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Cling: Running C++ in an interpreter (coldflake.com)
113 points by coldgrnd on Aug 12, 2012 | hide | past | web | favorite | 48 comments

To provide more precedents and a little history:

The first C "interpreters" I know of were for Lisp machines: Symbolics' C compiler (http://www.bitsavers.org/pdf/symbolics/software/genera_8/Use...) and Scott Burson's (hn user ScottBurson) ZetaC for TI Explorers/LMIs and Symbolics 3600s (now available under the public domain: http://www.bitsavers.org/bits/TI/Explorer/zeta-c/). Neither of them are interpreters, just "interactive" compilers like Lisp ones are.

I am writing a C to Common Lisp translator right now (https://github.com/vsedach/Vacietis). This is surprisingly easy because C is largely a small subset of Common Lisp. Pointers are trivial to implement with closures (Oleg explains how: http://okmij.org/ftp/Scheme/pointer-as-closure.txt but I discovered the technique independently around 2004). The only problem is how to deal with casting arrays of integers (or whatever) to arrays of bytes. But that's a problem for portable C software anyway. I think I'll also need a little source fudging magic for setjmp/longjmp. Otherwise the project is now where you can compile-file/load a C file just like you do a Lisp file by setting the readtable. There's a few things I need to finish with #includes, enums, stdlib and the variable-length struct hack, but that should be done in the next few weeks.

This should also extend to "compiling" C to other languages like JavaScript, without having to go through the whole "emulate LLVM or MIPS" garbage that other projects like that do. I think I figured out how to do gotos in JavaScript by using a trampoline with local CPS-rewriting, which is IMO the largest challenge for an interoperable C->JS translator.

As to how to do this for C++, don't ask me. According to the CERN people, CINT has "slightly less than 400,000 lines of code." (http://root.cern.ch/drupal/content/cint). What a joke.

That Oleg link has me nerd-snipped.

What I can't wrap my head around is how one would implement pointer-arithmetic with these closures? C pointers are not just references to cells, but those cells are guaranteed to be contiguous (up to a certain limit, be it an VM allocation unit, say "page", or all available system unit in VM-less systems)

That is to say, C pointers are not like ML references. Along with SET and REF they also allow addition, subtraction, scaling, etc.

For closures to model C pointers, wouldn't they need to order the allocation of cells in some manner? say, big array? And if so, this could get expensive very quickly (worst-case being "modeling" of entire memory, i.e. emulation) without certifying compiler or at least some exhaustive pointer analysis.

Hope I'm wrong on this.

> What I can't wrap my head around is how one would implement pointer-arithmetic with these closures?

  (defstruct memptr
    (ptr 0))

  (defun allocate-memory (size) ;; shared by malloc and static allocation
    (make-memptr :mem (make-array size :adjustable t :initial-element 0)))

  (defstruct place-ptr

  (defmacro vacietis.c:mkptr& (place) ;; need to deal w/function pointers
    (let ((new-value (gensym)))
      `(make-place-ptr :closure (lambda (&optional ,new-value)
                                  (if ,new-value
                                      (setf ,place ,new-value)

  (defun vacietis.c:deref* (ptr)
    (etypecase ptr
      (memptr (aref (memptr-mem ptr) (memptr-ptr ptr)))
      (place-ptr (funcall (place-ptr-closure ptr)))))

  (defun (setf vacietis.c:deref*) (new-value ptr)
    (etypecase ptr
      (memptr (setf (aref (memptr-mem ptr) (memptr-ptr ptr)) new-value))
      (plate-ptr (funcall (place-ptr-closure ptr) new-value))))

  (defmethod vacietis.c:+ ((x number) (y number))
    (+ x y))

  (defmethod vacietis.c:+ ((ptr memptr) (x integer))
    (make-memptr :mem (memptr-mem ptr) :ptr (+ x (memptr-ptr ptr))))

  (defmethod vacietis.c:- ((ptr1 memptr) (ptr2 memptr))
    (assert (eq (memptr-mem ptr1) (memptr-mem ptr2)) ()
            "Trying to subtract pointers from two different memory segments")
    (make-memptr :mem (memptr-mem ptr1) :ptr (- (memptr-ptr ptr1) (memptr-ptr ptr2))))
As you can see I don't do anything with type declarations, so arithmetic performance is going to be terrible. Multiple levels of pointer indirection work correctly with pointer arithmetic, since all pointers which can be legally added/subtracted are first-class objects.

