Add plugins list to mjSpec Python API.

Fixes #2061.

PiperOrigin-RevId: 677762985
Change-Id: I8e407373e7b505d9b0f1c023e97c5ea92169bbf6

doc/APIreference/functions.rst

@@ -4432,6 +4432,15 @@ mjs_asMaterial
 
 Safely cast an element as mjsMaterial, or return NULL if the element is not an mjsMaterial.
 
+.. _mjs_asPlugin:
+
+mjs_asPlugin
+~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_asPlugin
+
+Safely cast an element as mjsPlugin, or return NULL if the element is not an mjsPlugin.
+
 .. _AttributeSetters:
 
 Attribute setters

doc/includes/references.h

@@ -1738,7 +1738,7 @@ typedef struct mjsOrientation_ {   // alternative orientation specifiers
   double euler[3];                 // Euler angles
 } mjsOrientation;
 typedef struct mjsPlugin_ {        // plugin specification
-  mjsElement* instance;            // element type
+  mjsElement* element;             // element type
   mjString* name;                  // name
   mjString* instance_name;         // instance name
   int plugin_slot;                 // global registered slot number of the plugin
@@ -3622,6 +3622,7 @@ mjsHField* mjs_asHField(mjsElement* element);
 mjsSkin* mjs_asSkin(mjsElement* element);
 mjsTexture* mjs_asTexture(mjsElement* element);
 mjsMaterial* mjs_asMaterial(mjsElement* element);
+mjsPlugin* mjs_asPlugin(mjsElement* element);
 void mjs_setBuffer(mjByteVec* dest, const void* array, int size);
 void mjs_setString(mjString* dest, const char* text);
 void mjs_setStringVec(mjStringVec* dest, const char* text);

include/mujoco/mjspec.h

@@ -181,7 +181,7 @@ typedef struct mjsOrientation_ {   // alternative orientation specifiers
 
 
 typedef struct mjsPlugin_ {        // plugin specification
-  mjsElement* instance;            // element type
+  mjsElement* element;             // element type
   mjString* name;                  // name
   mjString* instance_name;         // instance name
   int plugin_slot;                 // global registered slot number of the plugin

include/mujoco/mujoco.h

@@ -1628,6 +1628,9 @@ MJAPI mjsTexture* mjs_asTexture(mjsElement* element);
 // Safely cast an element as mjsMaterial, or return NULL if the element is not an mjsMaterial.
 MJAPI mjsMaterial* mjs_asMaterial(mjsElement* element);
 
+// Safely cast an element as mjsPlugin, or return NULL if the element is not an mjsPlugin.
+MJAPI mjsPlugin* mjs_asPlugin(mjsElement* element);
+
 
 //---------------------------------- Attribute setters ---------------------------------------------
 

introspect/functions.py

@@ -10270,6 +10270,22 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
          ),
          doc='Safely cast an element as mjsMaterial, or return NULL if the element is not an mjsMaterial.',  # pylint: disable=line-too-long
      )),
+    ('mjs_asPlugin',
+     FunctionDecl(
+         name='mjs_asPlugin',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsPlugin'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsPlugin, or return NULL if the element is not an mjsPlugin.',  # pylint: disable=line-too-long
+     )),
     ('mjs_setBuffer',
      FunctionDecl(
          name='mjs_setBuffer',

introspect/structs.py

@@ -8565,7 +8565,7 @@ STRUCTS: Mapping[str, StructDecl] = dict([
          declname='struct mjsPlugin_',
          fields=(
              StructFieldDecl(
-                 name='instance',
+                 name='element',
                  type=PointerType(
                      inner_type=ValueType(name='mjsElement'),
                  ),

