diff --git a/src/user/user_composite.cc b/src/user/user_composite.cc
index 806593df..9cc75f58 100644
--- a/src/user/user_composite.cc
+++ b/src/user/user_composite.cc
@@ -1243,7 +1243,7 @@ void mjCComposite::MakeSkin2(mjCModel* model, mjtNum inflate) {
   mjCSkin* skin = model->AddSkin();
   mju::sprintf_arr(txt, "%sSkin", prefix.c_str());
   skin->name = txt;
-  skin->material = skinmaterial;
+  skin->set_material(skinmaterial);
   mjuu_copyvec(skin->rgba, skinrgba, 4);
   skin->inflate = inflate;
   skin->group = skingroup;
@@ -1845,7 +1845,7 @@ void mjCComposite::MakeSkin2Subgrid(mjCModel* model, mjtNum inflate) {
   mjCSkin* skin = model->AddSkin();
   mju::sprintf_arr(txt, "%sSkin", prefix.c_str());
   skin->name = txt;
-  skin->material = skinmaterial;
+  skin->set_material(skinmaterial);
   mjuu_copyvec(skin->rgba, skinrgba, 4);
   skin->inflate = inflate;
   skin->group = skingroup;
@@ -1998,7 +1998,7 @@ void mjCComposite::MakeSkin3(mjCModel* model) {
   mjCSkin* skin = model->AddSkin();
   mju::sprintf_arr(txt, "%sSkin", prefix.c_str());
   skin->name = txt;
-  skin->material = skinmaterial;
+  skin->set_material(skinmaterial);
   mjuu_copyvec(skin->rgba, skinrgba, 4);
   skin->inflate = skininflate;
   skin->group = skingroup;
diff --git a/src/user/user_mesh.cc b/src/user/user_mesh.cc
index 5da757ba..a6373022 100644
--- a/src/user/user_mesh.cc
+++ b/src/user/user_mesh.cc
@@ -1878,7 +1878,7 @@ mjCSkin::mjCSkin(mjCModel* _model) {
 
   // clear data
   file.clear();
-  material.clear();
+  material_.clear();
   rgba[0] = rgba[1] = rgba[2] = 0.5f;
   rgba[3] = 1.0f;
   inflate = 0;
@@ -1903,7 +1903,7 @@ mjCSkin::mjCSkin(mjCModel* _model) {
 // destructor
 mjCSkin::~mjCSkin() {
   file.clear();
-  material.clear();
+  material_.clear();
   vert.clear();
   texcoord.clear();
   face.clear();
@@ -2005,11 +2005,11 @@ void mjCSkin::Compile(const mjVFS* vfs) {
   }
 
   // resolve material name
-  mjCBase* pmat = model->FindObject(mjOBJ_MATERIAL, material);
+  mjCBase* pmat = model->FindObject(mjOBJ_MATERIAL, material_);
   if (pmat) {
     matid = pmat->id;
-  } else if (!material.empty()) {
-      throw mjCError(this, "unkown material '%s' in skin", material.c_str());
+  } else if (!material_.empty()) {
+      throw mjCError(this, "unkown material '%s' in skin", material_.c_str());
   }
 
   // set total vertex weights to 0
@@ -2239,7 +2239,7 @@ mjCFlex::mjCFlex(mjCModel* _model) {
   group = 0;
   edgestiffness = 0;
   edgedamping = 0;
-  material.clear();
+  material_.clear();
   rgba[0] = rgba[1] = rgba[2] = 0.5f;
   rgba[3] = 1.0f;
 
@@ -2314,11 +2314,11 @@ void mjCFlex::Compile(const mjVFS* vfs) {
   }
 
   // resolve material name
-  mjCBase* pmat = model->FindObject(mjOBJ_MATERIAL, material);
+  mjCBase* pmat = model->FindObject(mjOBJ_MATERIAL, material_);
   if (pmat) {
     matid = pmat->id;
-  } else if (!material.empty()) {
-      throw mjCError(this, "unkown material '%s' in flex", material.c_str());
+  } else if (!material_.empty()) {
+      throw mjCError(this, "unkown material '%s' in flex", material_.c_str());
   }
 
