ffi: two-argument form of sizeof.

If sizeof is given an extra argument (an expression which
evaluates to an object), it can calculate the dynamic size of
the FFI type in relation to representing that object.

* ffi.c (dyn_size): New static function.
(ffi_init): Register sys:dyn-size intrinsic.

* share/txr/stdlib/ffi.tl (sizeof): Support additional
argument, avoiding run-time compilation of the type expression
with the help of load-time.

* txr.1: Update documentation for sizeof macro.

1 files changed, 8 insertions, 0 deletions

diff --git a/ffi.c b/ffi.c
index d411dda7..c18b7efe 100644
--- a/ffi.c
+++ b/ffi.c
@@ -5494,6 +5494,13 @@ val fill_obj(val obj, val type, val stream)
   return tft->in(tft, 1, data, obj, self);
 }
 
+static val dyn_size(val type, val obj)
+{
+  val self = lit("sizeof");
+  struct txr_ffi_type *tft = ffi_type_struct_checked(self, type);
+  return num(tft->dynsize(tft, obj, self));
+}
+
 void ffi_init(void)
 {
   prot1(&ffi_typedef_hash);
@@ -5625,6 +5632,7 @@ void ffi_init(void)
   reg_fun(intern(lit("put-obj"), user_package), func_n3o(put_obj, 2));
   reg_fun(intern(lit("get-obj"), user_package), func_n2o(get_obj, 1));
   reg_fun(intern(lit("fill-obj"), user_package), func_n3o(fill_obj, 2));
+  reg_fun(intern(lit("dyn-size"), system_package), func_n2(dyn_size));
   ffi_typedef_hash = make_hash(nil, nil, nil);
   ffi_init_types();
   ffi_init_extra_types();