Spooky Garbage Collection

Stephen Brennan • 02 November 2015

Since Halloween was a few days ago, I wanted to write about a spooky topic: implementing garbage collection! I say spooky because garbage collection is one of those ubiquitous things that programmers use, but some may not understand (like shells). I’m a fan of demystifying these sorts of things, so I thought I’d share some of the experience I gained implementing it.

Intro to Garbage Collection

A quick refresher on garbage collection: Let’s imagine that you’re implementing a programming language interpreter in C. Say, your own implementation of a Lisp. When code executes, it will use memory resources. It will have strings, numbers, functions, and more, and all of that will need to reside somewhere in memory. More importantly, when the program is done with those resources, it will want to use the space they took up for other things. In programming languages like C, the programmer has to explicitly request and release memory. But in programming languages like Lisp (and Python, and Java, etc.), the programmer doesn’t have to think about it. Unfortunately, that means that you, the creator of this interpreter, will have to do the thinking for the programmers. You will have to come up with a system that will detect when program resources are no longer being used, and free them up. This is called garbage collection.

There are two main techniques people use to implement garbage collection. The first one is called reference counting, and the second one is called mark-and-sweep. I’m not going to talk about mark-and-sweep in this article, but the idea is pretty simple. At any point when you’re running a program, you know what variables are in scope. You can simply look at all the resources the program has, find all of the ones that are reachable from the variables in scope, and free up the rest. This approach is guaranteed to eliminate everything that is unneeded, but it requires you to “pause” the program while you separate out the reachable resources from the unreachable ones. Mark-and-sweep is used by Java.

The technique I’m going to demonstrate is reference counting. Here’s the idea behind it. Every memory resource that a program uses will be given a “reference count.” The reference count represents the number of different places where that resource is being used (or “referenced”). Whenever some code is done using a memory resource, it subtracts one from its reference count. When the reference count reaches zero, this means nothing is using the memory any more, so it can be freed. Reference counting has the advantage that unused memory can be freed as it becomes unused, and so you don’t need to “pause” the program to find unused memory. Unfortunately, there is at least one downside to reference counting, which I will explain at the end of this post.

Implementation of Reference Counting

Now that we have a pretty good understanding of reference counting, let’s dive into some C code that would allow our hypothetical Lisp interpreter to use reference counting garbage collection. First, let’s create a struct that will be included at the top of every data type in the programming language (think of it like a base class):

struct lisp_value {
  struct lisp_type *type;
  unsigned int refcount;
};

Both items are necessary for our garbage collection system! The refcount variable does exactly what you’d expect – it holds the number of things that own references to this object. As you might expect, this count always starts at one. It’s unsigned because it’ll never be negative – as soon as it becomes zero, the object will be freed.

But how do you free an object you don’t know anything about? At first, you might think you could just call free() on the pointer and be done with it. That works for something like an integer, but what if it’s a linked list? In that case, you’d have to also decrement the reference count of the object stored in the linked list node, and then do the same for each subsequent linked list node! Now, it seems clear that we can’t write a free() routine that would work on every possible data type in our Lisp. That’s where the type pointer in the struct lisp_value comes in. Here is what we define a struct lisp_type to be:

struct lisp_type {
  const char *tp_name;
  lisp_value* (*tp_alloc)(void);
  void (*tp_dealloc)(lisp_value*);
  void (*tp_print)(lisp_value*, FILE *, int);
};

This struct is really handy. It contains, among other things, a pointer (tp_dealloc) to a function that knows how to free objects of some type. We can create one (static) instance of this struct for each type we define. We put the implementation of the type’s tp_dealloc() routine in the struct, and then put a pointer to this struct into each instance of that type. Now, for any lisp_value, we know how to free it: just look for the tp_dealloc() function in its type object!