   // resolve body ids
diff --git a/src/user/user_model.cc b/src/user/user_model.cc
index d504eabc..7b484944 100644
--- a/src/user/user_model.cc
+++ b/src/user/user_model.cc
@@ -653,8 +653,8 @@ void mjCModel::MakeLists(mjCBody* body) {
 template <class T>
 void mjCModel::DeleteMaterial(std::vector<T*>& list, std::string_view name) {
   for (T* plist : list) {
-    if (name.empty() || plist->material == name) {
-      plist->material.clear();
+    if (name.empty() || plist->material_ == name) {
+      plist->material_.clear();
     }
   }
 }
@@ -775,12 +775,12 @@ void mjCModel::IndexAssets(bool discard) {
     mjCGeom* pgeom = geoms[i];
 
     // find material by name
-    if (!pgeom->material.empty()) {
-      mjCBase* m = FindObject(mjOBJ_MATERIAL, pgeom->material);
+    if (!pgeom->material_.empty()) {
+      mjCBase* m = FindObject(mjOBJ_MATERIAL, pgeom->material_);
       if (m) {
         pgeom->matid = m->id;
       } else {
-        throw mjCError(pgeom, "material '%s' not found in geom %d", pgeom->material.c_str(), i);
+        throw mjCError(pgeom, "material '%s' not found in geom %d", pgeom->material_.c_str(), i);
       }
     }
 
@@ -822,12 +822,12 @@ void mjCModel::IndexAssets(bool discard) {
     mjCSkin* pskin = skins[i];
 
     // find material by name
-    if (!pskin->material.empty()) {
-      mjCBase* m = FindObject(mjOBJ_MATERIAL, pskin->material);
+    if (!pskin->material_.empty()) {
+      mjCBase* m = FindObject(mjOBJ_MATERIAL, pskin->material_);
       if (m) {
         pskin->matid = m->id;
       } else {
-        throw mjCError(pskin, "material '%s' not found in skin %d", pskin->material.c_str(), i);
+        throw mjCError(pskin, "material '%s' not found in skin %d", pskin->material_.c_str(), i);
       }
     }
   }
@@ -837,12 +837,12 @@ void mjCModel::IndexAssets(bool discard) {
     mjCSite* psite = sites[i];
 
     // find material by name
-    if (!psite->material.empty()) {
-      mjCBase* m = FindObject(mjOBJ_MATERIAL, psite->material);
+    if (!psite->material_.empty()) {
+      mjCBase* m = FindObject(mjOBJ_MATERIAL, psite->get_material());
       if (m) {
         psite->matid = m->id;
       } else {
-        throw mjCError(psite, "material '%s' not found in site %d", psite->material.c_str(), i);
+        throw mjCError(psite, "material '%s' not found in site %d", psite->material_.c_str(), i);
       }
     }
   }
@@ -852,12 +852,12 @@ void mjCModel::IndexAssets(bool discard) {
     mjCTendon* pten = tendons[i];
 
     // find material by name
-    if (!pten->material.empty()) {
-      mjCBase* m = FindObject(mjOBJ_MATERIAL, pten->material);
+    if (!pten->material_.empty()) {
+      mjCBase* m = FindObject(mjOBJ_MATERIAL, pten->material_);
       if (m) {
         pten->matid = m->id;
       } else {
-        throw mjCError(pten, "material '%s' not found in tendon %d", pten->material.c_str(), i);
+        throw mjCError(pten, "material '%s' not found in tendon %d", pten->material_.c_str(), i);
       }
     }
   }
@@ -1718,7 +1718,7 @@ void mjCModel::CopyTree(mjModel* m) {
       copyvec(m->site_size+3*sid, ps->size, 3);
       copyvec(m->site_pos+3*sid, ps->pos, 3);
       copyvec(m->site_quat+4*sid, ps->quat, 4);
-      copyvec(m->site_user+nuser_site*sid, ps->userdata.data(), nuser_site);
+      copyvec(m->site_user+nuser_site*sid, ps->userdata_.data(), nuser_site);
       copyvec(m->site_rgba+4*sid, ps->rgba, 4);
 
