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https://github.com/orangeduck/tgc

A Tiny Garbage Collector for C
https://github.com/orangeduck/tgc

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A Tiny Garbage Collector for C

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README 

        
Tiny Garbage Collector

======================



About

-----



`tgc` is a tiny garbage collector for C written in ~500 lines of code and based 

on the [Cello Garbage Collector](http://libcello.org/learn/garbage-collection).



```c

#include "tgc.h"



static tgc_t gc;



static void example_function() {

  char *message = tgc_alloc(&gc, 64);

  strcpy(message, "No More Memory Leaks!");

}



int main(int argc, char **argv) {

  tgc_start(&gc, &argc);

  

  example_function();



  tgc_stop(&gc);

}

```



Usage

-----



`tgc` is a conservative, thread local, mark and sweep garbage collector,

which supports destructors, and automatically frees memory allocated by 

`tgc_alloc` and friends after it becomes _unreachable_.



A memory allocation is considered _reachable_ by `tgc` if...



* a pointer points to it, located on the stack at least one function call 

deeper than the call to `tgc_start`, or,

* a pointer points to it, inside memory allocated by `tgc_alloc` 

and friends.



Otherwise a memory allocation is considered _unreachable_.



Therefore some things that _don't_ qualify an allocation as _reachable_ are, 

if...



* a pointer points to an address inside of it, but not at the start of it, or,

* a pointer points to it from inside the `static` data segment, or, 

* a pointer points to it from memory allocated by `malloc`, 

`calloc`, `realloc` or any other non-`tgc` allocation methods, or, 

* a pointer points to it from a different thread, or, 

* a pointer points to it from any other unreachable location.



Given these conditions, `tgc` will free memory allocations some time after 

they become _unreachable_. To do this it performs an iteration of _mark and 

sweep_ when `tgc_alloc` is called and the number of memory allocations exceeds 

some threshold. It can also be run manually with `tgc_run`.



Memory allocated by `tgc_alloc` can be manually freed with `tgc_free`, and 

destructors (functions to be run just before memory is freed), can be 

registered with `tgc_set_dtor`.



Reference

---------



```c

void tgc_start(tgc_t *gc, void *stk);

```



Start the garbage collector on the current thread, beginning at the stack 

location given by the `stk` variable. Usually this can be found using the 

address of any local variable, and then the garbage collector will cover all 

memory at least one function call deeper.



* * *



```c

void tgc_stop(tgc_t *gc);

```



Stop the garbage collector and free its internal memory.



* * *



```c

void tgc_run(tgc_t *gc);

```



Run an iteration of the garbage collector, freeing any unreachable memory.



* * *



```c

void tgc_pause(tgc_t *gc);

void tgc_resume(tgc_t *gc);

```



Pause or resume the garbage collector. While paused the garbage collector will

not run during any allocations made.



* * * 



```c

void *tgc_alloc(gc_t *gc, size_t size);

```



Allocate memory via the garbage collector to be automatically freed once it

becomes unreachable.



* * *



```c

void *tgc_calloc(gc_t *gc, size_t num, size_t size);

```



Allocate memory via the garbage collector and initalise it to zero.



* * *



```c

void *tgc_realloc(gc_t *gc, void *ptr, size_t size);

```



Reallocate memory allocated by the garbage collector.



* * *



```c

void tgc_free(gc_t *gc, void *ptr);

```



Manually free an allocation made by the garbage collector. Runs any destructor

if registered.



* * *



```c

void *tgc_alloc_opt(tgc_t *gc, size_t size, int flags, void(*dtor)(void*));

```



Allocate memory via the garbage collector with the given flags and destructor.



For the `flags` argument, the flag `TGC_ROOT` may be specified to indicate that 

the allocation is a garbage collection _root_ and so should not be 

automatically freed and instead will be manually freed by the user with 

`tgc_free`. Because roots are not automatically freed, they can exist in 

normally unreachable locations such as in the `static` data segment or in 

memory allocated by `malloc`. 