The C standard basically only guarantees that pointer arithmetic works when the pointers involved all point to the same array object (it also allows for a pointer just off the end of an array object). Other pointer arithmetic or comparison is undefined behavior and an implementation can do whatever it pleases.

So I implemented this stricter definition of C pointers and it's neither interesting, nor representative of Real World uses that I know of. Need to investigate a bit more.

stricter definition of C pointers - this made me laugh, as the less-strict definition of C pointers is called undefined behavior.

This leads me to this statement based on what you said: interesting Real World C programs make use of undefined behavior

> interesting Real World C programs make use of undefined behavior

I assume you've worked with a significant amount of real world C programs? Because they surprisingly often do. The difficulty with porting many programs to 64-bit for instance, is due to relying on implementation defined behavior.

I assume you've worked with a significant amount of real world C programs?

Only embedded systems (AVR & PIC24). I have much more experience in C++, which I've used for both Desktop apps and telco server components.

The beauty of C (over C++) is that the standard is actually readable. C++ especially is a quagmire of undefined behavior. The scary thing about C/C++ is that its easy to hit undefined (or, at least, as you state, implementation defined) behavior and not even realize. Often the code looks valid, does what it looks like it does, yet is actually undefined or implementation defined and will break elsewhere.

With that said, while I don't expect everyone to have memorized the standard, I do hope most would have at least enough familiarity to avoid most cases of undefined behavior.

Most of what I learned about C++ I got from books, blogs and some great C++ guys (mostly from the boost community). I have to admit I did not read the standard at all.

could you give an example of a case where you hit undefined behavior since I hardly seem to recall a case where that bit me in the past? (I'm mostly working on embedded systems (PPC & ARM))

The cases that come to my mind for C++ all involve initialization...

Before I give you an example, I will define what the term undefined behavior means by quoting the standard - Section §1.3.12 (of the C standard, not the C++ standard which is horribly ginormous and hard to read):

    behaviour, such as might arise upon use of an erroneous program construct or erroneous data, for which this International Standard imposes no requirements 3.

    Undefined behaviour may also be expected when this International Standard omits the description of any explicit definition of behavior.
The reason undefined behavior is dangerous is that the standard does not guarantee any particular behavior and the implementation is free to do whatever it wants - ignore it, give an error message, delete everything on your hard drive.. whatever.

The two most commonly cited piece of undefined behavior is modifying a variable twice in one sequence point. The standard says:

    Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression.
This code snippet invokes undefined behavior:

    a = b++ * ++b;
because b is modified twice within one sequence point. More information here http://stackoverflow.com/questions/4176328/undefined-behavio...


For more real world examples of where undefined behavior may bite you in the ass in C++, take a look at Washu's simple C++ quiz. It's only four questions: http://www.scapecode.com/2011/05/a-simple-c-quiz/

Take a moment to answer the questions before looking at the answers.

Once you've done that, here are three more quizzes by the same guy - these ones are about OOP in C++, so may be much more relevant to your question: http://www.scapecode.com/2011/05/c-quiz-2/ and http://www.scapecode.com/2011/05/c-quiz-3/ and http://www.scapecode.com/2011/05/c-quiz-4/

Emscripten in no way emulates LLVM (as far as I know it's the only C-to-JS compiler that uses LLVM, and hence must be what you're referring to!) — it compiles LLVM to JS, with no emulation, and there's currently work going on to re-implement it as a LLVM backend (it currently is an entirely separate codebase that takes LLVM bitcode in and spits JS out).

My mistake, I thought Emscripten was an LLVM VM in JS. I'll take a look at it to see what the output actually does.

Looking at issues in https://github.com/kripken/emscripten/wiki/Filesystem-Guide it seems to be there would be a big win in having an LLVM VM - you'd be able to have synchronous IO and have threads.

