diff --git a/src/user/CMakeLists.txt b/src/user/CMakeLists.txt
index 5aa8981c..73c2050e 100644
--- a/src/user/CMakeLists.txt
+++ b/src/user/CMakeLists.txt
@@ -21,7 +21,7 @@ set(MUJOCO_USER_SRCS
     user_composite.h
     user_flexcomp.cc
     user_flexcomp.h
-    user_init.cc
+    user_init.c
     user_mesh.cc
     user_model.cc
     user_model.h
diff --git a/src/user/user_api.h b/src/user/user_api.h
index 59ec6d26..e23a5cb2 100644
--- a/src/user/user_api.h
+++ b/src/user/user_api.h
@@ -15,6 +15,7 @@
 #ifndef MUJOCO_SRC_USER_USER_API_H_
 #define MUJOCO_SRC_USER_USER_API_H_
 
+#include <math.h>
 #include <stddef.h>
 #include <mujoco/mjexport.h>
 #include <mujoco/mjmodel.h>
@@ -26,6 +27,8 @@
 extern "C" {
 #endif
 
+#define mjNAN NAN                  // used to mark undefined fields
+
 
 //---------------------------------- handles to internal objects -----------------------------------
 
@@ -144,7 +147,7 @@ typedef struct _mjSpec {           // model specification
   mjString modelfiledir;           // path to model file
 
   // other
-  bool hasImplicitPluginElem;      // already encountered an implicit plugin sensor/actuator
+  mjtByte hasImplicitPluginElem;   // already encountered an implicit plugin sensor/actuator
 } mjSpec;
 
 
@@ -747,7 +750,7 @@ typedef struct _mjsDefault {       // default specification
 //---------------------------------- Top-level spec manipulation -----------------------------------
 
 // Create spec.
-MJAPI mjSpec* mjs_createSpec();
+MJAPI mjSpec* mjs_createSpec(void);
 
 // Compile spec to model.
 MJAPI mjModel* mjs_compile(mjSpec* s, const mjVFS* vfs);
@@ -977,80 +980,80 @@ MJAPI const char* mjs_fullInertia(double quat[4], double inertia[3], const doubl
 
 //---------------------------------- Initialization  -----------------------------------------------
 
-// Default model attributes.
-MJAPI void mjs_defaultSpec(mjSpec& model);
+// Default spec attributes.
+MJAPI void mjs_defaultSpec(mjSpec* spec);
 
 // Default orientation attributes.
-MJAPI void mjs_defaultOrientation(mjsOrientation& orient);
+MJAPI void mjs_defaultOrientation(mjsOrientation* orient);
 
 // Default body attributes.
-MJAPI void mjs_defaultBody(mjsBody& body);
+MJAPI void mjs_defaultBody(mjsBody* body);
 
 // Default frame attributes.
-MJAPI void mjs_defaultFrame(mjsFrame& frame);
+MJAPI void mjs_defaultFrame(mjsFrame* frame);
 
 // Default joint attributes.
-MJAPI void mjs_defaultJoint(mjsJoint& joint);
+MJAPI void mjs_defaultJoint(mjsJoint* joint);
 
 // Default geom attributes.
-MJAPI void mjs_defaultGeom(mjsGeom& geom);
+MJAPI void mjs_defaultGeom(mjsGeom* geom);
 
 // Default site attributes.
-MJAPI void mjs_defaultSite(mjsSite& site);
+MJAPI void mjs_defaultSite(mjsSite* site);
 
 // Default camera attributes.
-MJAPI void mjs_defaultCamera(mjsCamera& camera);
+MJAPI void mjs_defaultCamera(mjsCamera* camera);
 
 // Default light attributes.
-MJAPI void mjs_defaultLight(mjsLight& light);
+MJAPI void mjs_defaultLight(mjsLight* light);
 
 // Default flex attributes.
-MJAPI void mjs_defaultFlex(mjsFlex& flex);
+MJAPI void mjs_defaultFlex(mjsFlex* flex);
 
 // Default mesh attributes.
-MJAPI void mjs_defaultMesh(mjsMesh& mesh);
+MJAPI void mjs_defaultMesh(mjsMesh* mesh);
 
 // Default height field attributes.
-MJAPI void mjs_defaultHField(mjsHField& hfield);
+MJAPI void mjs_defaultHField(mjsHField* hfield);
 
 // Default skin attributes.
-MJAPI void mjs_defaultSkin(mjsSkin& skin);
+MJAPI void mjs_defaultSkin(mjsSkin* skin);
 
 // Default texture attributes.
-MJAPI void mjs_defaultTexture(mjsTexture& texture);
+MJAPI void mjs_defaultTexture(mjsTexture* texture);
 
 // Default material attributes.
-MJAPI void mjs_defaultMaterial(mjsMaterial& material);
+MJAPI void mjs_defaultMaterial(mjsMaterial* material);
 
 // Default pair attributes.
-MJAPI void mjs_defaultPair(mjsPair& pair);
+MJAPI void mjs_defaultPair(mjsPair* pair);
 
 // Default equality attributes.
-MJAPI void mjs_defaultEquality(mjsEquality& equality);
+MJAPI void mjs_defaultEquality(mjsEquality* equality);
 
 // Default tendon attributes.
-MJAPI void mjs_defaultTendon(mjsTendon& tendon);
+MJAPI void mjs_defaultTendon(mjsTendon* tendon);
 
 // Default actuator attributes.
-MJAPI void mjs_defaultActuator(mjsActuator& actuator);
+MJAPI void mjs_defaultActuator(mjsActuator* actuator);
 
 // Default sensor attributes.
-MJAPI void mjs_defaultSensor(mjsSensor& sensor);
+MJAPI void mjs_defaultSensor(mjsSensor* sensor);
 
 // Default numeric attributes.
-MJAPI void mjs_defaultNumeric(mjsNumeric& numeric);
+MJAPI void mjs_defaultNumeric(mjsNumeric* numeric);
 
 // Default text attributes.
-MJAPI void mjs_defaultText(mjsText& text);
+MJAPI void mjs_defaultText(mjsText* text);
 
