Passing SCM values around as void *

[divoplade]

Hello guile users,

I have just found out a non-reproducible bug in my code on a (more
complex) version of this. Since it is not reproducible anyway, I will
not put a full example. This is just pseudo-code.

/* This function calls get with ctx until it is satisfied. get should
fill its third argument with as many bytes as its second argument. */
void work (void *ctx, int (*get) (void *, size_t, char *));

/* The guile callback returns a bytevector */
static void
get_as_scm (void *ctx, size_t n, char *dest)
{
  SCM *f = ctx;
  SCM ret = scm_call_1 (*f, scm_from_size_t (n));
  SCM_ASSERT (scm_c_bytevector_length (ret) == n, 
             *f, SCM_ARG1, "work");
  memcpy (dest, SCM_BYTEVECTOR_CONTENTS (ret), n);
}

static SCM
guile_work (SCM arg)
{
  void *ctx = &arg;
  work (ctx, get_as_scm);

  // This fixes the bug, it seems.
  scm_remember_upto_here_1 (arg);
}

I do not seem to have a bug if the callback does not allocate a return
value.

The underlying question is: how to pass SCM values as void *, and still
protect them from garbage collection? Do I need to call
scm_remember_upto_here_1 manually like I did, without any hint in the
manual, or is it just hiding the bug? It seems that the problem does
not happen with guile 2.2.

Best regards,

divoplade