The only problem is how to deal with casting arrays of integers (or whatever) to arrays of bytes. But that's a problem for portable C software anyway.

Byte-wise access to objects is legal and completely portable between conforming implementations. What isn't portable are arbitrary type conversions through pointer casts as these violate the effective typing rules. Such casts may break in practice due to mis-alignment or because of aliasing behind the optimizers back.

In a way, C is a strongly typed language - the type system is just really unsound.

> Byte-wise access to objects is legal and completely portable between conforming implementations.

That makes absolutely no sense. Just think about endianness for example. Type conversions in C are extremely tricky, and in many cases are not guaranteed to be portable across different compilers even on the same architecture. There is a good explanation of what you can and cannot count on in chapter 6 of Harbison and Steele's C A Reference Manual.

There's also a perfectly good list of what you can and cannot count on in the ISO C standard, no need for secondary literature.

While the values of the bytes are not specified, the ability to get at them is, and a conforming implementation needs to provide this ability.

I've used libtcc from tcc to do something similar on a prototype listening on a socket to do queries over a 2 gig memory mapped file. I'd pass over the query as a string of C that would be dynamically compiled and executed by libtcc. It worked really well but ultimately didn't go anywhere other than research. Here's an example from the distribution (first google result for the file): http://www.koders.com/c/fidC76C8B834DFF05F1D0BD61220AC19E246.... TCC can be found here: http://bellard.org/tcc/

I am surprised more projects don't use TCC for that. It's an awesome little compiler and using it that way saves way more time over writing DSLs. Even projects that have to compile C all the time (like Lisp->C compilers such as ECL and Gambit Scheme) use GCC, which is stupid slow.

Well, for anyone interested in Cling, check out this Google tech talk: http://www.youtube.com/watch?v=f9Xfh8pv3Fs

This is a CERN project and it uses Clang from the LLVM project. The idea is simple: use the clang to generate LLVM and then use LLVM just-in-time compiler.

Before that CINT was used (another C++ interpretter) - here is the page where they announce the future transition CINT -> CLING - http://root.cern.ch/drupal/content/cling

It's a cool idea, but his reason for creating it is kinda dumb.

Creating an entire "project" just to check a code snippet is just silly.

Just create a "testing" directory and throw your one off test files into it and compile/run them there.

I start mine with a comment explaining what I'm testing, why I'm testing it, and what special compilation flags are required, if any. I even have an Emacs macro that fills in the boilerplate includes and main function. The overhead involved is probably less than 15 seconds.

It has the advantages that I can test multiple compilers and I keep a history of the things I've tried.

The ROOT project has been using CINT for ages. Talk to your favorite physicist friend -- they'll be confused why the CS crowd _doesn't_ have this technology.

This is just an updating of what must be an awful hack to be built on LLVM infrastructure.

Although getting an interactive environment definitely is a nice thing, I'd argue you don't have to create a whole project directory, etc. to play with an idea; I usually write it in a single file.

Could you not just use a project directory template? All you need really is a .c(xx|pp|whatever) file, a Makefile, and then the workflow for a new idea is:

cd ~/src;

cp -R c-idea-template foobar;

cd foobar;

$EDITOR test.c*

and in your editor (say vim) just run :make

or write a .sh file with all of the above in it so it's just one step. No complex install procedure, you get all your normal tools and stuff.

Alternaitvely, create a 'test projects' git(hub) project, with the files you want in it, and create a new branch for each idea. That way you get backups as well.

@fferen, @jlarocco, @Mon_Ouie, @deckiedan: I know you can probably set up all you need within the blink of a second (using emacs or vim or a template setup). Actually I'm so hooked on this idea I even wrote my own rake based C/C++ Buildsystem (https://github.com/marcmo/cxxproject). But still sometimes I prefer not having to set up anything and not having to clean up anything after I end my experiments. ... Idea - drop into REPL and try out - exit - done.

To be clear, those empty #include's are typos, not Cling inferring desired header files. Both are:

    #include <iostream>

Uuhhh...damn! I messed that up. corrected now! thanks!