 // Default tuple attributes.
-MJAPI void mjs_defaultTuple(mjsTuple& tuple);
+MJAPI void mjs_defaultTuple(mjsTuple* tuple);
 
 // Default keyframe attributes.
-MJAPI void mjs_defaultKey(mjsKey& key);
+MJAPI void mjs_defaultKey(mjsKey* key);
 
 // Default plugin attributes.
-MJAPI void mjs_defaultPlugin(mjsPlugin& plugin);
+MJAPI void mjs_defaultPlugin(mjsPlugin* plugin);
 
 
 //---------------------------------- Compiler cache ------------------------------------------------
@@ -1061,7 +1064,7 @@ typedef struct _mjCache* mjCache;
 MJAPI void mj_setCacheSize(mjCache cache, size_t size);
 
 // Get internal global cache context.
-MJAPI mjCache mj_globalCache();
+MJAPI mjCache mj_globalCache(void);
 
 #ifdef __cplusplus
 }
diff --git a/src/user/user_composite.cc b/src/user/user_composite.cc
index 3a813b11..21c3cb6d 100644
--- a/src/user/user_composite.cc
+++ b/src/user/user_composite.cc
@@ -66,7 +66,7 @@ mjCComposite::mjCComposite(void) {
   mj_defaultSolRefImp(solrefsmooth, solimpsmooth);
 
   // plugin variables
-  mjs_defaultPlugin(plugin);
+  mjs_defaultPlugin(&plugin);
   plugin_name = "";
   plugin_instance_name = "";
   plugin.name = (mjString)&plugin_name;
diff --git a/src/user/user_flexcomp.cc b/src/user/user_flexcomp.cc
index a01f9a42..c319e1ab 100644
--- a/src/user/user_flexcomp.cc
+++ b/src/user/user_flexcomp.cc
@@ -87,8 +87,8 @@ mjCFlexcomp::mjCFlexcomp(void) {
   rigid = false;
   centered = false;
 
