What the Heck is a Stack Overflow

This is what happens when recursion goes wrong

Photo by Robert Anasch on Unsplash

I’m not talking about the stackoverflow you copy code from to do your job. I’m talking about what happens when I write bad recursion.

So what is a stack overflow?

According to wikipedia,

In software, a stack overflow occurs if the call stack pointer exceeds the stack bound. The call stack may consist of a limited amount of address space, often determined at the start of the program.

Translation: There is a limited amount of memory (the call stack) allocated to a program. When that is exceeded (when it overflows), we call it a stack overflow.

Every time I’ve seen it, it’s been the result of infinite recursion. Recursion is when a method/function calls itself.

Sometimes recursion is intentional (ie: iterating over a graph). Sometimes it’s unintentional (a callback triggers itself).

Now let’s cause intentionally cause stack overflows in a Python, Ruby and Javascript.

Python

In your favorite code editor, switch to python and run the following.

def do_recursion():
  do_recursion()
do_recursion()

This throws an error.

RecursionError: maximum recursion depth exceeded

Python throws a RecursionError when the stack gets out of bounds.

Ruby

Switch your editor to ruby and run this.

def do_recursion
  do_recursion
end
do_recursion

Which results in an error.

nodepad.rb:3:in `do_recursion': stack level too deep (SystemStackError)

Ruby throws a SystemStackError when the stack level get’s too deep.

Javascript

You can run this one in the JS console in chrome.

var do_recursion = function() {
  do_recursion()
}
do_recursion()

This throws an error.

Uncaught RangeError: Maximum call stack size exceeded

Javascript throws a RangeError when the stack is exceeded.

Conclusion

And that’s the real stack overflow.

In a nutshell, allocated memory is exceeded.

You won’t see it often unless you write a lot of recursive code. But it’s a good understanding to keep in your toolkit.