I just keep a fixed .cpp file with a bunch of common includes and directives, and alias the g++ command to link to common libraries, so my workflow goes:

> vim temp.cpp

> g++ temp.cpp

> ./a.out

Very similar to my process with Python, actually. Every time I use the REPL for some experimenting, the code ends up outgrowing it and I have to stuff it in a file anyway, so I may as well cut out the middleman to begin with. YMMV.

I too put the code in a file even in Python, but REPL is still valuable. I use "python -i" which runs your code (defines functions, classes, etc.) and then stop, and put you in that environment. This is very useful. It seems Cling could be used the same way: .x command.

I must be reading the wrong Python tutorials. Thank you for this

If you don't need any special compiler flags, it's even simpler:

    make temp

I think it is very useful if you want to know more about c++,especially some feature you 're not very sure about. interactive interpret make it very intuitive, and it will save you time,because you don't need to compile the code. but one limit is that now it doesn't support template , you still have to write a source file if you want to use template.

The LLVM infrastructure gets more amazing every day; Emscripten and Cling are very exciting projects. C++ gets a lot of flak but it's still got a huge amount of life in it.

Could this be used to make a c/c++ plugin for lightroom, once it is out and stable?

So, what are it's limitations, if any?

I'd say error recovery could be better. You often get kicked out of the session and than your environment is lost. Also: they claim the 'auto' keyword is implicit, does not seem to work for me. But it's pretty cool to use!

Why would I do that :S

Why not? It's great to quickly test C++, I have yet to find a developer who doesn't love a REPL for one.

C++ could also be used as an embedded extension language using something like this.

I share your enthusiasm but there are limits. C++ compilation speed (and thus interpretation speed) will make it quite impossible to use it as an extension language. And while I like and use C++ a lot I don't think it is the right language for this purpose. It can do quite a lot of things, but working in a homogeneous language in your extensions as well as your core product doesn't have enough benefits compared to using a systems programming language and an extension language.

> C++ compilation speed (and thus interpretation speed) will make it quite impossible to use it as an extension language.

I think you vastly underestimate the speed of the clang C++ compiler (or any modern C++ compiler at that). I can't imagine that the compilation time for anything that could be classified as an "extension" would be significant in any meaningful way.

clang doesn't outperform gcc on most projects I work on or that speed increase is not significant. I run quite a lot of test-suites - maybe I should try to measure them and provide some real numbers for reference instead of hand-wave myself through this argument.

If I where to provide extensions points to my C++ project they would certainly contain templates and that would also mean that they would require significant compilation time (compared to what you would expect). Even if I didn't, a single header that pulls in a huge preprocessor library could ruin speed. I'm inclined to believe you if we are talking about a Qt-style C++, but that is only a subset of possible code. I would be happy to be proven wrong, too.

> I'm inclined to believe you if we are talking about a Qt-style C++, but that is only a subset of possible code. I would be happy to be proven wrong, too.

If the alternative is to use a separate language altogether to implement extensions, then you are already limiting yourself to a subset of all possible (in C++) code, so what's wrong with doing that in C++ instead?

That being said, most of your problems can be solved with header optimization, we had gotten a full-rebuild of our multimillion line code-base down to about 2 minutes. So if an extension is only a file or two I'm sure you could manage very reasonable compile times, esp with optimization turned off.

> Even if I didn't, a single header that pulls in a huge preprocessor library could ruin speed.

In the case that you really need that header (If you are using as an extension language, it should be designed so you don't!) precompilation of certain units can greatly improve speed. clang in particular has very impressive precompilation support.

Well if it's possible for C++, it's definitely possible for D which is benchmarked to compile 100 times faster and has close-to-ruby's productivity in a close-to-metal language like C. An added benefit is that D already has most of the C++11 features and more.

I would very much like to use D as an extension language.

D has `rdmd`

I can see it being helpful if you are working in C++ all day and need to try small things out. I use LinqPad for doing the same w/ C#, helps when you need to test out some smaller ideas w/o all the cruft of projects and references / includes / etc.

Has anyone mentioned CH already (http://www.softintegration.com/)?

Kind of an interesting (hi-level) tool, too.

For small programs, something like CodeRunner is nice too: http://krillapps.com/coderunner/

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