The flag `TGC_LEAF` may be specified to indicate that the allocation is a 

garbage collection _leaf_ and so contains no pointers to other allocations

inside. This can benefit performance in many cases. For example, when 

allocating a large string there is no point the garbage collector scanning 

this allocation - it can take a long time and doesn't contain any pointers.



Otherwise the `flags` argument can be set to zero.



The `dtor` argument lets the user specify a _destructor_ function to be run 

just before the memory is freed. Destructors have many uses, for example they 

are often used to automatically release system resources (such as file handles)

when a data structure is finished with them. For no destructor the value `NULL` 

can be used.



* * *



```c

void *tgc_calloc_opt(tgc_t *gc, size_t num, size_t size, int flags, void(*dtor)(void*));

```



Allocate memory via the garbage collector with the given flags and destructor 

and initalise to zero.



* * *



```c

void tgc_set_dtor(tgc_t *gc, void *ptr, void(*dtor)(void*));

```



Register a destructor function to be called after the memory allocation `ptr`

becomes unreachable, and just before it is freed by the garbage collector.



* * *



```c

void tgc_set_flags(tgc_t *gc, void *ptr, int flags);

```



Set the flags associated with a memory allocation, for example the value 

`TGC_ROOT` can be used to specify that an allocation is a garbage collection 

root.



* * *



```c

int tgc_get_flags(tgc_t *gc, void *ptr);

```



Get the flags associated with a memory allocation.



* * *



```c

void(*tgc_get_dtor(tgc_t *gc, void *ptr))(void*);

```



Get the destructor associated with a memory allocation.



* * *



```c

size_t tgc_get_size(tgc_t *gc, void *ptr);

```



Get the size of a memory allocation.



F.A.Q

-----



### Is this real/safe/portable?



Definitely! While there is no way to create a _completely_ safe/portable 

garbage collector in C this collector doesn't use any platform specific tricks 

and only makes the most basic assumptions about the platform, such as that the 

architecture using a continuous call stack to implement function frames.



It _should_ be safe to use for more or less all reasonable architectures found 

in the wild and has been tested on Linux, Windows, and OSX, where it was easily 

integrated into several large real world programs (see `examples`) such as 

`bzip2` and `oggenc` without issue.



Saying all of that, there are the normal warnings - this library performs 

_undefined behaviour_ as specified by the C standard and so you use it at your 

own risk - there is no guarantee that something like a compiler or OS update 

wont mysteriously break it.



### What happens when some data just happens to look like a pointer?



In this unlikely case `tgc` will treat the data as a pointer and assume that 

the memory allocation it points to is still reachable. If this is causing your

application trouble by not allowing a large memory allocation to be freed 

consider freeing it manually with `tgc_free`.



### `tgc` isn't working when I increment pointers!



Due to the way `tgc` works, it always needs a pointer to the start of each 

memory allocation to be reachable. This can break algorithms such as the 

following, which work by incrementing a pointer.



```c

void bad_function(char *y) {

  char *x = tgc_alloc(&gc, strlen(y) + 1);

  strcpy(x, y);

  while (*x) {

    do_some_processsing(x);

    x++;

  }

}

```



Here, when `x` is incremented, it no longer points to the start of the memory 

allocation made by `tgc_alloc`. Then during `do_some_processing`, if a sweep 

is performed, `x` will be declared as unreachable and the memory freed.



If the pointer `x` is also stored elsewhere such as inside a heap structure 

there is no issue with incrementing a copy of it - so most of the time you 

don't need to worry, but occasionally you may need to adjust algorithms which

do significant pointer arithmetic. For example, in this case the pointer can be 

left as-is and an integer used to index it instead:



```c

void good_function(char *y) {

  int i;

  char *x = tgc_alloc(&gc, strlen(y) + 1);

  strcpy(x, y);

  for (i = 0; i < strlen(x); i++) {

    do_some_processsing(&x[i]);

  }

}

```



For now this is the behaviour of `tgc` until I think of a way to 

deal with offset pointers nicely.