-  mjs_defaultPlugin(plugin);
-  mjs_defaultOrientation(alt);
+  mjs_defaultPlugin(&plugin);
+  mjs_defaultOrientation(&alt);
   plugin_name = "";
   plugin_instance_name = "";
   plugin.name = (mjString)&plugin_name;
diff --git a/src/user/user_init.c b/src/user/user_init.c
new file mode 100644
index 00000000..02f8b8fa
--- /dev/null
+++ b/src/user/user_init.c
@@ -0,0 +1,404 @@
+// Copyright 2024 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <string.h>
+#include <mujoco/mjmodel.h>
+#include <mujoco/mjvisualize.h>
+#include "engine/engine_io.h"
+#include "user/user_api.h"
+
+
+
+// default model attributes
+void mjs_defaultSpec(mjSpec* spec) {
+  memset(spec, 0, sizeof(mjSpec));
+
+  // default statistics
+  spec->stat.meaninertia = mjNAN;
+  spec->stat.meanmass = mjNAN;
+  spec->stat.meansize = mjNAN;
+  spec->stat.extent = mjNAN;
+  spec->stat.center[0] = mjNAN;
+
+  // compiler settings
+  spec->autolimits = 1;
+  spec->settotalmass = -1;
+  spec->degree = 1;
+  spec->euler[0] = 'x';
+  spec->euler[1] = 'y';
+  spec->euler[2] = 'z';
+  spec->convexhull = 1;
+  spec->usethread = 1;
+  spec->inertiafromgeom = mjINERTIAFROMGEOM_AUTO;
+  spec->inertiagrouprange[1] = mjNGROUP-1;
+  mj_defaultLROpt(&spec->LRopt);
+
+  // engine data
+  mj_defaultOption(&spec->option);
+  mj_defaultVisual(&spec->visual);
+  spec->memory = -1;
+  spec->njmax = -1;
+  spec->nconmax = -1;
+  spec->nstack = -1;
+
+  // user fields
+  spec->nuser_body = -1;
+  spec->nuser_jnt = -1;
+  spec->nuser_geom = -1;
+  spec->nuser_site = -1;
+  spec->nuser_cam = -1;
+  spec->nuser_tendon = -1;
+  spec->nuser_actuator = -1;
+  spec->nuser_sensor = -1;
+}
+
+
+
+// default orientation attributes
+void mjs_defaultOrientation(mjsOrientation* orient) {
+  memset(orient, 0, sizeof(mjsOrientation));
+}
+
+
+
+// default body attributes
+void mjs_defaultBody(mjsBody* body) {
+  memset(body, 0, sizeof(mjsBody));
+
+  // body frame
+  body->pos[0] = mjNAN;
+  body->quat[0] = 1;
+
+  // inertial frame
+  body->ipos[0] = mjNAN;
+  body->iquat[0] = 1;
+  body->fullinertia[0] = mjNAN;
+}
+
+
+
+// default frame attributes
+void mjs_defaultFrame(mjsFrame* frame) {
+  memset(frame, 0, sizeof(mjsFrame));
+  frame->quat[0] = 1;
+}
+
+
+
+// default joint attributes
+void mjs_defaultJoint(mjsJoint* joint) {
+  memset(joint, 0, sizeof(mjsJoint));
+  joint->type = mjJNT_HINGE;
+  joint->axis[2] = 1;
+  joint->limited = mjLIMITED_AUTO;
+  joint->actfrclimited = mjLIMITED_AUTO;
+  mj_defaultSolRefImp(joint->solref_limit, joint->solimp_limit);
+  mj_defaultSolRefImp(joint->solref_friction, joint->solimp_friction);
+  joint->urdfeffort = -1;
+}
+
+
+
+// default geom attributes
+void mjs_defaultGeom(mjsGeom* geom) {
+  memset(geom, 0, sizeof(mjsGeom));
+
+  // type
+  geom->type = mjGEOM_SPHERE;
+
+  // frame
+  geom->quat[0] = 1;
+  geom->fromto[0] = mjNAN;
+
+  // contact-related
+  geom->contype = 1;
+  geom->conaffinity = 1;
+  geom->condim = 3;
+  geom->friction[0] = 1;
+  geom->friction[1] = 0.005;
+  geom->friction[2] = 0.0001;
+  geom->solmix = 1.0;
+  mj_defaultSolRefImp(geom->solref, geom->solimp);
+
+  // inertia-related
+  geom->mass = mjNAN;
+  geom->density = 1000;  // water density (1000 Kg / m^3)
+  geom->typeinertia = mjINERTIA_VOLUME;
+
+  // color
+  geom->rgba[0] = geom->rgba[1] = geom->rgba[2] = 0.5f;
+  geom->rgba[3] = 1.0f;
+
+  // fluid forces
+  geom->fluid_coefs[0] = 0.5;   // blunt drag
+  geom->fluid_coefs[1] = 0.25;  // slender drag
+  geom->fluid_coefs[2] = 1.5;   // angular drag
+  geom->fluid_coefs[3] = 1.0;   // kutta lift
+  geom->fluid_coefs[4] = 1.0;   // magnus lift
+
+  // other
+  geom->fitscale = 1;
+}
+
+
+
+// default site attributes
+void mjs_defaultSite(mjsSite* site) {
+  memset(site, 0, sizeof(mjsSite));
+
+  // type
+  site->type = mjGEOM_SPHERE;
+
+  // frame
+  site->quat[0] = 1;
+  site->size[0] = site->size[1] = site->size[2] = 0.005;
+  site->fromto[0] = mjNAN;
+
+  // color
+  site->rgba[0] = site->rgba[1] = site->rgba[2] = 0.5f;
+  site->rgba[3] = 1.0f;
+}
+
+
+
+// default cam attributes
+void mjs_defaultCamera(mjsCamera* cam) {
+  memset(cam, 0, sizeof(mjsCamera));
+
+  // mode
+  cam->mode = mjCAMLIGHT_FIXED;
+
+  // extrinsics
+  cam->quat[0] = 1;
+
+  // intrinsics
+  cam->fovy = 45;
+  cam->ipd = 0.068;
+  cam->resolution[0] = cam->resolution[1] = 1;
+}
+
+
+
+// default light attributes
+void mjs_defaultLight(mjsLight* light) {
+  memset(light, 0, sizeof(mjsLight));
+
+  // mode
+  light->mode = mjCAMLIGHT_FIXED;
+
+  // extrinsics
+  light->dir[2] = -1;
+
+  // intrinsics
+  light->castshadow = 1;
+  light->bulbradius = 0.02;
+  light->active = 1;
+  light->attenuation[0] = 1;
+  light->cutoff = 45;
+  light->exponent = 10;
+  light->diffuse[0] = light->diffuse[1] = light->diffuse[2] = 0.7;
+  light->specular[0] = light->specular[1] = light->specular[2] = 0.3;
+}
+
+
+
+// default flex attributes
+void mjs_defaultFlex(mjsFlex* flex) {
+  memset(flex, 0, sizeof(mjsFlex));
