fegetenv() and fesetenv() functions in C++

In this tutorial, we will learn about fegetenv() and fesetenv() functions in C++. These are inbuilt functions and are defined in <cfenv> header file.

fegetenv()

The syntax of this function is as follows:

int fegetenv( fenv_t* envp)

As given in the syntax, the function accepts one parameter which is a pointer to an object of type fenv_t that stores the status of the floating-point environment. It returns 0 on success and a non-zero value on failure. The fegetenv() function attempts to store the current status of the floating-point environment in a fenv_t object.

Have a look at the following C++ program to understand the working of the fegetenv() function.

#include <iostream>
#include <cfenv>

using namespace std;

void print_env()
{
    cout << "Raised Exception: ";   
    
    if (fetestexcept(FE_ALL_EXCEPT)) { 
        if (fetestexcept(FE_DIVBYZERO)) 
            cout << "FE_DIVBYZERO " << endl; 
        if (fetestexcept(FE_INEXACT)) 
            cout << "FE_INEXACT " << endl; 
        if (fetestexcept(FE_INVALID)) 
            cout << "FE_INVALID " << endl; 
        if (fetestexcept(FE_OVERFLOW)) 
            cout << "FE_OVERFLOW " << endl; 
        if (fetestexcept(FE_UNDERFLOW)) 
            cout << "FE_UNDERFLOW " << endl; 
    } 
    else
        cout << "None" << endl;
        
    cout << "Rounding direction: ";
  switch (fegetround()) {
    case FE_TONEAREST:
      cout << "FE_TONEAREST";
      break;
    case FE_DOWNWARD:
      cout << "FE_DOWNWARD";
      break;
    case FE_UPWARD:
      cout << "FE_UPWARD";
      break;
    case FE_TOWARDZERO:
      cout << "FE_TOWARDZERO";
      break;
    default:
      cout << "unknown";
  };
  cout << "\n";
}

int main()
{
    fenv_t envp;
    
    cout << "Initial Environment:--" << endl;
    
    print_env();
    
    fegetenv(&envp); 
    feraiseexcept(FE_INEXACT); 
    fesetround(FE_DOWNWARD); 
    
    cout << "Final Environment:--" << endl;
    
    print_env();
    
    return 0;
}

Output:

Initial Environment:--
Raised Exception: None
Rounding direction: FE_TONEAREST
Final Environment:--
Raised Exception: FE_INEXACT 
Rounding direction: FE_DOWNWARD

fesetenv()

The syntax of this function is as follows:

int fesetenv( fenv_t* envp)

As given in the syntax, the function accepts one parameter which is a pointer to an object of type fenv_t that stores the status of the floating-point environment. Similar to fegetenv() function, it returns 0 on success and a non-zero value on failure. The fesetenv() function attempts to set the current status of the floating-point environment from a fenv_t object.

In the above example program, we can retrieve the initial environment using fesetenv() function. See the code given below.

#include <iostream>
#include <cfenv>

using namespace std;

void print_env()
{
    cout << "Raised Exception: ";   
    
    if (fetestexcept(FE_ALL_EXCEPT)) { 
        if (fetestexcept(FE_DIVBYZERO)) 
            cout << "FE_DIVBYZERO " << endl; 
        if (fetestexcept(FE_INEXACT)) 
            cout << "FE_INEXACT " << endl; 
        if (fetestexcept(FE_INVALID)) 
            cout << "FE_INVALID " << endl; 
        if (fetestexcept(FE_OVERFLOW)) 
            cout << "FE_OVERFLOW " << endl; 
        if (fetestexcept(FE_UNDERFLOW)) 
            cout << "FE_UNDERFLOW " << endl; 
    } 
    else
        cout << "None" << endl;
        
    cout << "Rounding direction: ";
  switch (fegetround()) {
    case FE_TONEAREST:
      cout << "FE_TONEAREST";
      break;
    case FE_DOWNWARD:
      cout << "FE_DOWNWARD";
      break;
    case FE_UPWARD:
      cout << "FE_UPWARD";
      break;
    case FE_TOWARDZERO:
      cout << "FE_TOWARDZERO";
      break;
    default:
      cout << "unknown";
  };
  cout << "\n";
}

int main()
{
    fenv_t envp;
    
    cout << "Initial Environment:--" << endl;
    
    print_env();
    
    fegetenv(&envp); 
    feraiseexcept(FE_INEXACT); 
    fesetround(FE_DOWNWARD); 
    
    cout << "Final Environment:--" << endl;
    
    print_env();
    
    cout << "Retrieving Initial Environment using fesetenv():--" << endl;
    fesetenv(&envp);
    
    print_env();
    
    return 0;
}

Output:

Initial Environment:--
Raised Exception: None
Rounding direction: FE_TONEAREST
Final Environment:--
Raised Exception: FE_INEXACT 
Rounding direction: FE_DOWNWARD
Retrieving Initial Environment using fesetenv():--
Raised Exception: None
Rounding direction: FE_TONEAREST

Also read: fesetround() and fegetround() in C++