python/mujoco/specs.cc

@@ -274,6 +274,18 @@ PYBIND11_MODULE(_specs, m) {
   mjSpec.def("detach_body", [](MjSpec& self, raw::MjsBody& body) {
     mjs_detachBody(self.ptr, &body);
   });
+    mjSpec.def_property_readonly(
+      "plugins",
+      [](MjSpec& self) -> py::list {
+        py::list list;
+        raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_PLUGIN);
+        while (el) {
+          list.append(mjs_asPlugin(el));
+          el = mjs_nextElement(self.ptr, el);
+        }
+        return list;
+      },
+      py::return_value_policy::reference_internal);
   mjSpec.def_property_readonly(
       "actuators",
       [](MjSpec& self) -> py::list {
@@ -900,12 +912,12 @@ PYBIND11_MODULE(_specs, m) {
   // ============================= MJSPLUGIN ===================================
   mjsPlugin.def_property(
       "id",
-      [](raw::MjsPlugin& self) -> int { return mjs_getId(self.instance); },
+      [](raw::MjsPlugin& self) -> int { return mjs_getId(self.element); },
       [](raw::MjsPlugin& self, raw::MjsPlugin* other) {
-        self.instance = other->instance;
+        self.element = other->element;
       });
   mjsPlugin.def("delete",
-                [](raw::MjsPlugin& self) { mjs_delete(self.instance); });
+                [](raw::MjsPlugin& self) { mjs_delete(self.element); });
 
 #include "specs.cc.inc"
 }  // PYBIND11_MODULE // NOLINT

python/mujoco/specs_test.py

@@ -690,5 +690,32 @@ class SpecsTest(absltest.TestCase):
     ):
       spec.recompile(model, data)
 
+  def test_delete_unused_plugin(self):
+    spec = mujoco.MjSpec()
+    spec.from_string(textwrap.dedent("""
+      <mujoco model="MuJoCo Model">
+        <extension>
+          <plugin plugin="mujoco.pid">
+            <instance name="pid1">
+              <config key="kp" value="4.0"/>
+            </instance>
+          </plugin>
+        </extension>
+
+        <worldbody>
+          <body>
+            <geom size="1"/>
+          </body>
+        </worldbody>
+      </mujoco>
+    """))
+    plugin = spec.plugins[0]
+    self.assertIsNotNone(plugin)
+    plugin.delete()
+
+    model = spec.compile()
+    self.assertIsNotNone(model)
+    self.assertEqual(model.nplugin, 0)
+
 if __name__ == '__main__':
   absltest.main()

src/user/user_api.cc

@@ -471,7 +471,7 @@ mjsKey* mjs_addKey(mjSpec* s) {
 mjsPlugin* mjs_addPlugin(mjSpec* s) {
   mjCModel* modelC = static_cast<mjCModel*>(s->element);
   mjCPlugin* plugin = modelC->AddPlugin();
-  plugin->spec.instance = static_cast<mjsElement*>(plugin);
+  plugin->spec.element = static_cast<mjsElement*>(plugin);
   return &plugin->spec;
 }
 
@@ -887,6 +887,16 @@ mjsMaterial* mjs_asMaterial(mjsElement* element) {
 
 
 
+// return plugin given mjsElement
+mjsPlugin* mjs_asPlugin(mjsElement* element) {
+  if (element && element->elemtype == mjOBJ_PLUGIN) {
+    return &(static_cast<mjCPlugin*>(element)->spec);
+  }
+  return nullptr;
+}
+
+
+
 // copy buffer to destination buffer
 void mjs_setBuffer(mjByteVec* dest, const void* array, int size) {
   const std::byte* buffer = static_cast<const std::byte*>(array);
@@ -995,7 +1005,7 @@ const double* mjs_getDouble(const mjDoubleVec* source, int* size) {
 
 // set plugin attributes
 void mjs_setPluginAttributes(mjsPlugin* plugin, void* attributes) {
-  mjCPlugin* pluginC = static_cast<mjCPlugin*>(plugin->instance);
+  mjCPlugin* pluginC = static_cast<mjCPlugin*>(plugin->element);
   std::map<std::string, std::string, std::less<>>* config_attribs =
       reinterpret_cast<std::map<std::string, std::string, std::less<>>*>(attributes);
   pluginC->config_attribs = std::move(*config_attribs);

src/user/user_api.h

@@ -296,6 +296,9 @@ MJAPI mjsTexture* mjs_asTexture(mjsElement* element);
 // Safely cast an element as mjsMaterial, or return NULL if the element is not an mjsMaterial.
 MJAPI mjsMaterial* mjs_asMaterial(mjsElement* element);
 