+
+  // set contact defaults
+  flex->contype = 1;
+  flex->conaffinity = 1;
+  flex->condim = 3;
+  flex->friction[0] = 1;
+  flex->friction[1] = 0.005;
+  flex->friction[2] = 0.0001;
+  flex->solmix = 1.0;
+  mj_defaultSolRefImp(flex->solref, flex->solimp);
+
+  // set other defaults
+  flex->dim = 2;
+  flex->radius = 0.005;
+  flex->internal = 1;
+  flex->selfcollide = mjFLEXSELF_AUTO;
+  flex->activelayers = 1;
+  flex->rgba[0] = flex->rgba[1] = flex->rgba[2] = 0.5f;
+  flex->rgba[3] = 1.0f;
+}
+
+
+
+// default mesh attributes
+void mjs_defaultMesh(mjsMesh* mesh) {
+  memset(mesh, 0, sizeof(mjsMesh));
+  mesh->refquat[0] = 1;
+  mesh->scale[0] = mesh->scale[1] = mesh->scale[2] = 1;
+}
+
+
+
+// default height field attributes
+void mjs_defaultHField(mjsHField* hfield) {
+  memset(hfield, 0, sizeof(mjsHField));
+}
+
+
+
+// default skin attributes
+void mjs_defaultSkin(mjsSkin* skin) {
+  memset(skin, 0, sizeof(mjsSkin));
+  skin->rgba[0] = skin->rgba[1] = skin->rgba[2] = 0.5f;
+  skin->rgba[3] = 1.0f;
+}
+
+
+
+// default texture attributes
+void mjs_defaultTexture(mjsTexture* texture) {
+  memset(texture, 0, sizeof(mjsTexture));
+  texture->type = mjTEXTURE_CUBE;
+  texture->rgb1[0] = texture->rgb1[1] = texture->rgb1[2] = 0.8;
+  texture->rgb2[0] = texture->rgb2[1] = texture->rgb2[2] = 0.5;
+  texture->random = 0.01;
+  texture->gridsize[0] = texture->gridsize[1] = 1;
+  char defaultlayout[sizeof(texture->gridlayout)] = "............";
+  strncpy(texture->gridlayout, defaultlayout, sizeof(texture->gridlayout));
+}
+
+
+
+// default material attributes
+void mjs_defaultMaterial(mjsMaterial* material) {
+  memset(material, 0, sizeof(mjsMaterial));
+  material->texrepeat[0] = material->texrepeat[1] = 1;
+  material->specular = 0.5;
+  material->shininess = 0.5;
+  material->metallic = 1.;
+  material->roughness = 1.0;
+  material->rgba[0] = material->rgba[1] = material->rgba[2] = material->rgba[3] = 1;
+}
+
+
+
+// default pair attributes
+void mjs_defaultPair(mjsPair* pair) {
+  memset(pair, 0, sizeof(mjsPair));
+  pair->condim = 3;
+  mj_defaultSolRefImp(pair->solref, pair->solimp);
+  pair->friction[0] = 1;
+  pair->friction[1] = 1;
+  pair->friction[2] = 0.005;
+  pair->friction[3] = 0.0001;
+  pair->friction[4] = 0.0001;
+}
+
+
+
+// default equality attributes
+void mjs_defaultEquality(mjsEquality* equality) {
+  memset(equality, 0, sizeof(mjsEquality));
+  equality->type = mjEQ_CONNECT;
+  equality->active = 1;
+  mj_defaultSolRefImp(equality->solref, equality->solimp);
+  equality->data[1] = 1;
+  equality->data[10] = 1;  // torque:force ratio
+}
+
+
+
+// default tendon attributes
+void mjs_defaultTendon(mjsTendon* tendon) {
+  memset(tendon, 0, sizeof(mjsTendon));
+  tendon->limited = mjLIMITED_AUTO;
+  tendon->springlength[0] = tendon->springlength[1] = -1;
+  mj_defaultSolRefImp(tendon->solref_limit, tendon->solimp_limit);
+  mj_defaultSolRefImp(tendon->solref_friction, tendon->solimp_friction);
+  tendon->width = 0.003;
+  tendon->rgba[0] = tendon->rgba[1] = tendon->rgba[2] = 0.5f;
+  tendon->rgba[3] = 1.0f;
+}
+
+
+
+// default actuator attributes
+void mjs_defaultActuator(mjsActuator* actuator) {
+  memset(actuator, 0, sizeof(mjsActuator));
+
+  // gain, bias
+  actuator->gaintype = mjGAIN_FIXED;
+  actuator->gainprm[0] = 1;
+  actuator->biastype = mjBIAS_NONE;
+
+  // activation state
+  actuator->dyntype = mjDYN_NONE;
+  actuator->dynprm[0] = 1;
+  actuator->actdim = -1;
+
+  // transmission
+  actuator->trntype = mjTRN_UNDEFINED;
+  actuator->gear[0] = 1;
+
+  // input/output clamping
+  actuator->ctrllimited = mjLIMITED_AUTO;
+  actuator->forcelimited = mjLIMITED_AUTO;
+  actuator->actlimited = mjLIMITED_AUTO;
+}
+
+
+
+// default sensor attributes
+void mjs_defaultSensor(mjsSensor* sensor) {
+  memset(sensor, 0, sizeof(mjsSensor));
+
+  sensor->type = mjSENS_TOUCH;
+  sensor->datatype = mjDATATYPE_REAL;
+  sensor->needstage = mjSTAGE_ACC;
+}
+
+
+
+// Default numeric attributes.
+void mjs_defaultNumeric(mjsNumeric* numeric) {
+  memset(numeric, 0, sizeof(mjsNumeric));
+}
+
+
+
+// Default text attributes.
+void mjs_defaultText(mjsText* text) {
+  memset(text, 0, sizeof(mjsText));
+}
+
+
+
+// Default tuple attributes.
+void mjs_defaultTuple(mjsTuple* tuple) {
+  memset(tuple, 0, sizeof(mjsTuple));
+}
+
+
+
+// Default keyframe attributes.
+void mjs_defaultKey(mjsKey* key) {
+  memset(key, 0, sizeof(mjsKey));
+}
+
+
+
+// default plugin attributes
+void mjs_defaultPlugin(mjsPlugin* plugin) {
+  memset(plugin, 0, sizeof(mjsPlugin));
+  plugin->plugin_slot = -1;
+}
+
diff --git a/src/user/user_init.cc b/src/user/user_init.cc
deleted file mode 100644
index 73ab11b4..00000000
--- a/src/user/user_init.cc
+++ /dev/null
@@ -1,425 +0,0 @@
-// Copyright 2024 DeepMind Technologies Limited
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include <cstring>
-#include <mujoco/mjmodel.h>
-#include <mujoco/mujoco.h>
-#include "cc/array_safety.h"
