How Does Stack Unwinding Ensure Resource Cleanup in C Exception Handling?

Stack Unwinding

Stack unwinding is an essential mechanism in exception handling, especially in C . It allows the controlled termination of active functions and the cleanup of resources.

Consider the following example:

void func(int x)
{
    char* pleak = new char[1024]; // might leak if exception is thrown
    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
}

int main()
{
    try
    {
        func(10);
    }
    catch (const std::exception& e)
    {
        return 1;
    }

    return 0;
}

In this scenario, the memory allocated for pleak will be lost if an exception is thrown. However, s will be properly cleaned up by its destructor, as it's an automatic (stack) variable.

When an exception is thrown, the stack is unwound by the compiler. This involves:

• Calling destructors of all automatic variables in the current function
• Returning control to the calling function and continuing stack unwinding

This process ensures that resources allocated on the stack are properly released, even if an exception is thrown.

The concept of stack unwinding is central to the RAII (Resource Acquisition Is Initialization) idiom, which provides a consistent approach to resource management in C . By acquiring resources in the constructor and releasing them in the destructor, developers can establish exception safety guarantees and handle resources consistently.

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