ffi: fix semantics of ptr-out-d.

Since the callee allocates the buffer for a ptr-out-d, the FFI
mechanism should not also be allocating a buffer to receive
the object. The buffer pointer will just be overwritten by the
callee with its own dynamic pointer. We should pass a null
pointer which the callee fills in (making a ptr-out-d not
suitable as a by-value parameter).

Initially, ptr-out-d was envisioned for return values only
and for use in callbacks, which is why I neglected this
aspect.

* ffi.c (ffi_ptr_out_null_put): New static function.
(ffi_type_compile): Use ffi_ptr_out_null_put for the
ffi_ptr_out_d case, so caller places a null pointer, then
frees the pointer that the callee places there.

1 files changed, 7 insertions, 1 deletions

diff --git a/ffi.c b/ffi.c
index 29d943be..8664ea64 100644
--- a/ffi.c
+++ b/ffi.c
@@ -780,6 +780,12 @@ static void ffi_ptr_in_put(struct txr_ffi_type *tft, val s, mem_t *dst,
   }
 }
 
+static void ffi_ptr_out_null_put(struct txr_ffi_type *tft, val s, mem_t *dst,
+                                 val self)
+{
+  *coerce(mem_t **, dst) =  0;
+}
+
 static val ffi_struct_in(struct txr_ffi_type *tft, mem_t *src, val strct,
                          val self)
 {
@@ -1282,7 +1288,7 @@ val ffi_type_compile(val syntax)
       val target_type = ffi_type_compile(cadr(syntax));
       return make_ffi_type_pointer(syntax, cptr_s, sizeof (mem_t *),
                                    &ffi_type_pointer,
-                                   ffi_ptr_out_put, ffi_ptr_d_get,
+                                   ffi_ptr_out_null_put, ffi_ptr_d_get,
                                    ffi_ptr_out_in, ffi_ptr_out_out,
                                    target_type);
     } else if (sym == ptr_s) {