-#include "user/user_api.h"
-#include "user/user_util.h"
-
-
-
-// default model attributes
-void mjs_defaultSpec(mjSpec& model) {
-  memset(&model, 0, sizeof(mjSpec));
-
-  // default statistics
-  model.stat.meaninertia = mjNAN;
-  model.stat.meanmass = mjNAN;
-  model.stat.meansize = mjNAN;
-  model.stat.extent = mjNAN;
-  model.stat.center[0] = mjNAN;
-
-  // compiler settings
-  model.autolimits = true;
-  model.boundmass = 0;
-  model.boundinertia = 0;
-  model.settotalmass = -1;
-  model.balanceinertia = false;
-  model.strippath = false;
-  model.fitaabb = false;
-  model.degree = true;
-  model.euler[0] = 'x';
-  model.euler[1] = 'y';
-  model.euler[2] = 'z';
-  model.discardvisual = false;
-  model.convexhull = true;
-  model.usethread = true;
-  model.fusestatic = false;
-  model.inertiafromgeom = mjINERTIAFROMGEOM_AUTO;
-  model.inertiagrouprange[0] = 0;
-  model.inertiagrouprange[1] = mjNGROUP-1;
-  model.exactmeshinertia = false;
-  mj_defaultLROpt(&model.LRopt);
-
-  // engine data
-  mj_defaultOption(&model.option);
-  mj_defaultVisual(&model.visual);
-  model.memory = -1;
-  model.nemax = 0;
-  model.njmax = -1;
-  model.nconmax = -1;
-  model.nstack = -1;
-  model.nuserdata = 0;
-  model.nuser_body = -1;
-  model.nuser_jnt = -1;
-  model.nuser_geom = -1;
-  model.nuser_site = -1;
-  model.nuser_cam = -1;
-  model.nuser_tendon = -1;
-  model.nuser_actuator = -1;
-  model.nuser_sensor = -1;
-}
-
-
-
-// default orientation attributes
-void mjs_defaultOrientation(mjsOrientation& orient) {
-  memset(&orient, 0, sizeof(mjsOrientation));
-}
-
-
-
-// default body attributes
-void mjs_defaultBody(mjsBody& body) {
-  memset(&body, 0, sizeof(mjsBody));
-
-  // body frame
-  body.pos[0] = mjNAN;
-  body.quat[0] = 1;
-  mjs_defaultOrientation(body.alt);
-
-  // inertial frame
-  body.ipos[0] = mjNAN;
-  body.iquat[0] = 1;
-  mjs_defaultOrientation(body.ialt);
-  body.fullinertia[0] = mjNAN;
-}
-
-
-
-// default frame attributes
-void mjs_defaultFrame(mjsFrame& frame) {
-  memset(&frame, 0, sizeof(mjsFrame));
-  mju_zero3(frame.pos);
-  mjuu_setvec(frame.quat, 1, 0, 0, 0);
-  mjs_defaultOrientation(frame.alt);
-}
-
-
-
-// default joint attributes
-void mjs_defaultJoint(mjsJoint& joint) {
-  memset(&joint, 0, sizeof(mjsJoint));
-
-  joint.type = mjJNT_HINGE;
-  joint.axis[2] = 1;
-  joint.limited = mjLIMITED_AUTO;
-  joint.actfrclimited = mjLIMITED_AUTO;
-  mj_defaultSolRefImp(joint.solref_limit, joint.solimp_limit);
-  mj_defaultSolRefImp(joint.solref_friction, joint.solimp_friction);
-  joint.urdfeffort = -1;
-}
-
-
-
-// default geom attributes
-void mjs_defaultGeom(mjsGeom& geom) {
-  memset(&geom, 0, sizeof(mjsGeom));
-
-  // type
-  geom.type = mjGEOM_SPHERE;
-
-  // frame
-  geom.quat[0] = 1;
-  geom.fromto[0] = mjNAN;
-  mjs_defaultOrientation(geom.alt);
-
-  // contact-related
-  geom.contype = 1;
-  geom.conaffinity = 1;
-  geom.condim = 3;
-  geom.friction[0] = 1;
-  geom.friction[1] = 0.005;
-  geom.friction[2] = 0.0001;
-  geom.solmix = 1.0;
-  mj_defaultSolRefImp(geom.solref, geom.solimp);
-
-  // inertia-related
-  geom.mass = mjNAN;
-  geom.density = 1000;  // water density (1000 Kg / m^3)
-  geom.typeinertia = mjINERTIA_VOLUME;
-
-  // color
-  geom.rgba[0] = geom.rgba[1] = geom.rgba[2] = 0.5f;
-  geom.rgba[3] = 1.0f;
-
-  // fluid forces
-  geom.fluid_coefs[0] = 0.5;   // blunt drag
-  geom.fluid_coefs[1] = 0.25;  // slender drag
-  geom.fluid_coefs[2] = 1.5;   // angular drag
-  geom.fluid_coefs[3] = 1.0;   // kutta lift
-  geom.fluid_coefs[4] = 1.0;   // magnus lift
-
-  // other
-  geom.fitscale = 1;
-}
-
-
-
-// default site attributes
-void mjs_defaultSite(mjsSite& site) {
-  // set everything to 0 / false / NULL
-  memset(&site, 0, sizeof(mjsSite));
-
-  // type
-  site.type = mjGEOM_SPHERE;
-
-  // frame
-  site.quat[0] = 1;
-  site.size[0] = site.size[1] = site.size[2] = 0.005;
-  site.fromto[0] = mjNAN;
-  mjs_defaultOrientation(site.alt);
-
-  // color
-  site.rgba[0] = site.rgba[1] = site.rgba[2] = 0.5f;
-  site.rgba[3] = 1.0f;
-}
-
-
-
-// default cam attributes
-void mjs_defaultCamera(mjsCamera& cam) {
-  memset(&cam, 0, sizeof(mjsCamera));
-
-  // type
-  cam.mode = mjCAMLIGHT_FIXED;
-
-  // extrinsics
-  cam.quat[0] = 1;
-  mjs_defaultOrientation(cam.alt);
-
-  // intrinsics
-  cam.fovy = 45;
-  cam.ipd = 0.068;
-  cam.resolution[0] = cam.resolution[1] = 1;
-}
-
-
-
-// default light attributes
-void mjs_defaultLight(mjsLight& light) {
-  memset(&light, 0, sizeof(mjsLight));
-
-  // mode
-  light.mode = mjCAMLIGHT_FIXED;
-
-  // intrinsics
-  light.castshadow = 1;
-  light.bulbradius = 0.02;
-  light.active = 1;
-  light.dir[2] = -1;
-  light.attenuation[0] = 1;
-  light.cutoff = 45;
-  light.exponent = 10;
-  light.diffuse[0] = light.diffuse[1] = light.diffuse[2] = 0.7;
-  light.specular[0] = light.specular[1] = light.specular[2] = 0.3;
-}
-
-
-
-// default flex attributes
-void mjs_defaultFlex(mjsFlex& flex) {