       // determine sameframe
@@ -2901,7 +2901,7 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
   if (nuser_site == -1) {
     nuser_site = 0;
     for (int i=0; i<sites.size(); i++) {
-      nuser_site = mjMAX(nuser_site, sites[i]->spec.userdata.size());
+      nuser_site = mjMAX(nuser_site, sites[i]->spec_userdata_.size());
     }
   }
   if (nuser_cam == -1) {
@@ -3407,7 +3407,7 @@ bool mjCModel::CopyBack(const mjModel* m) {
     copyvec(sites[i]->rgba, m->site_rgba+4*i, 4);
 
     if (nuser_site) {
-      copyvec(sites[i]->userdata.data(), m->site_user + nuser_site*i, nuser_site);
+      copyvec(sites[i]->userdata_.data(), m->site_user + nuser_site*i, nuser_site);
     }
   }
 
diff --git a/src/user/user_objects.cc b/src/user/user_objects.cc
index 6db1c8f1..bd539ec7 100644
--- a/src/user/user_objects.cc
+++ b/src/user/user_objects.cc
@@ -497,7 +497,7 @@ void mjCDef::Compile(const mjCModel* model) {
   // enforce length of all default userdata arrays
   joint.userdata.resize(model->nuser_jnt);
   geom.userdata.resize(model->nuser_geom);
-  site.userdata.resize(model->nuser_site);
+  site.userdata_.resize(model->nuser_site);
   camera.userdata.resize(model->nuser_cam);
   tendon.userdata.resize(model->nuser_tendon);
   actuator.userdata.resize(model->nuser_actuator);
@@ -1356,7 +1356,7 @@ mjCGeom::mjCGeom(mjCModel* _model, mjCDef* _def) {
   density = 1000;             // water density (1000 kg / m^3)
   meshname.clear();
   fitscale = 1;
-  material.clear();
+  material_.clear();
   rgba[0] = rgba[1] = rgba[2] = 0.5f;
   rgba[3] = 1.0f;
   userdata.clear();
@@ -1931,14 +1931,13 @@ mjCSite::mjCSite(mjCModel* _model, mjCDef* _def) {
   spec.group = 0;
   mjuu_setvec(spec.quat, 1, 0, 0, 0);
   mjuu_setvec(spec.pos, 0, 0, 0);
-  spec.material.clear();
+  spec.material = nullptr;
   spec.rgba[0] = spec.rgba[1] = spec.rgba[2] = 0.5f;
   spec.rgba[3] = 1.0f;
   spec.fromto[0] = mjNAN;
-  spec.userdata.clear();
+  spec.userdata = nullptr;
 
   // clear internal variables
-  spec.material.clear();
   body = 0;
   matid = -1;
 
@@ -1960,6 +1959,10 @@ mjCSite::mjCSite(mjCModel* _model, mjCDef* _def) {
 
 void mjCSite::CopyFromSpec() {
   *static_cast<mjmSite*>(this) = spec;
+  userdata_ = spec_userdata_;
+  material_ = spec_material_;
+  userdata = userdata_.data();
+  material = material_.data();
 }
 
 
@@ -1969,11 +1972,11 @@ void mjCSite::Compile(void) {
   CopyFromSpec();
 
   // resize userdata
-  if (userdata.size() > model->nuser_site) {
+  if (userdata_.size() > model->nuser_site) {
     throw mjCError(this, "user has more values than nuser_site in site '%s' (id = %d)",
                    name.c_str(), id);
   }
-  userdata.resize(model->nuser_site);
+  userdata_.resize(model->nuser_site);
 
