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From: Damien Doligez <damien.doligez@inria.fr>
Subject: Re: [Caml-list] How to find out free diskspace?
Date: Sun, 3 Jul 2005 13:54:15 +0200
To: caml users <caml-list@inria.fr>
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On Jul 1, 2005, at 20:19, Richard Jones wrote:

> static value
> copy_statfs (struct statfs *buf)
> {
>   CAMLparam0 ();
>   CAMLlocal1 (bufv);
>   bufv = caml_alloc (9, 0);
>   caml_modify (&Field (bufv, 0), copy_int64 (buf->f_type));
>   [...]
>

There's a nasty bug lurking in this code.  Depending on your
C compiler, you might be computing &Field (bufv, 0) before
the call to copy_int64, which can trigger a GC and change the
value of bufv, hence invalidating the address you've just
computed.

You should do it this way:

static value
copy_statfs (struct statfs *buf)
{
   CAMLparam0 ();
   CAMLlocal1 (bufv, v);
   bufv = caml_alloc (9, 0);
   v = copy_int64 (buf->f_type); caml_modify (&Field (bufv, 0), v);
   v = copy_int64 (buf->f_bsize); caml_modify (&Field (bufv, 1), v);
   v = copy_int64 (buf->f_blocks); caml_modify (&Field (bufv, 2), v);
   v = copy_int64 (buf->f_bfree); caml_modify (&Field (bufv, 3), v);
   v = copy_int64 (buf->f_bavail); caml_modify (&Field (bufv, 4), v);
   v = copy_int64 (buf->f_files); caml_modify (&Field (bufv, 5), v);
   v = copy_int64 (buf->f_ffree); caml_modify (&Field (bufv, 6), v);
   caml_modify (&Field (bufv, 7), Val_unit);
   v = copy_int64 (buf->f_namelen); caml_modify (&Field (bufv, 8), v);
   CAMLreturn (bufv);
}

-- Damien