-  memset(&flex, 0, sizeof(mjsFlex));
-
-  // set contact defaults
-  flex.contype = 1;
-  flex.conaffinity = 1;
-  flex.condim = 3;
-  mjuu_setvec(flex.friction, 1, 0.005, 0.0001);
-  flex.solmix = 1.0;
-  mj_defaultSolRefImp(flex.solref, flex.solimp);
-
-  // set other defaults
-  flex.dim = 2;
-  flex.radius = 0.005;
-  flex.internal = true;
-  flex.selfcollide = mjFLEXSELF_AUTO;
-  flex.activelayers = 1;
-  flex.rgba[0] = flex.rgba[1] = flex.rgba[2] = 0.5f;
-  flex.rgba[3] = 1.0f;
-}
-
-
-
-// default mesh attributes
-void mjs_defaultMesh(mjsMesh& mesh) {
-  memset(&mesh, 0, sizeof(mjsMesh));
-  mjuu_setvec(mesh.refpos, 0, 0, 0);
-  mjuu_setvec(mesh.refquat, 1, 0, 0, 0);
-  mjuu_setvec(mesh.scale, 1, 1, 1);
-  mesh.smoothnormal = false;
-}
-
-
-
-// default height field attributes
-void mjs_defaultHField(mjsHField& hfield) {
-  memset(&hfield, 0, sizeof(mjsHField));
-}
-
-
-
-// default skin attributes
-void mjs_defaultSkin(mjsSkin& skin) {
-  memset(&skin, 0, sizeof(mjsSkin));
-  skin.rgba[0] = skin.rgba[1] = skin.rgba[2] = 0.5f;
-  skin.rgba[3] = 1.0f;
-  skin.inflate = 0;
-  skin.group = 0;
-}
-
-
-
-// default texture attributes
-void mjs_defaultTexture(mjsTexture& texture) {
-  memset(&texture, 0, sizeof(mjsTexture));
-  texture.type = mjTEXTURE_CUBE;
-  mjuu_setvec(texture.rgb1, 0.8, 0.8, 0.8);
-  mjuu_setvec(texture.rgb2, 0.5, 0.5, 0.5);
-  mjuu_setvec(texture.markrgb, 0, 0, 0);
-  texture.random = 0.01;
-  texture.gridsize[0] = texture.gridsize[1] = 1;
-  mujoco::util::strcpy_arr(texture.gridlayout, "............");
-}
-
-
-
-// default material attributes
-void mjs_defaultMaterial(mjsMaterial& material) {
-  memset(&material, 0, sizeof(mjsMaterial));
-  material.texuniform = false;
-  material.texrepeat[0] = material.texrepeat[1] = 1;
-  material.emission = 0;
-  material.specular = 0.5;
-  material.shininess = 0.5;
-  material.reflectance = 0;
-  material.metallic = 1.0;
-  material.roughness = 1.0;
-  material.rgba[0] = material.rgba[1] = material.rgba[2] = material.rgba[3] = 1;
-}
-
-
-
-// default pair attributes
-void mjs_defaultPair(mjsPair& pair) {
-  memset(&pair, 0, sizeof(mjsPair));
-  pair.condim = 3;
-  mj_defaultSolRefImp(pair.solref, pair.solimp);
-  pair.friction[0] = 1;
-  pair.friction[1] = 1;
-  pair.friction[2] = 0.005;
-  pair.friction[3] = 0.0001;
-  pair.friction[4] = 0.0001;
-}
-
-
-
-// default equality attributes
-void mjs_defaultEquality(mjsEquality& equality) {
-  memset(&equality, 0, sizeof(mjsEquality));
-  equality.type = mjEQ_CONNECT;
-  equality.active = 1;
-  mj_defaultSolRefImp(equality.solref, equality.solimp);
-  equality.data[1] = 1;
-  equality.data[10] = 1;  // torque:force ratio
-}
-
-
-
-// default tendon attributes
-void mjs_defaultTendon(mjsTendon& tendon) {
-  memset(&tendon, 0, sizeof(mjsTendon));
-  tendon.limited = mjLIMITED_AUTO;
-  tendon.springlength[0] = tendon.springlength[1] = -1;
-  mj_defaultSolRefImp(tendon.solref_limit, tendon.solimp_limit);
-  mj_defaultSolRefImp(tendon.solref_friction, tendon.solimp_friction);
-  tendon.width = 0.003;
-  tendon.rgba[0] = tendon.rgba[1] = tendon.rgba[2] = 0.5f;
-  tendon.rgba[3] = 1.0f;
-}
-
-
-
-// default actuator attributes
-void mjs_defaultActuator(mjsActuator& actuator) {
-  memset(&actuator, 0, sizeof(mjsActuator));
-
-  // gain, bias
-  actuator.gaintype = mjGAIN_FIXED;
-  actuator.gainprm[0] = 1;
-  actuator.biastype = mjBIAS_NONE;
-
-  // activation state
-  actuator.dyntype = mjDYN_NONE;
-  actuator.dynprm[0] = 1;
-  actuator.actdim = -1;
-
-  // transmission
-  actuator.trntype = mjTRN_UNDEFINED;
-  actuator.gear[0] = 1;
-
-  // input/output clamping
-  actuator.ctrllimited = mjLIMITED_AUTO;
-  actuator.forcelimited = mjLIMITED_AUTO;
-  actuator.actlimited = mjLIMITED_AUTO;
-}
-
-
-
-// default sensor attributes
-void mjs_defaultSensor(mjsSensor& sensor) {
-  memset(&sensor, 0, sizeof(mjsSensor));
-  sensor.type = mjSENS_TOUCH;
-  sensor.datatype = mjDATATYPE_REAL;
-  sensor.needstage = mjSTAGE_ACC;
-}
-
-
-
-// Default numeric attributes.
-void mjs_defaultNumeric(mjsNumeric& numeric) {
-  memset(&numeric, 0, sizeof(mjsNumeric));
-}
-
-
-
-// Default text attributes.
-void mjs_defaultText(mjsText& text) {
-  memset(&text, 0, sizeof(mjsText));
-}
-
-
-
-// Default tuple attributes.
-void mjs_defaultTuple(mjsTuple& tuple) {
-  memset(&tuple, 0, sizeof(mjsTuple));
-}
-
-
-
-// Default keyframe attributes.
-void mjs_defaultKey(mjsKey& key) {
-  memset(&key, 0, sizeof(mjsKey));
-}
-
-
-
-// default plugin attributes
-void mjs_defaultPlugin(mjsPlugin& plugin) {
-  memset(&plugin, 0, sizeof(mjsPlugin));
-  plugin.plugin_slot = -1;
-}
-
diff --git a/src/user/user_mesh.cc b/src/user/user_mesh.cc
index ed2f7d72..00103c73 100644
--- a/src/user/user_mesh.cc
+++ b/src/user/user_mesh.cc
@@ -131,7 +131,7 @@ static void ReadFromBuffer(T* dst, const char* src) {
 //------------------ class mjCMesh implementation --------------------------------------------------
 