   // check type
   if (type<0 || type>=mjNGEOMTYPES) {
@@ -3500,7 +3503,7 @@ void mjCEquality::Compile(void) {
 mjCTendon::mjCTendon(mjCModel* _model, mjCDef* _def) {
   // tendon defaults
   group = 0;
-  material.clear();
+  material_.clear();
   width = 0.003;
   limited = 2;
   range[0] = 0;
diff --git a/src/user/user_objects.h b/src/user/user_objects.h
index 5d0daff3..5fac7b3a 100644
--- a/src/user/user_objects.h
+++ b/src/user/user_objects.h
@@ -390,6 +390,9 @@ class mjCGeom : public mjCBase {
   bool IsVisual(void) const { return visual_; }
   void SetNotVisual(void) { visual_ = false; }
 
+  void set_material(std::string _material) { material_ = _material; }
+  std::string& get_material() { return material_; }
+
   // Compute all coefs modeling the interaction with the surrounding fluid.
   void SetFluidCoefs(void);
   // Compute the kappa coefs of the added inertia due to the surrounding fluid.
@@ -418,7 +421,6 @@ class mjCGeom : public mjCBase {
   std::string hfieldname;         // hfield attached to geom
   std::string meshname;           // mesh attached to geom
   double fitscale;                // scale mesh uniformly
-  std::string material;           // name of material used for rendering
   std::vector<double> userdata;   // user data
   float rgba[4];                  // rgba when material is omitted
   mjtMeshType typeinertia;        // selects between surface and volume inertia
@@ -440,6 +442,7 @@ class mjCGeom : public mjCBase {
   double GetRBound(void);             // compute bounding sphere radius
   void ComputeAABB(void);             // compute axis-aligned bounding box
 
+  std::string material_;              // name of material used for rendering
   bool visual_;                       // true: geom does not collide and is unreferenced
   int matid;                          // id of geom's material
   mjCMesh* mesh;                      // geom's mesh
@@ -461,8 +464,8 @@ typedef struct _mjmSite {
   double size[3];                 // geom size for rendering
   double pos[3];                  // position
   double quat[4];                 // orientation
-  std::string material;           // name of material for rendering
-  std::vector<double> userdata;   // user data
+  char* material;                 // name of material for rendering
+  double* userdata;               // user data
   float rgba[4];                  // rgba when material is omitted
   double fromto[6];               // alternative for capsule, cylinder, box, ellipsoid
   mjCAlternative alt;             // alternative orientation specification
@@ -478,12 +481,30 @@ class mjCSite : public mjCBase, private mjmSite {
  public:
   mjmSite spec;                   // variables set by user
 
+  void set_material(std::string _material) {
+    spec_material_ = _material;
+    spec.material = spec_material_.data();
+  }
+  void set_userdata(std::vector<double> _userdata) {
+    spec_userdata_ = _userdata;
+    spec.userdata = spec_userdata_.data();
+  }
+  std::vector<double>& get_userdata() { return userdata_; }
+  std::string& get_material() { return material_; }
+
   // variables computed by 'compile' and 'mjCBody::addSite'
  private:
   mjCSite(mjCModel* = 0, mjCDef* = 0);    // constructor
   void Compile(void);                     // compiler
   void CopyFromSpec();                    // copy spec into attributes
 
