors
(function)
restores the floating-point environment and raises the previously raise exceptions
(function)
default floating-point environment
(macro constant)