feholdexcept() in C++

Floating point operations in computer programs often involve approximations that may lead to inaccuracies and exceptional situations. These exceptions can cause unwanted program termination or incorrect output when performing sensitive numerical calculations. The C++ programming language provides mechanisms to handle these floating-point exceptions and functions to temporarily turn off exception handling when needed.

One such Function for controlling floating-point exceptions is feholdexcept(). This proper Function allows the preservation of the current floating-point environment, clearing the flags to turn off exceptions temporarily during code execution and restoring the environment afterwards. By isolating sections of code that may produce spurious exceptions, a program can be made more robust and fault tolerant.

The feholdexcept() Function is declared in the <fenv.h> header allows saving and clearing the floating point exception flags specified by its fenv_t environment argument. After the risky code has been executed with exceptions disabled, the previous environment can be restored with fenv_setenv(). This article explains the feholdexcept() Function, demonstrates its usage, and how it can help manage numerical exceptions.

By understanding functions like feholdexcept(), developers can isolate unstable numerical operations in their C++ programs while maintaining exception-handling behavior in other sections. It aids in writing correct; fault-tolerant code that avoids unexpected terminations due to unavoidable floating-point inaccuracies.

What is feholdexcept function ():

The feholdexcept() function enables saving and clearing the current floating-point environment to temporarily turn off floating-point exceptions. Its declaration in the C++ language is:

fenv_t feholdexcept(fenv_t env);

It takes one argument, env, and fenv_t object to store the existing state. fenv_t is a type defined in <fenv.h> representing the entire floating-point environment.

Inside feholdexcept(), the current exception flags are saved into the env argument. These flags indicate when a floating point exception has been raised, such as an invalid operation, divide by zero, overflow, or other numerical error.

The available floating-point exceptions are:

• FE_DIVBYZERO - Divide by zero
• FE_INEXACT - Inexact result
• FE_INVALID - Invalid operation
• FE_OVERFLOW - Numeric overflow
• FE_UNDERFLOW - Numeric underflow

After storing the existing flags, feholdexcept() clears the specified exception flags passed to it. Typically, FE_ALL_EXCEPT would be passed to clear all flags at once temporarily.

With the flags cleared, any code inside the protected section will not have exceptions raised, even for mathematically erroneous operations. The previous environment state stored in the env can later be restored by calling fenv_setenv(), which reinstates the flags. This isolates code sections needing lax handling while maintaining strict behaviour everywhere else.

In essence, feholdexcept() allows exception throwing to be turned off for a particular block of code, providing control over where numerical exceptions get handled in a C++ program. It facilitates better management of floating point behaviour.

Uses of the feholdexcept() function:

• Temporarily turn off floating-point exceptions in sensitive numerical code: Call feholdexcept() before code sections that may undesirably throw exceptions due to unavoidable inaccuracies. It prevents unwanted program termination.
• Isolate buggy floating point code: Use feholdexcept()/fenv_setenv() around code you suspect contains numerical bugs to allow execution. The later correction will then enable exceptions again for that code.
• Improve computational performance: Exception handling can reduce the speed of math-intensive programs. feholdexcept() effectively turns off this handling in non-critical sections to improve speed.
• Let iterative calculations continue: Certain algorithms may hit occasional exceptions but produce usable interim results. Wrapping iterations in feholdexcept() allows completion rather than early termination.
• Maintain surrounding exception behaviour: By only clearing flags locally, code external to the feholdexcept() protected section will keep behaving regularly and throw exceptions when warranted.
• Implement custom floating point exception handling: Save and restore the environment around sections of custom exception handling code rather than using C++ default mechanisms.
• Gradually enable exceptions: When migrating or testing floating point code, temporarily use feholdexcept(), and then "shrink" its scope to fix and expose issues incrementally.
• Reduce debugging complexity: Disable exceptions in some code during debugging to independently simplify the analysis of known numeric issue sections.

Implementation of the feholdexcept() function:

Let's take a C++ Program to illustrate the feholdexcept() function.

Output:

Result of division: inf
No exceptions raised during division.

Explanation:

1. Include the necessary headers:
   1. iostream: It is used for input/output stream functionality
   2. cfenv: It is used for floating-point environment functions like feholdexcept()
   3. cmath: It contains various math functions.
2. Define a safeDivision() function that takes two doubles as arguments (dividend and divisor) and returns their division result as a double.
3. Inside safeDivision(): Declare a fenv_t object called currentEnv to save the current floating-point environment. b. Call fegetenv() and pass the address of currentEnv. It saves the current floating-point state into currentEnv.
4. Perform the actual division operation between the passed arguments and store the result.
5. Declare another fenv_t object called newEnv. It will be used with feholdexcept().
6. Call feholdexcept(), passing the address of newEnv. It saves the current environment into newEnv and clears the flags.
7. Explicitly clear the overflow and divide-by-zero flags using feclearexcept(). It is done because the previous environment stored in newEnv may have had these flags set from earlier operations. We want a clean environment inside this Function.
8. Return the division result. At this point, any floating-point exceptions during the division would not generate exceptions due to the cleared environment.
9. In main() function:
10. Clear all floating point exception flags using feclearexcept().
11. Define the dividend and divisor values. The divisor is set to 0 to cause a divide-by-zero.
12. Call safeDivision(), which returns the result.
13. Print out the result.
14. Use fetestexcept() to check if any exceptions were raised during safeDivision(). f. Print out an appropriate message based on the previous step.

Program 2:

Let's take another C++ Program to illustrate the feholdexcept() function.

Output:

Exceptions raised: FE_DIVBYZERO
Exceptions raised: No exceptions found
Exceptions raised: FE_DIVBYZERO

Explanation:

1. Include necessary headers - iostream for I/O, cfenv for floating point environment functions
2. Define a displayExceptions() Function to print any currently raised floating-point exceptions using fetestexcept()
3. In the main() function:
4. Declare a fenv_t object called current environment to hold the environment
5. Explicitly raise a "divide by zero" exception using feraiseexcept() to simulate an error
6. Call displayExceptions() first time to print the raised exception
7. Declare another fenv_t object called newEnvironment
8. Call feholdexcept(), passing address of newEnvironment. This will:
   1. Save the current environment with flags into newEnvironment
   2. Clear the flags in the current environment
9. Call displayExceptions() This time, it will not show any raised exceptions due to step 8.
10. Restore the saved environment in newEnvironment back into the current environment using feupdateenv()
11. Call displayExceptions() third time, now showing "FE_DIVBYZERO" exception again since restored

In summary:

• Raised an exception.
• Used feholdexcept() to save and clean the environment
• After that, the code was executed with no raised exceptions.
• Finally, restored the original environment using a pointer, bringing back the exception, too.

It demonstrates how feholdexcept() can temporarily disable and resume exception handling when needed.

Conclusion:

The feholdexcept() function provides C++ developers fine-grained control over floating point exception handling when performing numerical computations. By clearing flags after saving the environment, exceptions can be locally disabled during sensitive or unstable calculations, allowing the code to continue instead of terminating abruptly. Later, the original state can be restored with fenv_setenv(), so the rest of the program usually functions. This approach isolates messy mathematical code and limits the scope of relaxed exceptions while maintaining strict behaviour in other sections. In essence, feholdexcept() and related functions empower developers to better manage unavoidable floating point inaccuracies and exceptions. They facilitate writing correct, fault-tolerant numerical code to handle occasional anomalies without disruption. By wisely leveraging such environmental control mechanisms, C++ programs can become more robust and production-ready for today's complex computational challenges.