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Using OCaml's run-time from LLVM-generated native code
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Date: 2008-02-04 (10:58)
From: Jon Harrop <jon@f...>
Subject: Re: [Caml-list] Using OCaml's run-time from LLVM-generated native code
On Monday 04 February 2008 07:03:17 Alain Frisch wrote:
> Jon Harrop wrote:
> > How does OCaml's stack walker work with C code, for example? In
> > particular, how does it know what is a pointer into the heap from a C
> > stack frame? Must it be explicitly disabled?
> The OCaml runtime does not scan the stack frames corresponding to C
> functions.

How does it know which stack frames correspond to C functions?

> Jon, it is somewhat weird that you spend so much time writing about
> forking OCaml and do not take a few minutes to read the source code. The
> macros CAMLparam*, CAMLlocal* are not really that mysterious.

Despite the availability of that code it seems that few people can use it 
correctly and I am one of them.

This seems to work even though it calls full_major aggressively:

#include <stdio.h>
#include <string.h>
#include <caml/mlvalues.h>
#include <caml/alloc.h>
#include <caml/memory.h>
#include <caml/fail.h>
#include <caml/callback.h>
#include <caml/custom.h>
#include <caml/intext.h>

extern value caml_gc_full_major(value v);

CAMLprim value fib(value nv) {
  CAMLlocal5(a, b, c, d, e);
  int64 n = Int64_val(nv);
  if (n < 2) CAMLreturn(nv);
  a = copy_int64(n-1);
  b = copy_int64(n-2);
  c = fib(a);
  d = fib(b);
  e = copy_int64(Int64_val(c) + Int64_val(d));

int apply(int n) {
  CAMLlocal2(nv, fibn);
  nv = copy_int64(n);
  fibn = fib(nv);
  return Int64_val(fib(nv));

int main(int argc, char* argv[]) {
  printf("%d\n", apply(argc == 2 ? atoi(argv[1]) : 10));
  return 0;

Is that correct code?

Rather than messing around with these macros in each and every function it is 
probably easier and more efficient to register a single global root at entry, 
pointing to a shadow stack and push and pop elements to and from that 

Dr Jon D Harrop, Flying Frog Consultancy Ltd.