+  // variable-size data
+  std::string material_;
+  std::vector<double> userdata_;
+
+  std::string spec_material_;
+  std::vector<double> spec_userdata_;
+
   mjCBody* body;                  // site's body
   int matid;                      // material id for rendering
 };
@@ -573,6 +594,9 @@ class mjCFlex: public mjCBase {
   friend class mjXWriter;
 
  public:
+  void set_material(std::string _material) { material_ = _material; }
+  std::string& get_material() { return material_; }
+
   // contact properties
   int contype;                    // contact type
   int conaffinity;                // contact affinity
@@ -595,7 +619,6 @@ class mjCFlex: public mjCBase {
   int group;                      // group for visualizatioh
   double edgestiffness;           // edge stiffness
   double edgedamping;             // edge damping
-  std::string material;           // name of material used for rendering
   float rgba[4];                  // rgba when material is omitted
 
   std::vector<std::string> vertbody;  // vertex body names
@@ -626,6 +649,7 @@ class mjCFlex: public mjCBase {
   std::vector<int> evpair;                // element-vertex pairs
   std::vector<mjtNum> vertxpos;           // global vertex positions
   mjCBoundingVolumeHierarchy tree;        // bounding volume hierarchy
+  std::string material_;                  // name of material used for rendering
 };
 
 
@@ -799,9 +823,10 @@ class mjCSkin: public mjCBase {
 
  public:
   std::string get_file() const { return file; }
+  void set_material(std::string _material) { material_ = _material; }
+  std::string& get_material() { return material_; }
 
   std::string file;                   // skin file
-  std::string material;               // name of material used for rendering
   float rgba[4];                      // rgba when material is omitted
   float inflate;                      // inflate in normal direction
   int group;                          // group for visualization
@@ -824,6 +849,7 @@ class mjCSkin: public mjCBase {
   void Compile(const mjVFS* vfs);             // compiler
   void LoadSKN(mjResource* resource);         // load skin in SKN BIN format
 
+  std::string material_;              // name of material used for rendering
   int matid;                          // material id
   std::vector<int> bodyid;            // body ids
 };
@@ -1059,6 +1085,9 @@ class mjCTendon : public mjCBase {
   friend class mjXWriter;
 
  public:
+  void set_material(std::string _material) { material_ = _material; }
+  std::string& get_material() { return material_; }
+
   // API for adding wrapping objects
   void WrapSite(std::string name, int row=-1, int col=-1);                    // site
   void WrapGeom(std::string name, std::string side, int row=-1, int col=-1);  // geom
@@ -1071,7 +1100,6 @@ class mjCTendon : public mjCBase {
 
   // variables set by user
   int group;                       // group for visualization
-  std::string material;            // name of material for rendering
   int limited;                     // does tendon have limits: 0 false, 1 true, 2 auto
   double width;                    // width for rendering
   mjtNum solref_limit[mjNREF];     // solver reference: tendon limits
@@ -1092,6 +1120,7 @@ class mjCTendon : public mjCBase {
   ~mjCTendon();                               // destructor
   void Compile(void);                         // compiler
 
