On: Show HN: A small virtual machine written in C

[akkartik]

Can you give some directions for how to run it?

  $ git clone https://github.com/tekknolagi/carp
  $ make
  $ ./carp
  $ ./carp --help  # after peeking in carp.c
  help msg

There's a tantalizing target called 'tests' in the Makefile, but it doesn't work.

There's some examples, but no makefile to build them. How do you run them?

I see you used to have some .carp files but you just deleted them: https://github.com/tekknolagi/carp/commit/fa16eeb443

I tried restoring one, but it doesn't work:

  $ git checkout fa16eeb443~1 examples/reg.carp
  $ ./carp -f examples/reg.carp
  Unknown label <add>

So at this point I'm ready to give up..

[tekknolagi → akkartik]

Hi there! Sorry for this. I actually have a help message but for some reason I forgot to commit it. It's coming soon!

You run the examples like so: carp -f examples/carp/call.carp

You can compile C files that use the Carp API, but I have not written any documentation yet.

[tekknolagi → tekknolagi]

Oh that's weird. the carp folder from examples disappeared. Gimme a sec.

[proxyswapi → akkartik]

A year (or two) ago, I wrote a simple virtual machine [1] of my own. Fun time. Interesting exercise. Wish I had courage to post it here and get feedback from the HN. Anyways, yours is much cleaner and concise code. Kudos.

[1] https://github.com/swapi/z9

[tekknolagi → proxyswapi]

Definitely post it! I've written muuuuch worse and it's still public :D

Check out http://github.com/tekknolagi/gecho — shudder

[proxyswapi → tekknolagi]

Posted my reply on wrong comment (Got confused between akkartik and tekknolagi) :). Anyways, thanks for sharing. Are you planning to add memory management opcodes (Allocate memory, free memory)? I see DBS, DBG instruction, but it is like key-value store (since it uses carp_ht internally).

[tekknolagi → proxyswapi]

Umm perhaps. Could you give a use case? Not sure how/why.

[tekknolagi → akkartik]

Alright, try now. Also — thanks for the pull request!

[akkartik → tekknolagi]

You're welcome. I pulled the new commits, but I'm still having trouble running the examples:

  $ ./carp -f examples/carp/call.carp 
  Unknown label <add>

(I am very interested in building easy-to-understand codebases: http://akkartik.name/post/wart)

[tekknolagi → akkartik]

That's strange: http://i.imgur.com/TbF7Izc.png

Try rebuilding carp? o.O

That was some very nice documentation!

[akkartik → tekknolagi]

Nope, rebuilding didn't help.. never mind, I'll look into it later tonight.

Edit: I just noticed you changed the target (why?) and are also immediately deleting carp.out. So I was indeed using the stale version in spite of using 'make clean' :)

Anyways, so I did this, but still no luck.

  $ git diff
  diff --git a/Makefile b/Makefile
  index f921d96..7c55b4e 100644
  --- a/Makefile
  +++ b/Makefile
  @@ -10,7 +10,6 @@ all:
	  $(CC) $(CFLAGS) $(SRCS)
	  ar cr libcarp.a $(OBJS)
	  $(CC) src/carp.c libcarp.a -o $(PROG)
  -       make clean_objs
   
   #.PHONY: tests
   
  $ make
  gcc  -c -std=c99 -Wall -Werror -Wno-unused-variable -Wno-format-security -O3 -static   src/carp_instructions.c src/carp_lexer.c src/carp_machine.c src/carp_tokenizer.c src/lib/carp_stack.c src/lib/carp_ht.c
  ar cr libcarp.a *.o
  gcc  src/carp.c libcarp.a -o carp.out
  $ ./carp.out -f reg.carp 
  Unknown label <add>

[tekknolagi → akkartik]

Yeah I realized that I had changed PROG to carp.out but not changed clean_objs. Already pushed a fix!

[tekknolagi → akkartik]

So strange.

[akkartik → tekknolagi]

In the process of debugging it I noticed that you're compiling but not linking in c99 mode. When I fix that the problem seems to go away:

  $ git diff
  diff --git a/Makefile b/Makefile
  index 1263dd8..6592c59 100644
  --- a/Makefile
  +++ b/Makefile
  @@ -1,16 +1,16 @@
   CC = gcc #/usr/local/bin/gcc-4.2
   PREFIX = /usr/local
   NDEBUG ?= 
  -CFLAGS = -c -std=c99 -Wall -Werror -Wno-unused-variable -Wno-format-security -O3 -static $(NDEBUG) 
  +CFLAGS = -std=c99 -Wall -Werror -Wno-unused-variable -Wno-format-security -O3 -static $(NDEBUG) 
   SRCS = src/carp_instructions.c src/carp_lexer.c src/carp_machine.c src/carp_tokenizer.c src/lib/carp_stack.c src/lib/carp_ht.c
   #$(wildcard src/*.c src/lib/*.c)
   OBJS = *.o
   PROG = carp.out
   
   all:
  -       $(CC) $(CFLAGS) $(SRCS)
  +       $(CC) -c $(CFLAGS) $(SRCS)
	  ar cr libcarp.a $(OBJS)
  -       $(CC) src/carp.c libcarp.a -o $(PROG)
  +       $(CC) $(CFLAGS) src/carp.c libcarp.a -o $(PROG)
   
   #.PHONY: tests

Still, I'm concerned that you might be triggering some sort of undefined behavior, which means it might work for you but not on a slightly different machine or compiler version. I tried printing out token lexemes in carp_run_program, right after the call to tokenize(), and some of the tokens printed binary garbage, suggesting that they might be missing a terminating null, or worse. Does this look right?

  $ git diff
  ...
  diff --git a/src/carp.c b/src/carp.c
  index 9268d98..19e16d9 100644
  --- a/src/carp.c
  +++ b/src/carp.c
  @@ -88,6 +88,8 @@ void carp_print_conditions () {
   
   void carp_run_program (char *fn) {
     carp_tok *tokens = carp_lex_tokenize(fn);
  +  for (carp_tok* tt = tokens; tt; tt=tt->next)
  +    printf("%s %d\n", tt->lexeme, tt->type);
     if (tokens == NULL) {
       fprintf(stderr, "Something went wrong with tokenization.\n");
       exit(1);

  $ make && ./carp.out -f ./examples/carp/call.carp
  gcc  -c -std=c99 -Wall -Werror -Wno-unused-variable -Wno-format-security -O3 -static   src/carp_instructions.c src/carp_lexer.c src/carp_machine.c src/carp_tokenizer.c src/lib/carp_stack.c src/lib/carp_ht.c
  ar cr libcarp.a *.o
  gcc  -std=c99 -Wall -Werror -Wno-unused-variable -Wno-format-security -O3 -static   src/carp.c libcarp.a -o carp.out
  add 3
  gload 6
  -5l 1
  gload 6
  -4l 1
  add 6
  ret 6
  main 3
  push 6
  7�l 1
  push 6
  9�l 1
  call 6
  add 4
  2 1
  ptop 6
  halt 6
  0 1
  16

The trouble with C/C++ is that it's so easy to end up with undefined behavior :/

[tekknolagi → akkartik]

Actually I can't link with the same CFLAGS; I get an error about missing lcrt0.o or something of the sort.

Also, yeah your code looks fine; I added it in and it worked fine. That's unfortunate. I'll debug soon.

[akkartik → tekknolagi]

What machine/os are you on?

(Perhaps we should take this offline. My email is in my profile.)

[tekknolagi → akkartik]

Sent an email :)