### `tgc` isn't working when optimisations are enabled!



Variables are only considered reachable if they are one function call shallower 

than the call to `tgc_start`. If optimisations are enabled sometimes the 

compiler will inline functions which removes this one level of indirection.



The most portable way to get compilers not to inline functions is to call them 

through `volatile` function pointers.



```c

static tgc_t gc;



void please_dont_inline(void) {

  ...

}



int main(int argc, char **argv) {

  

  tgc_start(&gc, &argc);



  void (*volatile func)(void) = please_dont_inline;

  func();

  

  tgc_stop(&gc);



  return 1;

}

```



### `tgc` isn't working with `setjmp` and `longjmp`!



Unfortunately `tgc` doesn't work properly with `setjmp` and `longjmp` since 

these functions can cause complex stack behaviour. One simple option is to 

disable the garbage collector while using these functions and to re-enable

it afterwards.



### Why do I get _uninitialised values_ warnings with Valgrind?



The garbage collector scans the stack memory and this naturally contains 

uninitialised values. It scans memory safely, but if you are running through 

Valgrind these accesses will be reported as warnings/errors. Other than this 

`tgc` shouldn't have any memory errors in Valgrind, so the easiest way to 

disable these to examine any real problems is to run Valgrind with the option 

`--undef-value-errors=no`.



### Is `tgc` fast?



At the moment `tgc` has decent performance - it is competative with many 

existing memory management systems - but definitely can't claim to be the 

fastest garbage collector on the market. Saying that, there is a fair amount of 

low hanging fruit for anyone interested in optimising it - so some potential to

be faster exists.



How it Works

------------



For a basic _mark and sweep_ garbage collector two things are required. The 

first thing is a list of all of the allocations made by the program. The second 

is a list of all the allocations _in use_ by the program at any given time. 

With these two things the algorithm is simple - compare the two lists and free 

any allocations which are in the first list, but not in the second - exactly 

those allocations which are no longer in use.



To get a list of all the allocations made by the progam is relatively 

simple. We make the programmer use a special function we've prepared (in this

case `tgc_alloc`) which allocates memory, and then adds a pointer to that 

memory to an internal list. If at any point this allocation is freed (such as 

by `tgc_free`), it is removed from the list.



The second list is the difficult one - the list of allocations _in use_ by the 

program. At first, with C's semantics, pointer arithematic, and all the crazy 

flexibility that comes with it, it might seem like finding all the allocations 

in use by the program at any point in time is impossible, and to some extent 

you'd be right. It can actually be shown that this problem reduces to the 

halting problem in the most general case - even for languages saner than C - 

but by slightly adjusting our problem statement, and assuming we are only 

dealing with a set of _well behaved_ C programs of some form, we can come up 

with something that works.



First we have to relax our goal a little. Instead of trying to find all of 

the memory allocations _in use_ by a program, we can instead try to find all 

the _reachable_ memory allocations - those allocations which have a pointer 

pointing to them somewhere in the program's memory. The distinction here is 

subtle but important. For example, I _could_ write a C program which makes an 

allocation, encodes the returned pointer as a string, and performs `rot13` on 

that string, later on decoding the string, casting it back to a pointer, 

and using the memory as if nothing had happened. This is a perfectly valid, C 

program, and the crazy memory allocation is _is use_ throughout. It is just 

that during the pointer's `rot13` encoding there is no practical way to know 

that this memory allocation is still going to be used later on.



So instead we want to make a list of all memory allocations which are pointed 

to by pointers in the program's memory. For most _well behaved_ C programs this

is enough to tell if an allocation is in use.



