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A C-like language with explicit free which is nonetheless safe
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The Eightebed Programming Language

Language version 1.1


While discussing Cyclone, Gregor Richards stated that in order for a language to support explicit malloc()ing and free()ing of allocated memory, while also being safe (in the sense of not being able to execute or dereference incorrectly-populated memory) would require that language to either support garbage collection, or to not implement free(). In his words:

A C-like language which provides a true explicit free() cannot be safe. (By "true" I mean that you can get that memory back in a later malloc().) To be safe a language must either never free (which is bad) or be GC'd. [C-like languages being] imperative languages with pointers at arbitrary data, where safety is defined as not seeing that data as a different type.

Eightebed was designed as a counterexample to that claim. Eightebed is a small, C-like language with explicit malloc() and free(). Memory is actually freed by free() and might be re-allocated by a future malloc(). Yet Eightebed is a safe language, requiring only a modicum of static analysis and runtime support, and in particular, it neither specifies nor requires garbage collection:

  • Garbage, reasonably defined as "any unreachable block of memory", is disregarded and considered a memory leak, as is good and proper (or at least accepted) in a language with explicit memory management; and
  • Nothing is collected in any way.

Without Loss of Generality

We place some restrictions on Eightebed in order that our implementation of a compiler and analyzer for it may be simplified. These restrictions do not, we assert, prevent the language from being "C-like", as it would be possible to extend the language to include them; the only thing we would be adding if we were to do so would be additional complexity in implementation. These restrictions are:

  • There are no functions in Eightebed. Common functionality can be repeated verbatim inline, and recursion can be replaced with while loops.
  • Pointers may only point to named types, not integers or other pointers, and only structures may be named. The effect of a pointer to an integer or pointer may be easily achieved by pointing to a named structure which consists of only an integer or pointer itself.
  • Structures may not contain structures. Again, this can be easily simulated by "flattening" the structure into a single structure with perhaps differentiated names.


EBNF Grammar

Note that where this grammar is a little weird, it is only to support being fully LL(1) to ease parser construction. Notably, the syntax to access a member of a structure uses both square brackets around the structure and a dot between structure and member. Unlike C, there is no syntax like -> to dereference and access a member in one go; you need to dereference with @, then access the member with []..

Eightebed ::= {TypeDecl} {VarDecl} Block.
Block     ::= "{" {Stmt} "}".
TypeDecl  ::= "type" NameType Type ";"
Type      ::= "int"
            | "struct" "{" {Decl} "}"
            | "ptr" "to" Type
            | NameType.
Decl      ::= Type Name ";".
VarDecl   ::= "var" Decl.
Stmt      ::= "while" Expr Block
            | "if" Expr Block ["else" Block]
            | "free" Ref ";"
            | "print" Expr ";"
            | Ref "=" Expr ";".
Ref       ::= "[" Ref "]" "." Name
            | "@" Ref
            | Name.
Expr      ::= "(" Expr ("+"|"-"|"*"|"/"|"="|">"|"&"|"|") Expr ")"
            | "malloc" NameType
            | "valid" Expr
            | IntLit
            | Ref.

Example Program

type node struct {
    int value;
    ptr to node next;
var ptr to node jim;
var ptr to node george;
    jim = malloc node;
    if valid jim {
        [@jim].value = (1 + 4);
        george = jim;
    if valid george {
        print [@george].value;
    free george;
    free jim;

How it Works

Static Analysis

Dereferencing a pointer x must only occur at the safe start of the "then" part of an if statement whose test condition consists only of the expression valid x. The safe start of a block is the set of statements preceding and including the first assignment statement or free. (This is on the [admittedly somewhat pessimistic] assumption that any assignment could invalidate x.) (New in 1.1: the safe start must precede the first free statement, to prevent creation of dangling aliased pointers. Thanks Gregor!) To simplify implementation, we limit x to a simple variable name rather than a full expression. (This too is without loss of generality, as it is a simple matter to use a temporary variable to store the result of a pointer expression.) Any attempt to dereference a pointer which does not follow these rules is caught by the static checker and disallowed.