 mjCMesh::mjCMesh(mjCModel* _model, mjCDef* _def) {
-  mjs_defaultMesh(spec);
+  mjs_defaultMesh(&spec);
   elemtype = mjOBJ_MESH;
 
   // clear internal variables
@@ -2059,7 +2059,7 @@ void mjCMesh::MakeCenter(void) {
 
 // constructor
 mjCSkin::mjCSkin(mjCModel* _model) {
-  mjs_defaultSkin(spec);
+  mjs_defaultSkin(&spec);
   elemtype = mjOBJ_SKIN;
 
   // set model pointer
@@ -2484,7 +2484,7 @@ constexpr int eledge[3][6][2] = {{{ 0,  1}, {-1, -1}, {-1, -1},
 
 // constructor
 mjCFlex::mjCFlex(mjCModel* _model) {
-  mjs_defaultFlex(spec);
+  mjs_defaultFlex(&spec);
   elemtype = mjOBJ_FLEX;
 
   // set model
diff --git a/src/user/user_model.cc b/src/user/user_model.cc
index 6c1e547c..e8a3791e 100644
--- a/src/user/user_model.cc
+++ b/src/user/user_model.cc
@@ -83,7 +83,7 @@ static void copyvec(T1* dest, T2* src, int n) {
 
 // constructor
 mjCModel::mjCModel() {
-  mjs_defaultSpec(spec);
+  mjs_defaultSpec(&spec);
   elemtype = mjOBJ_UNKNOWN;
   spec_comment_.clear();
   spec_modelfiledir_.clear();
diff --git a/src/user/user_objects.cc b/src/user/user_objects.cc
index 35db7cbd..636b4921 100644
--- a/src/user/user_objects.cc
+++ b/src/user/user_objects.cc
@@ -474,18 +474,18 @@ mjCDef::mjCDef(void) {
   name.clear();
   parentid = -1;
   childid.clear();
-  mjs_defaultJoint(joint.spec);
-  mjs_defaultGeom(geom.spec);
-  mjs_defaultSite(site.spec);
-  mjs_defaultCamera(camera.spec);
-  mjs_defaultLight(light.spec);
-  mjs_defaultFlex(flex.spec);
-  mjs_defaultMesh(mesh.spec);
-  mjs_defaultMaterial(material.spec);
-  mjs_defaultPair(pair.spec);
-  mjs_defaultEquality(equality.spec);
-  mjs_defaultTendon(tendon.spec);
-  mjs_defaultActuator(actuator.spec);
+  mjs_defaultJoint(&joint.spec);
+  mjs_defaultGeom(&geom.spec);
+  mjs_defaultSite(&site.spec);
+  mjs_defaultCamera(&camera.spec);
+  mjs_defaultLight(&light.spec);
+  mjs_defaultFlex(&flex.spec);
+  mjs_defaultMesh(&mesh.spec);
+  mjs_defaultMaterial(&material.spec);
+  mjs_defaultPair(&pair.spec);
+  mjs_defaultEquality(&equality.spec);
+  mjs_defaultTendon(&tendon.spec);
+  mjs_defaultActuator(&actuator.spec);
 
   // make sure all the pointers are local
   PointToLocal();
@@ -675,7 +675,7 @@ mjCBody::mjCBody(mjCModel* _model) {
   // set model pointer
   model = _model;
 
-  mjs_defaultBody(spec);
+  mjs_defaultBody(&spec);
   elemtype = mjOBJ_BODY;
   parentid = -1;
   weldid = -1;
@@ -1451,7 +1451,7 @@ void mjCBody::Compile(void) {
 
 // initialize frame
 mjCFrame::mjCFrame(mjCModel* _model, mjCFrame* _frame) {
-  mjs_defaultFrame(spec);
+  mjs_defaultFrame(&spec);
   elemtype = mjOBJ_FRAME;
   compiled = false;
   model = _model;
@@ -1568,7 +1568,7 @@ void mjCFrame::Compile() {
 
 // initialize default joint
 mjCJoint::mjCJoint(mjCModel* _model, mjCDef* _def) {
-  mjs_defaultJoint(spec);
+  mjs_defaultJoint(&spec);
   elemtype = mjOBJ_JOINT;
 
   // clear internal variables
@@ -1758,7 +1758,7 @@ int mjCJoint::Compile(void) {
 
 // initialize default geom
 mjCGeom::mjCGeom(mjCModel* _model, mjCDef* _def) {
-  mjs_defaultGeom(spec);
+  mjs_defaultGeom(&spec);
   elemtype = mjOBJ_GEOM;
 
   mass_ = 0;
@@ -2405,7 +2405,7 @@ void mjCGeom::Compile(void) {
 
 // initialize default site
 mjCSite::mjCSite(mjCModel* _model, mjCDef* _def) {
-  mjs_defaultSite(spec);
+  mjs_defaultSite(&spec);
   elemtype = mjOBJ_SITE;
 
   // clear internal variables
@@ -2558,7 +2558,7 @@ void mjCSite::Compile(void) {
 
 // initialize defaults
 mjCCamera::mjCCamera(mjCModel* _model, mjCDef* _def) {
-  mjs_defaultCamera(spec);
+  mjs_defaultCamera(&spec);
   elemtype = mjOBJ_CAMERA;
 
   // clear private variables
@@ -2712,7 +2712,7 @@ void mjCCamera::Compile(void) {
 
 // initialize defaults
 mjCLight::mjCLight(mjCModel* _model, mjCDef* _def) {
-  mjs_defaultLight(spec);
+  mjs_defaultLight(&spec);
   elemtype = mjOBJ_LIGHT;
 
   // clear private variables
@@ -2815,7 +2815,7 @@ void mjCLight::Compile(void) {
 
 // constructor
 mjCHField::mjCHField(mjCModel* _model) {
-  mjs_defaultHField(spec);
+  mjs_defaultHField(&spec);
   elemtype = mjOBJ_HFIELD;
 
   // set model pointer
@@ -3066,7 +3066,7 @@ void mjCHField::Compile(const mjVFS* vfs) {
 
 // initialize defaults
 mjCTexture::mjCTexture(mjCModel* _model) {
-  mjs_defaultTexture(spec);
+  mjs_defaultTexture(&spec);
   elemtype = mjOBJ_TEXTURE;
 
   // set model pointer
@@ -3797,7 +3797,7 @@ void mjCTexture::Compile(const mjVFS* vfs) {
 