+// Safely cast an element as mjsPlugin, or return NULL if the element is not an mjsPlugin.
+MJAPI mjsPlugin* mjs_asPlugin(mjsElement* element);
+
 
 //---------------------------------- Attribute setters ---------------------------------------------
 

src/user/user_composite.cc

@@ -315,7 +315,7 @@ bool mjCComposite::Make(mjSpec* spec, mjsBody* body, char* error, int error_sz)
   // overwrite plugin name
   if (plugin_instance_name.empty() && plugin.active) {
     plugin_instance_name = "composite" + prefix;
-    (static_cast<mjCPlugin*>(plugin.instance))->name = plugin_instance_name;
+    (static_cast<mjCPlugin*>(plugin.element))->name = plugin_instance_name;
   }
 
   // dispatch
@@ -714,7 +714,7 @@ mjsBody* mjCComposite::AddCableBody(mjCModel* model, mjsBody* body, int ix,
   if (plugin.active) {
     mjsPlugin* pplugin = &body->plugin;
     pplugin->active = true;
-    pplugin->instance = plugin.instance;
+    pplugin->element = plugin.element;
     mjs_setString(pplugin->name, mjs_getString(plugin.name));
     mjs_setString(pplugin->instance_name, plugin_instance_name.c_str());
   }

src/user/user_flexcomp.cc

@@ -422,7 +422,7 @@ bool mjCFlexcomp::Make(mjSpec* spec, mjsBody* body, char* error, int error_sz) {
   // overwrite plugin name
   if (plugin.active && plugin_instance_name.empty()) {
     plugin_instance_name = "flexcomp_" + name;
-    static_cast<mjCPlugin*>(plugin.instance)->name = plugin_instance_name;
+    static_cast<mjCPlugin*>(plugin.element)->name = plugin_instance_name;
   }
 
   // create bodies, construct flex vert and vertbody
@@ -440,7 +440,7 @@ bool mjCFlexcomp::Make(mjSpec* spec, mjsBody* body, char* error, int error_sz) {
       if (plugin.active) {
         mjsPlugin* pplugin = &body->plugin;
         pplugin->active = true;
-        pplugin->instance = static_cast<mjsElement*>(plugin.instance);
+        pplugin->element = static_cast<mjsElement*>(plugin.element);
         mjs_setString(pplugin->name, mjs_getString(plugin.name));
         mjs_setString(pplugin->instance_name, plugin_instance_name.c_str());
       }
@@ -504,7 +504,7 @@ bool mjCFlexcomp::Make(mjSpec* spec, mjsBody* body, char* error, int error_sz) {
       if (plugin.active) {
         mjsPlugin* pplugin = &pb->plugin;
         pplugin->active = true;
-        pplugin->instance = static_cast<mjsElement*>(plugin.instance);
+        pplugin->element = static_cast<mjsElement*>(plugin.element);
         mjs_setString(pplugin->name, mjs_getString(plugin.name));
         mjs_setString(pplugin->instance_name, plugin_instance_name.c_str());
       }

src/user/user_mesh.cc

@@ -246,7 +246,7 @@ void mjCMesh::CopyFromSpec() {
   facetexcoord_ = spec_facetexcoord_;
   maxhullvert_ = spec.maxhullvert;
   plugin.active = spec.plugin.active;
-  plugin.instance = spec.plugin.instance;
+  plugin.element = spec.plugin.element;
   plugin.name = spec.plugin.name;
   plugin.instance_name = spec.plugin.instance_name;
 
@@ -270,7 +270,7 @@ mjCMesh::~mjCMesh() {
   if (center_) mju_free(center_);
   if (graph_) mju_free(graph_);
   if (spec.plugin.active && spec.plugin.instance_name->empty()) {
-    model->DeleteElement(spec.plugin.instance);
+    model->DeleteElement(spec.plugin.element);
   }
 }
 
@@ -289,9 +289,9 @@ void mjCMesh::LoadSDF() {
                    name.c_str(), id);
   }
 
