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fegetenv, fesetenv

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< c‎ | numeric‎ | fenv
Revision as of 05:58, 8 April 2014 by Newatthis (Talk | contribs)

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)

Contents

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()
{
    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

See also

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]
default floating-point environment
(macro constant) [edit]
C++ documentation for fegetenv, fesetenv