In general, memory in C exists in three different segments. We have the stack,

the heap, and the data segment. This means - if a pointer to a certain 

allocation exists in the program's memory it must be in one of these locations.

Now the challenge is to find these locations, and scan them for pointers.



The data segment is the most difficult - there is no portable way to get the 

bounds of this segment. But because the data segment is somewhat limited in use 

we can choose to ignore it - we tell users that allocations only pointed to 

from the data segment are not considered reachable.



As an aside, for programmers coming from other languages, this might seem like 

a poor solution - to simply ask the programmer not to store pointers to 

allocations in this segment - and in many ways it is. It is never a good 

interface to _request_ the programmer do something in the documentation - 

instead it is better to handle every edge case to make it impossible for them 

to create an error. But this is C - in C programmers are constantly asked _not_

to do things which are perfectly possible. In fact - one of the very things 

this library is trying to deal with is the fact that programmers are only 

_asked_ to make sure they free dynamically allocated memory - there is no 

system in place to enforce this. So _for C_ this is a perfectly reasonable 

interface. And there is an added advantage - it makes the implementation far 

more simple - far more adaptable. In other words - [Worse Is Better](https://en.wikipedia.org/wiki/Worse_is_better).



With the data segment covered we have the heap and the stack. If we consider

only the heap allocations which have been made via `tgc_alloc` and friends then

our job is again made easy - in our list of all allocations we also store the

size of each allocation. Then, if we need to scan one of the memory regions 

we've allocated, the task is made easy.



With the heap and the data segment covered, this leaves us with the stack - 

this is the most tricky segment. The stack is something we don't have any 

control over, but we do know that for most reasonable implementations of C, the

stack is a continuous area of memory that is expanded downwards (or for some

implementations upwards, but it doesn't matter) for each function call. It 

contains the most important memory in regards to reachability - all of the 

local variables used in functions.



If we can get the memory addresses of the top and the bottom of the stack we 

can scan the memory inbetween as if it were heap memory, and add to our list of 

reachable pointers all those found inbetween.



Assuming the stack grows from top to bottom we can get a conservative 

approximation of the bottom of the stack by just taking the address of some 

local variable.



```c

void *stack_bottom(void) {

  int x;

  return &x;

}

```



This address should cover the memory of all the local variables for whichever

function calls it. For this reason we need to ensure two things before we 

actually do call it. First we want to make sure we flush all of the values in 

the registers onto the stack so that we don't miss a pointer hiding in a 

register, and secondly we want to make sure the call to `stack_bottom` isn't 

inlined by the compiler.



We can spill the registers into stack memory in a somewhat portable way with 

`setjmp` - which puts the registers into a `jmp_buf` variable. And we can 

ensure that the function is not inlined by only calling it via a volatile 

function pointer. The `volatile` keyword forces the compiler to always manually

read the pointer value from memory before calling the function, ensuring it

cannot be inlined.



```c

void *get_stack_bottom(void) {

  jmt_buf env;

  setjmp(env);

  void *(*volatile f)(void) = stack_bottom;

  return f();

}

```



To get the top of the stack we can again get the address of a local variable.

This time it is easier if we simply ask the programmer to supply us with one.

If the programmer wishes for the garbage collector to scan the whole stack he 

can give the address of a local variable in `main`. This address should cover 

all function calls one deeper than `main`. This we can store in some global 

(or local) variable.



```c

static void *stack_top = NULL;



int main(int argc, char **argv) {

  stack_top = &argc;

  run_program(argc, argv);

  return 1;

}

```



Now, at any point we can get a safe approximate upper and lower bound of the 

stack memory, allowing us to scan it for pointers. We interprit each bound as a

`void **` - a pointer to an array of pointers, and iterate, interpriting the

memory inbetween as pointers.



```c

void mark(void) {

  void **p;

  void **t = stack_top;

  void **b = get_stack_bottom();

  

  for (p = t; p < b; p++) {

    scan(*p);

  }

}

```