In case this was a bit confusing, let’s get concrete. Let’s look at an integer type:

struct lisp_int {
  lisp_value lv;
  long int value;
};

The first element of the struct includes our lisp_value “base class” (i.e., the refcount and pointer to type object). The second element is the integer value. Now, here is what the definition of the type object looks like:

struct lisp_type tp_int = {
  .tp_name = "int",
  // one function to allocate them:
  .tp_alloc = &lisp_int_alloc,
  // one function to free them:
  .tp_dealloc = &generic_dealloc,
  // one function to print them:
  .tp_print = &lisp_int_print
  // ... and in the darkness, bind them
};

Whenever we create a new instance of an integer (say, in lisp_int_alloc()), we simply set the refcount and type fields correctly:

static struct lisp_value *lisp_int_alloc(void)
{
  struct lisp_int *rv = malloc(sizeof(struct lisp_int));
  rv->lv.type = &tp_int; // ptr to type object!
  rv->lv.refcount = 1; // the caller owns a reference
  rv->value = 0;
  return (struct lisp_value *)rv;
}

So long as we do this for every type we define in our language, we know that every object’s lifetime can be managed through the reference counting system. All you have to do from here is use functions like these to increment and decrement reference counts:

void lisp_incref(struct lisp_value *lv)
{
  if (lv == NULL) return;
  lv->refcount += 1;
}

void lisp_decref(struct lisp_value *lv)
{
  if (lv == NULL) return;
  lv->refcount -= 1;
  if (lv->refcount == 0) {
    // use the type object's function pointer to free it:
    lv->type->tp_dealloc(lv);
  }
}

As a side note, the strategy of creating “type” objects with pointers to functions is a solid way to start implementing a lot of high level programming language features. This is one of the few good ways of achieving “dynamic dispatch” (not knowing what function you’re calling until runtime) for objects in C. Dynamic dispatch is one of the important pieces of an object oriented programming language. So if you ever wanted to implement an object oriented programming language with C, chances are you should start with type objects similar to these!

Reference Counting Semantics

You may think that we’re done now. After all, we have objects we know we can always free, and we have functions to update reference counts and free objects when we no longer need them. But one major question remains to be answered: “when do we increment and decrement reference counts?” This is pretty critical; if you mess up your reference counting, one of two things will happen:

Sadly, there’s no real “automatic” way to get it right. The best way to avoid these errors is by creating a standard set of rules for dealing with reference counts, and then following them rigidly. Here are rules that I use:

These rules are actually remarkably similar to the rules used by Python’s standard implementation, CPython. In fact, my entire implementation of reference counting is based on CPython’s implementation! So if you really want to learn more, you should check out the CPython source. I learned a ton by working on a C extension to the interpreter, and I highly recommend that as a learning experience!

Downside of Reference Counting

Unfortunately, there is one major downside to reference counting. The downside is that there are some ways that you can “fool” simple reference counting into not freeing memory that is unused. Here is an example of Python code that would “fool” a simple reference counter:

def some_function():
    person_a = Person('Stephen')
    # person_a now owns a reference to Stephen (count=1)
    person_b = Person('Tyler')
    # person_b now owns a reference to Tyler (count=1)
    person_a.friend = person_b
    # now, Stephen owns a reference to Tyler (count=2)
    person_b.friend = person_a
    # similarly, Tyler now owns a reference to Stephen (count=2)
    return
    # person_a and person_b go out of scope, so Stephen and Tyler now have count=1

As you can see, at the end of this function, “Stephen” and “Tyler” are unused, and should be gotten rid of. But since they own references to each other, their reference counts never reach zero, and they never get freed. There are ways of fixing this problem (for instance, CPython has a fix for this issue), but for this article, I’ll leave it to your (and my) imagination how to do it.

Conclusion

You know how this whole time I was writing code for a “hypothetical” Lisp interpreter? Turns out, it’s not really hypothetical. All this code is taken from my lisp project on GitHub. You should really check that out, because then you can see this simple reference counting code in action!

Easter Egg: There is currently at least one memory leak in my Lisp implementation (as of commit c99f4b5345ec651c9d6cd51358e2d9595c71c356). See if you can find it! My only hints:

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