 // initialize defaults
 mjCMaterial::mjCMaterial(mjCModel* _model, mjCDef* _def) {
-  mjs_defaultMaterial(spec);
+  mjs_defaultMaterial(&spec);
   elemtype = mjOBJ_MATERIAL;
 
   // clear internal
@@ -3880,7 +3880,7 @@ void mjCMaterial::Compile(void) {
 
 // constructor
 mjCPair::mjCPair(mjCModel* _model, mjCDef* _def) {
-  mjs_defaultPair(spec);
+  mjs_defaultPair(&spec);
   elemtype = mjOBJ_PAIR;
 
   // set defaults
@@ -4245,7 +4245,7 @@ void mjCBodyPair::Compile(void) {
 
 // initialize default constraint
 mjCEquality::mjCEquality(mjCModel* _model, mjCDef* _def) {
-  mjs_defaultEquality(spec);
+  mjs_defaultEquality(&spec);
   elemtype = mjOBJ_EQUALITY;
 
   // clear internal variables
@@ -4406,7 +4406,7 @@ void mjCEquality::Compile(void) {
 
 // constructor
 mjCTendon::mjCTendon(mjCModel* _model, mjCDef* _def) {
-  mjs_defaultTendon(spec);
+  mjs_defaultTendon(&spec);
   elemtype = mjOBJ_TENDON;
 
   // clear internal variables
@@ -4883,7 +4883,7 @@ void mjCWrap::ResolveReferences(const mjCModel* m) {
 
 // initialize defaults
 mjCActuator::mjCActuator(mjCModel* _model, mjCDef* _def) {
-  mjs_defaultActuator(spec);
+  mjs_defaultActuator(&spec);
   elemtype = mjOBJ_ACTUATOR;
 
   // clear private variables
@@ -5226,7 +5226,7 @@ void mjCActuator::Compile(void) {
 
 // initialize defaults
 mjCSensor::mjCSensor(mjCModel* _model) {
-  mjs_defaultSensor(spec);
+  mjs_defaultSensor(&spec);
   elemtype = mjOBJ_SENSOR;
 
   // set model
@@ -5708,7 +5708,7 @@ void mjCSensor::Compile(void) {
 
 // constructor
 mjCNumeric::mjCNumeric(mjCModel* _model) {
-  mjs_defaultNumeric(spec);
+  mjs_defaultNumeric(&spec);
   elemtype = mjOBJ_NUMERIC;
 
   // set model pointer
@@ -5797,7 +5797,7 @@ void mjCNumeric::Compile(void) {
 
 // constructor
 mjCText::mjCText(mjCModel* _model) {
-  mjs_defaultText(spec);
+  mjs_defaultText(&spec);
   elemtype = mjOBJ_TEXT;
 
   // set model pointer
@@ -5874,7 +5874,7 @@ void mjCText::Compile(void) {
 
 // constructor
 mjCTuple::mjCTuple(mjCModel* _model) {
-  mjs_defaultTuple(spec);
+  mjs_defaultTuple(&spec);
   elemtype = mjOBJ_TUPLE;
 
   // set model pointer
@@ -6007,7 +6007,7 @@ void mjCTuple::Compile(void) {
 
 // constructor
 mjCKey::mjCKey(mjCModel* _model) {
-  mjs_defaultKey(spec);
+  mjs_defaultKey(&spec);
   elemtype = mjOBJ_KEY;
 
   // set model pointer
@@ -6191,7 +6191,7 @@ mjCPlugin::mjCPlugin(mjCModel* _model) {
   instance_name.clear();
 
   // public interface
-  mjs_defaultPlugin(spec);
+  mjs_defaultPlugin(&spec);
   elemtype = mjOBJ_PLUGIN;
   spec.name = (mjString)&name;
   spec.instance_name = (mjString)&instance_name;
diff --git a/src/user/user_util.cc b/src/user/user_util.cc
index 35e5568d..a2a91ee9 100644
--- a/src/user/user_util.cc
+++ b/src/user/user_util.cc
@@ -33,10 +33,6 @@ using std::string;
 using std::numeric_limits;
 
 
-// set value of NAN here; needs <limits>
-const double mjNAN = numeric_limits<double>::quiet_NaN();
-
-
 // check if numeric variable is defined
 bool mjuu_defined(const double num) {
   return !isnan(num);
diff --git a/src/user/user_util.h b/src/user/user_util.h
index 2cede571..29ff5dd8 100644
--- a/src/user/user_util.h
+++ b/src/user/user_util.h
@@ -20,7 +20,6 @@
 #include <string_view>
 
 
-extern const double mjNAN;                      // used to mark undefined fields
 const double mjEPS = 1E-14;                     // minimum value in various calculations
 const double mjMINMASS = 1E-6;                  // minimum mass allowed
 
diff --git a/src/xml/xml_native_reader.cc b/src/xml/xml_native_reader.cc
index 1c742a45..1a12d3d8 100644
--- a/src/xml/xml_native_reader.cc
+++ b/src/xml/xml_native_reader.cc
@@ -3394,7 +3394,7 @@ void mjXReader::Body(XMLElement* section, mjsBody* pbody, mjsFrame* frame) {
 
       // store rotation difference
       mjsOrientation alt;
-      mjs_defaultOrientation(alt);
+      mjs_defaultOrientation(&alt);
       alt.type = mjORIENTATION_EULER;
       mjuu_copyvec(alt.euler, euler, 3);
       double rotation[4] = {1, 0, 0, 0};
diff --git a/src/xml/xml_urdf.cc b/src/xml/xml_urdf.cc
index 30739848..e7d281ce 100644
--- a/src/xml/xml_urdf.cc
+++ b/src/xml/xml_urdf.cc
@@ -633,7 +633,7 @@ void mjXURDF::Origin(XMLElement* origin_elem, double* pos, double* quat) {
 
     // orientation
     mjsOrientation alt;
-    mjs_defaultOrientation(alt);
+    mjs_defaultOrientation(&alt);
     if (ReadAttr(temp, "rpy", 3, alt.euler, text)) {
       alt.type = mjORIENTATION_EULER;
       mjs_resolveOrientation(quat, 0, "XYZ", &alt);