Programming Fundamentals › Programming Basics

Stack Overflow

Crashing when the call stack runs out of space, usually from runaway recursion.

Also known as: stack overflow error, call stack overflow, RecursionError, maximum call stack size exceeded

A stack overflow happens when the call stack runs out of space. Each function call adds a frame to the stack (its arguments and local variables), and frames are removed when the function returns. Too many nested calls, and the stack fills up.

def forever(n):
    return forever(n + 1)      # no base case

forever(0)
# RecursionError: maximum recursion depth exceeded

// RangeError: Maximum call stack size exceeded

In languages like C, Java and Go, you may see a crash or a StackOverflowError.

Usual causes

• Recursion with no base case, or one that’s never reached (base case).
• Recursion that is correct but too deep: a recursive walk over a list of 100,000 items.
• Accidental mutual recursion: a() calls b() which calls a(). Common with getters or toString calling each other, or an event handler that triggers itself.
• Very large local variables in each frame.

Fixing it

1. Find the loop. The stack trace repeats the same few functions many times.
2. Check the base case and that each call gets closer to it.
3. Switch to a loop with your own stack or queue for deep structures.
4. Make sure that the language can optimize tail calls before relying on it. Most mainstream ones don’t.
5. As a last resort, raise the limit. That only postpones the problem.

Don’t confuse it with the Q&A site of the same name. This is the error.