As for wrapping them in C functions: the problem is that the most basic Arc threading function isn't 'thread, it's 'new-thread, which accepts a function as input. 'thread is defined as:

  (mac thread body
    `(new-thread (fn () ,@body)))

  (new-thread
   (if
    (something)
      (fn () (ha-ha-ha))
    (something-else)
      (fn () (he-he-he))
      (fn () (je-je-je))))

It would be possible to implement if pthreads or whatnot can pass even just a single pointer to the newly-threaded function, but if it can't (why not?) then our alternative is to create a bunch of C functions for each Arc function, which just calls the execute() function with the correct number.

---------

Edit: okay, I did a little eensy-weensy bit of research on pthreads, and it seems that pthreads can pass a pointer to the called C function.

So I suppose we can modify execute's signature:

  obj execute(long pc, obj stack[], obj *sp);
  int main(void){
    /*init...*/
    obj *stack = GC_MALLOC(sizeof(obj)*MAX_STACK);
    obj *sp = stack;
    execute(0, stack, sp);
  }

  typedef struct{
    long pc;
    obj *stack;
    obj *sp;
  } newthreadpasser;
  #define T_THREAD 9999 //some random number
  typedef struct{
    long type; /*T_THREAD*/
    pthread_t val;
  } thread;
  #define NEWTHREAD() \
   {obj *newstack = GC_MALLOC(sizeof(obj) * MAX_STACK);\
    obj *newsp = &newstack[2]; \
    newstack[0] = POP(); \
    newstack[1] = THREAD_EXIT_CONTINUATION;\
    newthreadpasser * np = GC_MALLOC(sizeof(newthreadpasser));\
    np->pc = ((obj*)newstack[0])[0];\
    np->stack = newstack;\
    np->sp = newsp;\
    thread * threadobj = GC_MALLOC(sizeof(thread));\
    threadobj->type = T_THREAD;\
    pthread_create(&(threadobj->val), NULL, threadcall, (void*) np)\
    PUSH((obj)threadobj);}

  void *threadcall(void * args){
    newthreadpasser * np = (newthreadpasser *) args;
    execute(np->pc, np->stack, np->sp);
  }