Program memory: the stack and the heap

A running program's memory is split into regions: the code itself, global data, the stack and the heap. The stack holds one frame for each function call. A frame keeps that call's local variables and the address to go back to when it ends.

The stack grows when a function is called and shrinks when it returns, all automatically. The heap holds data whose size or lifetime is only known while the program runs, such as a list that grows as users sign up. In C you manage heap memory yourself.

In Java, Python or JavaScript a garbage collector frees it for you. This explains three common bugs. Very deep recursion runs out of stack space, a stack overflow.

Memory you never release piles up, a memory leak. And returning a pointer to a local variable is a bug, because that frame is gone once the function returns. The operating systems stage builds on this when it maps these regions into virtual memory.