-  mjCPlugin* plugin_instance = static_cast<mjCPlugin*>(plugin.instance);
+  mjCPlugin* plugin_instance = static_cast<mjCPlugin*>(plugin.element);
   model->ResolvePlugin(this, plugin_name, plugin_instance_name, &plugin_instance);
-  plugin.instance = plugin_instance;
+  plugin.element = plugin_instance;
   const mjpPlugin* pplugin = mjp_getPluginAtSlot(plugin_instance->spec.plugin_slot);
   if (!(pplugin->capabilityflags & mjPLUGIN_SDF)) {
     throw mjCError(this, "plugin '%s' does not support signed distance fields", pplugin->name);
@@ -2919,9 +2919,9 @@ void mjCFlex::Compile(const mjVFS* vfs) {
   useredge = VectorToString(edgeidx_);
 
   for (const auto& vbodyid : vertbodyid) {
-    if (model->Bodies()[vbodyid]->plugin.instance) {
+    if (model->Bodies()[vbodyid]->plugin.element) {
       mjCPlugin* plugin_instance =
-          static_cast<mjCPlugin*>(model->Bodies()[vbodyid]->plugin.instance);
+          static_cast<mjCPlugin*>(model->Bodies()[vbodyid]->plugin.element);
       if (damping > 0) {
         plugin_instance->config_attribs["damping"] = std::to_string(damping);
       }

src/user/user_model.cc

@@ -938,6 +938,8 @@ mjsElement* mjCModel::NextObject(mjsElement* object, mjtObj type) {
       return GetNext(textures_, object);
     case mjOBJ_MATERIAL:
       return GetNext(materials_, object);
+    case mjOBJ_PLUGIN:
+      return GetNext(plugins_, object);
     default:
       return nullptr;
   }
@@ -3932,7 +3934,7 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
     std::vector<std::vector<int>> plugin_to_actuators(nplugin);
     for (int i = 0; i < nu; ++i) {
       if (actuators_[i]->plugin.active) {
-        int actuator_plugin = static_cast<mjCPlugin*>(actuators_[i]->plugin.instance)->id;
+        int actuator_plugin = static_cast<mjCPlugin*>(actuators_[i]->plugin.element)->id;
         m->actuator_plugin[i] = actuator_plugin;
         plugin_to_actuators[actuator_plugin].push_back(i);
       } else {
@@ -3942,7 +3944,7 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
 
     for (int i = 0; i < nbody; ++i) {
       if (bodies_[i]->plugin.active) {
-        m->body_plugin[i] = static_cast<mjCPlugin*>(bodies_[i]->plugin.instance)->id;
+        m->body_plugin[i] = static_cast<mjCPlugin*>(bodies_[i]->plugin.element)->id;
       } else {
         m->body_plugin[i] = -1;
       }
@@ -3950,7 +3952,7 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
 
     for (int i = 0; i < ngeom; ++i) {
       if (geoms_[i]->plugin.active) {
-        m->geom_plugin[i] = static_cast<mjCPlugin*>(geoms_[i]->plugin.instance)->id;
+        m->geom_plugin[i] = static_cast<mjCPlugin*>(geoms_[i]->plugin.element)->id;
       } else {
         m->geom_plugin[i] = -1;
       }
@@ -3959,7 +3961,7 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
     std::vector<std::vector<int>> plugin_to_sensors(nplugin);
     for (int i = 0; i < nsensor; ++i) {
       if (sensors_[i]->type == mjSENS_PLUGIN) {
-        int sensor_plugin = static_cast<mjCPlugin*>(sensors_[i]->plugin.instance)->id;
+        int sensor_plugin = static_cast<mjCPlugin*>(sensors_[i]->plugin.element)->id;
         m->sensor_plugin[i] = sensor_plugin;
         plugin_to_sensors[sensor_plugin].push_back(i);
       } else {

src/user/user_objects.cc

@@ -959,7 +959,7 @@ void mjCBody::CopyFromSpec() {
   *static_cast<mjsBody*>(this) = spec;
   userdata_ = spec_userdata_;
   plugin.active = spec.plugin.active;
-  plugin.instance = spec.plugin.instance;
+  plugin.element = spec.plugin.element;
   plugin.name = spec.plugin.name;
   plugin.instance_name = spec.plugin.instance_name;
 }
@@ -986,7 +986,7 @@ mjCBody::~mjCBody() {
   lights.clear();
 
   if (spec.plugin.active && spec.plugin.instance_name->empty()) {
-    model->DeleteElement(spec.plugin.instance);
+    model->DeleteElement(spec.plugin.element);
   }
 }
 