+  std::string material_;          // name of material for rendering
   int matid;                      // material id for rendering
   std::vector<mjCWrap*> path;     // wrapping objects
 };
diff --git a/src/xml/xml_native_reader.cc b/src/xml/xml_native_reader.cc
index 6df5e7a5..9e9ba1d5 100644
--- a/src/xml/xml_native_reader.cc
+++ b/src/xml/xml_native_reader.cc
@@ -1279,7 +1279,7 @@ void mjXReader::OneFlex(XMLElement* elem, mjCFlex* pflex) {
   // read attributes
   ReadAttrTxt(elem, "name", pflex->name);
   ReadAttr(elem, "radius", 1, &pflex->radius, text);
-  ReadAttrTxt(elem, "material", pflex->material);
+  ReadAttrTxt(elem, "material", pflex->get_material());
   ReadAttr(elem, "rgba", 4, pflex->rgba, text);
   if (MapValue(elem, "flatskin", &n, bool_map, 2)) {
     pflex->flatskin = (n==1);
@@ -1381,7 +1381,7 @@ void mjXReader::OneSkin(XMLElement* elem, mjCSkin* pskin) {
   // read attributes
   ReadAttrTxt(elem, "name", pskin->name);
   ReadAttrTxt(elem, "file", pskin->file);
-  ReadAttrTxt(elem, "material", pskin->material);
+  ReadAttrTxt(elem, "material", pskin->get_material());
   ReadAttrInt(elem, "group", &pskin->group);
   if (pskin->group<0 || pskin->group>=mjNGROUP) {
     throw mjXError(elem, "skin group must be between 0 and 5");
@@ -1528,7 +1528,7 @@ void mjXReader::OneGeom(XMLElement* elem, mjCGeom* pgeom) {
   ReadAttrTxt(elem, "hfield", pgeom->hfieldname);
   ReadAttrTxt(elem, "mesh", pgeom->meshname);
   ReadAttr(elem, "fitscale", 1, &pgeom->fitscale, text);
-  ReadAttrTxt(elem, "material", pgeom->material);
+  ReadAttrTxt(elem, "material", pgeom->get_material());
   ReadAttr(elem, "rgba", 4, pgeom->rgba, text);
   if (MapValue(elem, "fluidshape", &n, fluid_map, 2)) {
     pgeom->fluid_switch = (n == 1);
@@ -1567,6 +1567,8 @@ void mjXReader::OneSite(XMLElement* elem, mjCSite* site) {
   int n;
   string text;
   mjmSite* psite = &site->spec;
+  std::vector<double> userdata;
+  std::string material;
 
   // read attributes
   ReadAttrTxt(elem, "name", site->name);
@@ -1578,13 +1580,15 @@ void mjXReader::OneSite(XMLElement* elem, mjCSite* site) {
   ReadAttrInt(elem, "group", &psite->group);
   ReadAttr(elem, "pos", 3, psite->pos, text);
   ReadQuat(elem, "quat", psite->quat, text);
-  ReadAttrTxt(elem, "material", psite->material);
+  ReadAttrTxt(elem, "material", material);
   ReadAttr(elem, "rgba", 4, psite->rgba, text);
   ReadAttr(elem, "fromto", 6, psite->fromto, text);
   ReadAlternative(elem, psite->alt);
+  ReadVector(elem, "user", userdata, text);
 
-  // read userdata
-  ReadVector(elem, "user", psite->userdata, text);
+  // set variable-size attributes
+  site->set_userdata(userdata);
+  site->set_material(material);
 
   GetXMLPos(elem, site);
 }
@@ -1773,7 +1777,7 @@ void mjXReader::OneTendon(XMLElement* elem, mjCTendon* pten) {
   ReadAttrTxt(elem, "name", pten->name);
   ReadAttrTxt(elem, "class", pten->classname);
   ReadAttrInt(elem, "group", &pten->group);
-  ReadAttrTxt(elem, "material", pten->material);
+  ReadAttrTxt(elem, "material", pten->get_material());
   MapValue(elem, "limited", &pten->limited, TFAuto_map, 3);
   ReadAttr(elem, "width", 1, &pten->width, text);
   ReadAttr(elem, "solreflimit", mjNREF, pten->solref_limit, text, false, false);
@@ -2158,7 +2162,7 @@ void mjXReader::OneComposite(XMLElement* elem, mjCBody* pbody, mjCDef* def) {
     ReadAttr(egeom, "solimp", mjNIMP, comp.def[0].geom.solimp, text, false, false);
     ReadAttr(egeom, "margin", 1, &comp.def[0].geom.margin, text);
     ReadAttr(egeom, "gap", 1, &comp.def[0].geom.gap, text);
-    ReadAttrTxt(egeom, "material", comp.def[0].geom.material);
+    ReadAttrTxt(egeom, "material", comp.def[0].geom.get_material());
     ReadAttr(egeom, "rgba", 4, comp.def[0].geom.rgba, text);
     ReadAttr(egeom, "mass", 1, &comp.def[0].geom._mass, text);
     ReadAttr(egeom, "density", 1, &comp.def[0].geom.density, text);
@@ -2167,11 +2171,13 @@ void mjXReader::OneComposite(XMLElement* elem, mjCBody* pbody, mjCDef* def) {
   // site
   XMLElement* esite = elem->FirstChildElement("site");
   if (esite) {
+    std::string material;
     mjmSite& dsite = comp.def[0].site.spec;
     ReadAttr(esite, "size", 3, dsite.size, text, false, false);
     ReadAttrInt(esite, "group", &dsite.group);
-    ReadAttrTxt(esite, "material", dsite.material);
+    ReadAttrTxt(esite, "material", material);
     ReadAttr(esite, "rgba", 4, dsite.rgba, text);
+    comp.def[0].site.set_material(material);
   }
 
