How Does Stack Unwinding Ensure Exception Safety and Resource Management in C ?

Stack Unwinding: A Comprehensive Guide

Stack unwinding is an integral part of exception handling in programming. It refers to the systematic process of cleaning up the stack when an exception occurs.

Imagine a function called func that does the following:

void func(int x) {
    char* pleak = new char[1024]; // might be lost -> memory leak
    std::string s("hello world"); // will be properly destructed

    if (x) throw std::runtime_error("boom");

    delete [] pleak; // will only get here if x == 0. if x!=0, throw exception
}

Within the func function, the pointer pleak is allocated dynamically, and a std::string object s is created on the stack. Assuming x is non-zero, an exception is thrown. In this scenario, the memory allocated to pleak will be lost, leading to a memory leak.

Stack unwinding solves this issue. When an exception is thrown, the program must unwind the stack to clean up any objects allocated on it. In the above example, the destructor of std::string will be called correctly, ensuring the resources associated with s are released.

This is where the RAII (Resource Acquisition Is Initialization) concept comes into play. RAII emphasizes allocating resources during initialization and releasing them in destructors. This ensures that resources are properly managed even if an exception occurs.

Stack unwinding and RAII work together to provide exception safety, guaranteeing that resources are cleaned up consistently, regardless of the reason for termination.

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