@@ -1669,9 +1669,9 @@ void mjCBody::Compile(void) {
           name.c_str(), id);
     }
 
-    mjCPlugin* plugin_instance = static_cast<mjCPlugin*>(plugin.instance);
+    mjCPlugin* plugin_instance = static_cast<mjCPlugin*>(plugin.element);
     model->ResolvePlugin(this, plugin_name, plugin_instance_name, &plugin_instance);
-    plugin.instance = plugin_instance;
+    plugin.element = plugin_instance;
     const mjpPlugin* pplugin = mjp_getPluginAtSlot(plugin_instance->spec.plugin_slot);
     if (!(pplugin->capabilityflags & mjPLUGIN_PASSIVE)) {
       throw mjCError(this, "plugin '%s' does not support passive forces", pplugin->name);
@@ -2132,7 +2132,7 @@ mjCGeom::mjCGeom(const mjCGeom& other) {
 
 mjCGeom::~mjCGeom() {
   if (spec.plugin.active && spec.plugin.instance_name->empty()) {
-    model->DeleteElement(spec.plugin.instance);
+    model->DeleteElement(spec.plugin.element);
   }
 }
 
@@ -2176,7 +2176,7 @@ void mjCGeom::CopyFromSpec() {
   meshname_ = spec_meshname_;
   material_ = spec_material_;
   plugin.active = spec.plugin.active;
-  plugin.instance = spec.plugin.instance;
+  plugin.element = spec.plugin.element;
   plugin.name = spec.plugin.name;
   plugin.instance_name = spec.plugin.instance_name;
 }
@@ -2913,9 +2913,9 @@ void mjCGeom::Compile(void) {
           this, "neither 'plugin' nor 'instance' is specified for geom");
     }
 
-    mjCPlugin* plugin_instance = static_cast<mjCPlugin*>(plugin.instance);
+    mjCPlugin* plugin_instance = static_cast<mjCPlugin*>(plugin.element);
     model->ResolvePlugin(this, plugin_name, plugin_instance_name, &plugin_instance);
-    plugin.instance = plugin_instance;
+    plugin.element = plugin_instance;
     const mjpPlugin* pplugin = mjp_getPluginAtSlot(plugin_instance->spec.plugin_slot);
     if (!(pplugin->capabilityflags & mjPLUGIN_SDF)) {
       throw mjCError(this, "plugin '%s' does not support sign distance fields", pplugin->name);
@@ -5554,7 +5554,7 @@ mjCActuator::mjCActuator(const mjCActuator& other) {
 
 mjCActuator::~mjCActuator() {
   if (spec.plugin.active && spec.plugin.instance_name->empty()) {
-    model->DeleteElement(spec.plugin.instance);
+    model->DeleteElement(spec.plugin.element);
   }
 }
 
@@ -5640,7 +5640,7 @@ void mjCActuator::CopyFromSpec() {
   refsite_ = spec_refsite_;
   slidersite_ = spec_slidersite_;
   plugin.active = spec.plugin.active;
-  plugin.instance = spec.plugin.instance;
+  plugin.element = spec.plugin.element;
   plugin.name = spec.plugin.name;
   plugin.instance_name = spec.plugin.instance_name;
 }
@@ -5869,9 +5869,9 @@ void mjCActuator::Compile(void) {
           name.c_str(), id);
     }
 
-    mjCPlugin* plugin_instance = static_cast<mjCPlugin*>(plugin.instance);
+    mjCPlugin* plugin_instance = static_cast<mjCPlugin*>(plugin.element);
     model->ResolvePlugin(this, plugin_name, plugin_instance_name, &plugin_instance);
-    plugin.instance = plugin_instance;
+    plugin.element = plugin_instance;
     const mjpPlugin* pplugin = mjp_getPluginAtSlot(plugin_instance->spec.plugin_slot);
     if (!(pplugin->capabilityflags & mjPLUGIN_ACTUATOR)) {
       throw mjCError(this, "plugin '%s' does not support actuators", pplugin->name);
@@ -5916,7 +5916,7 @@ mjCSensor::mjCSensor(const mjCSensor& other) {
 
 mjCSensor::~mjCSensor() {
   if (spec.plugin.active && spec.plugin.instance_name->empty()) {
-    model->DeleteElement(spec.plugin.instance);
+    model->DeleteElement(spec.plugin.element);
   }
 }
 