Runtime Support

Every pointer in the Eightebed language is implemented internally as a structure of a machine pointer (obtained, for instance, by C's malloc()) coupled with a boolean flag called valid. When a chunk of memory is initially successfully allocated, valid is set to true. Freeing a pointer first checks this flag; freeing the machine pointer is only attempted if valid is true. In addition, just before freeing the machine pointer, we invalidate all aliases to that pointer. (Starting with the "root set" of the program's global variables, we traverse all memory blocks reachable by following valid pointers from them, looking for pointers which match the pointer about to be freed; any we find, we set their valid flags to false.) After freeing a pointer, we set its valid to false.

Why this Works

Because of the static analysis, it is not possible to dereference a pointer at a point in the program where we do not know for certain that it is valid (i.e., it is not possible to dereference an invalid pointer.) Because of the runtime support, as soon as a pointer becomes invalid, all aliases of it become invalid as well. (All reachable aliases, that is – but if an alias isn't reachable, it can't be dereferenced anyway.) Add both of these together, and you get memory that can leak without any risk of being reused.

And no, this isn't garbage collection, because (as stated already) we don't care about garbage and we don't collect anything. Yes, the runtime support looks a bit like the mark phase of a mark-and-sweep garbage collector, but even it has a different job: not marking everything that is reachable, rather invalidating all aliases of a given pointer.

And finally, yes, I realize how little this proves. Long live loopholes.

16:19:38 <Gregor> We implement this without a GC by stuffing most of a
                  GC into the free function, thereby making it just as
                  slow as a GC'd language with none of the advantages!
16:25:29 <Gregor> So yes, although you have managed to fit my
                  requirements, I am wildly underwhelmed :P

Reference Implementation

Cat's Eye Technologies provides a cockamamie reference implementation of Eightebed called Written in Python 2.6, it compiles Eightebed code to C, and for convenience will optionally compile that C with the C compiler of your choice and run the resulting executable. ships with a fairly extensive (for a language like this!) suite of test programs, which can of course double as example sources; these can be found in the eightebed.tests module. also ships with a parser combinator module called which, being a single file and in the public domain, can be dropped into and used in any Python project that requires an LL(1) recursive-descent parser, if that's your sort of thing.

For an appreciation of just how cockamamie is, run --help and read through the command-line options it provides.

Legal Issues

The name Eightebed started life as a typo for the word "enlightened" made on an iPhone by a mysterious individual known only as Alise. (Well, perhaps not only.) Alise has aggressively asserted her intellectual property rights by copyrighting [sic] the name Eightebed. Cat's Eye Technologies has pursued permission to use the name for this language, only to be told that the procedure for obtaining such permission "involves five yaks, a Golden toad that hasn't eaten for five days, five boxes of antique confetti (not stripped of uranium), dye number 90 (blood green), a very confused weasel, and three pieces of A4.15 paper."

Cat's Eye Technologies' legal-and-yak-husbandry team is currently investigating the feasibility of this arrangement, and as of this writing, official permission is still pending. If complications persist, another, less contentious name (such as "Microsoft Windows 7") may need to be chosen for this language.

17:52:08 <alise> cpressey: I request that all harm is done to animals
                 in the making of this production.

Future Work

In which we reveal the outline of a grand plan for a blockbuster sequel to Eightebed which will never materialize

  • To be titled Eightebed: Ascension or Eightebed: Generations. At least, title should have one of those bad-ass colons in it. Possibly Eightebed: Eightebed.
  • To support functions, analysis of arbitrary expressions as the condition in an if valid, pointers to unnamed types, structures which contain other structures, and all that other boring stuff that we just said doesn't matter.
  • To have a literate specification written in SUPER ITALIAN, thus giving all programs the power of UNMATCHED PROPHETIC SNEEZING.
  • To be co-authored with Frank Zappa (note: turns out Mr. Zappa is dead. Maybe Tipper Gore instead? Yes, that should work.)
  • To include a garbage collector.
  • Puppets???

Happy leaking!
Chris Pressey
September 1, 2010
Evanston, IL

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