What is Safety in Programming Languages

In the context of programming languages, safety refers to design features of the language that prevent or reduce the likelihood of undefined behavior during execution of a program due to programming errors. It means that a "safe" language enforces rules that help developers avoid common mistakes, making programs more robust, predictable, and secure. The programming language which guarantees some level of safety allows you to focus on implementing your core logic rather than spending time on testing and refactoring to make your code safe. The safe languages ensures that programming errors (not logic errors) are trapped or avoided and the software crashes or throws an exception when it occurs.

Depending upon the various types of programming errors, following are the main types of safety ensured by programming languages:

1. Type Safety

Type Safety a.k.a Type Soundness is the property of a programming language to prevent the type errors. The type errors are undefined behaviors during runtime due to incorrect operations on datatypes e.g. adding an integer to a string without explicit conversion or casting. The type unsafe programming languages will not be able to handle such kind of errors and may produce undefined scenarios during program execution. In type-unsafe languages like C, pointers can be used to bypass type checks, potentially causing runtime errors. In type-safe languages like Java or Python, you can't assign an integer to a variable expected to hold a string without explicit conversion. To understand the concept, here’s a simple example in Python:

Consider the following piece of code, the function add_numbers is defined to take two integers (a and b) and return an integer. 

def add_numbers (a: int, b: int) -> int:

    return a + b

Passing arguments of the correct type (e.g., integer) ensures the program runs safely and as expected. However, if you pass a string instead of an integer (e.g., "10" instead of 10), Python will raise a runtime error since the operation 5 + "10" is invalid. This is an example of Type Safety, where the type error is handled gracefully at runtime. For improved type safety, some static type checker tools may be used to identify and catch such kind of type errors before runtime and ensure type safety throughout the code.

2. Memory Safety

Memory Safety is the property of a programming language to prevent or minimize memory errors. It means that the programs written in that language restrict reading / writing unauthorized memory locations. In a memory-safe language, when you access a variable or an item in an array, you can be sure that you are indeed accessing what you meant to or are allowed to access. In other words, you will not be reading from or writing into the memory of another variable or pointer by mistake, regardless of what you do in your program. Languages like Rust are designed with strict memory safety rules. In C or C++, memory safety can be violated if developers don't manage memory carefully. Some types of common errors related to memory read / write include:

• Buffer Overflows: Accessing memory outside the allocated range.

• Use After Free: Accessing memory that has already been deallocated.

• Dangling Pointers: Referencing invalid memory locations.

A simple example demonstrating memory safety can be shown in Rust, a programming language designed with memory safety guarantees. Here’s a basic example:

fn main() {

    let numbers = vec![1, 2, 3, 4, 5];

    // Accessing elements safely using Rust's bounds checking

    for i in 0..numbers.len() {

        println!("Element {}: {}", i, numbers[i]);

    }

}

In the example code above, Rust's Vec type manages memory automatically, ensuring proper allocation and deallocation. Moreover, if you attempt to access an element outside the valid range, Rust will panic at runtime rather than allowing undefined behavior. Rust also ensures memory safety through its ownership model, ensuring no dangling pointers or use-after-free errors.

3. Exception Safety

Exception safety refers to the features of the programming language to handle program crashes or any unpredictable behavior when the exceptions occur. An exception is an error or unexpected event which disrupts normal program execution e.g. Null Pointer Exception or Array Index Out of Bounds Exception. Ensuring exception safety means handling these situations gracefully without causing resource leaks, corruption of data, or unpredictable behavior. Exception safety involves well-defined mechanisms for exception handling and cleanup guarantees (e.g., finally blocks in Java). There are generally four levels of exception safety guarantees:

• No Guarantee: The program may enter an undefined or corrupted state when an exception occurs. This level is avoided in robust software design.

• Basic Guarantee: The program remains in a valid, consistent state after an exception, but some operations may not be completed as expected. Resources such as memory or file handles are not leaked.

• Strong Guarantee: Operations are either completed successfully or have no side effects, ensuring a rollback to the original state if an exception occurs. This is akin to an "all-or-nothing" behavior.

• No-Throw Guarantee: Operations are guaranteed not to throw any exceptions, ensuring complete stability. This is often used in destructors or critical system components.

Achieving exception safety involves techniques like resource acquisition is initialization (RAII), careful use of try-catch blocks, and designing functions to minimize side effects during failure scenarios. Exception safety is crucial for building reliable, maintainable, and fault-tolerant software systems. Languages like Kotlin and Swift incorporate null safety into their type systems to avoid null pointer exceptions.

Here’s a simple example of exception safety in Java. It demonstrates how to use a try-catch-finally block to ensure resources are properly handled, even if an exception occurs:

import java.io.BufferedReader;

import java.io.FileReader;

import java.io.IOException;

public class ExceptionSafetyExample {

    public static void main(String[] args) {

        BufferedReader reader = null;

        try {

            // Attempt to open a file and read its content

            reader = new BufferedReader(new FileReader("example.txt"));

            String line;

            while ((line = reader.readLine()) != null) {

                System.out.println(line);

            }

        } catch (IOException e) {

            // Handle any I/O exceptions that occur

            System.err.println("An error occurred while reading the file: " + e.getMessage());

        } finally {

            // Ensure the resource is closed, even if an exception was thrown

            if (reader != null) {

                try {

                    reader.close();

                } catch (IOException e) {

                    System.err.println("Failed to close the file reader: " + e.getMessage());

                }

            }

        }

    }

}

In the example Java code above, Try Block contains the code that might throw an exception, Catch Block handles the exception, ensuring the program can continue or fail gracefully, Finally Block ensures cleanup (like closing the file) which happens regardless of whether an exception was thrown.

4. Concurrency Safety

Concurrency or thread safety in a programming language ensures that concurrent operations do not lead to race conditions, deadlocks, or other synchronization problems. For example, a thread-safe function or class works properly when accessed by multiple threads regardless of the scheduling or interleaving of program execution. Languages like Go and Rust provide tools and abstractions to handle concurrency safely.

As an example, in the following piece of Java code, the keyword synchronized makes the method thread-safe:

class Counter {

    private int i = 0;

    public synchronized void inc() {

        i++;

    }

}

Safety in a programming language ensures that programs can handle runtime errors gracefully without leaving the system in an inconsistent state. Safety may involve trade-offs: Unsafe languages, like C or assembly, offer more control but require the programmer to handle safety concerns manually, increasing the risk of bugs and vulnerabilities. By incorporating these features, safe programming languages aim to make programs more reliable and maintainable, especially in critical systems like avionics, healthcare, and financial software.