@@ -5968,7 +5968,7 @@ void mjCSensor::CopyFromSpec() {
   objname_ = spec_objname_;
   refname_ = spec_refname_;
   plugin.active = spec.plugin.active;
-  plugin.instance = spec.plugin.instance;
+  plugin.element = spec.plugin.element;
   plugin.name = spec.plugin.name;
   plugin.instance_name = spec.plugin.instance_name;
 }
@@ -6373,9 +6373,9 @@ void mjCSensor::Compile(void) {
 
     // resolve plugin instance, or create one if using the "plugin" attribute shortcut
     {
-      mjCPlugin* plugin_instance = static_cast<mjCPlugin*>(plugin.instance);
+      mjCPlugin* plugin_instance = static_cast<mjCPlugin*>(plugin.element);
       model->ResolvePlugin(this, plugin_name, plugin_instance_name, &plugin_instance);
-      plugin.instance = plugin_instance;
+      plugin.element = plugin_instance;
       const mjpPlugin* pplugin = mjp_getPluginAtSlot(plugin_instance->spec.plugin_slot);
       if (!(pplugin->capabilityflags & mjPLUGIN_SENSOR)) {
         throw mjCError(this, "plugin '%s' does not support sensors", pplugin->name);

src/xml/xml_native_reader.cc

@@ -2767,7 +2767,7 @@ void mjXReader::OnePlugin(XMLElement* elem, mjsPlugin* plugin) {
   mjs_setString(plugin->name, name.c_str());
   mjs_setString(plugin->instance_name, instance_name.c_str());
   if (instance_name.empty()) {
-    plugin->instance = mjs_addPlugin(spec)->instance;
+    plugin->element = mjs_addPlugin(spec)->element;
     ReadPluginConfigs(elem, plugin);
   } else {
     spec->hasImplicitPluginElem = true;

src/xml/xml_native_writer.cc

@@ -827,8 +827,8 @@ void mjXWriter::OnePlugin(XMLElement* elem, const mjsPlugin* plugin) {
   } else {
     WriteAttrTxt(elem, "plugin", plugin_name);
     const mjpPlugin* pplugin = mjp_getPluginAtSlot(
-        static_cast<mjCPlugin*>(plugin->instance)->spec.plugin_slot);
-    const char* c = &(static_cast<mjCPlugin*>(plugin->instance)->flattened_attributes[0]);
+        static_cast<mjCPlugin*>(plugin->element)->spec.plugin_slot);
+    const char* c = &(static_cast<mjCPlugin*>(plugin->element)->flattened_attributes[0]);
     for (int i = 0; i < pplugin->nattribute; ++i) {
       string value(c);
       if (!value.empty()) {

test/user/user_api_test.cc

@@ -147,7 +147,7 @@ TEST_F(PluginTest, ActivatePlugin) {
   // associate plugin to body
   mjsBody* body = mjs_addBody(mjs_findBody(spec, "world"), 0);
   mjs_setString(body->plugin.name, plugin_name.c_str());
-  body->plugin.instance = mjs_addPlugin(spec)->instance;
+  body->plugin.element = mjs_addPlugin(spec)->element;
   body->plugin.active = true;
   mjsGeom* geom = mjs_addGeom(body, 0);
   geom->type = mjGEOM_BOX;
@@ -191,7 +191,7 @@ TEST_F(PluginTest, DeletePlugin) {
   mjsActuator* actuator = mjs_addActuator(spec, 0);
   mjs_setString(actuator->target, "j1");
   mjs_setString(actuator->plugin.name, plugin_name.c_str());
-  actuator->plugin.instance = mjs_addPlugin(spec)->instance;
+  actuator->plugin.element = mjs_addPlugin(spec)->element;
   actuator->plugin.active = true;
   actuator->trntype = mjTRN_JOINT;