   // joint
@@ -2249,7 +2255,7 @@ void mjXReader::OneComposite(XMLElement* elem, mjCBody* pbody, mjCDef* def) {
     ReadAttr(eten, "stiffness", 1, &comp.def[kind].tendon.stiffness, text);
     ReadAttr(eten, "damping", 1, &comp.def[kind].tendon.damping, text);
     ReadAttr(eten, "frictionloss", 1, &comp.def[kind].tendon.frictionloss, text);
-    ReadAttrTxt(eten, "material", comp.def[kind].tendon.material);
+    ReadAttrTxt(eten, "material", comp.def[kind].tendon.get_material());
     ReadAttr(eten, "rgba", 4, comp.def[kind].tendon.rgba, text);
     ReadAttr(eten, "width", 1, &comp.def[kind].tendon.width, text);
 
@@ -2303,7 +2309,7 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjCBody* pbody) {
   ReadAttr(elem, "mass", 1, &fcomp.mass, text);
   ReadAttr(elem, "inertiabox", 1, &fcomp.inertiabox, text);
   ReadAttrTxt(elem, "file", fcomp.file);
-  ReadAttrTxt(elem, "material", fcomp.def.flex.material);
+  ReadAttrTxt(elem, "material", fcomp.def.flex.get_material());
   ReadAttr(elem, "rgba", 4, fcomp.def.flex.rgba, text);
   if (MapValue(elem, "flatskin", &n, bool_map, 2)) {
     fcomp.def.flex.flatskin = (n==1);
diff --git a/src/xml/xml_native_writer.cc b/src/xml/xml_native_writer.cc
index da41f82c..98f9f793 100644
--- a/src/xml/xml_native_writer.cc
+++ b/src/xml/xml_native_writer.cc
@@ -90,8 +90,8 @@ void mjXWriter::OneFlex(XMLElement* elem, mjCFlex* pflex) {
   // common attributes
   WriteAttrTxt(elem, "name", pflex->name);
   WriteAttr(elem, "radius", 1, &pflex->radius, &defflex.radius);
-  if (pflex->material != defflex.material) {
-    WriteAttrTxt(elem, "material", pflex->material);
+  if (pflex->get_material() != defflex.get_material()) {
+    WriteAttrTxt(elem, "material", pflex->get_material());
   }
   WriteAttr(elem, "rgba", 4, pflex->rgba, defflex.rgba);
   WriteAttrKey(elem, "flatskin", bool_map, 2, pflex->flatskin, defflex.flatskin);
@@ -204,7 +204,7 @@ void mjXWriter::OneSkin(XMLElement* elem, mjCSkin* pskin) {
   // write attributes
   WriteAttrTxt(elem, "name", pskin->name);
   WriteAttrTxt(elem, "file", pskin->file);
-  WriteAttrTxt(elem, "material", pskin->material);
+  WriteAttrTxt(elem, "material", pskin->get_material());
   WriteAttrInt(elem, "group", pskin->group, 0);
   WriteAttr(elem, "rgba", 4, pskin->rgba, mydef.geom.rgba);
   WriteAttr(elem, "inflate", 1, &pskin->inflate, &zero);
@@ -396,8 +396,8 @@ void mjXWriter::OneGeom(XMLElement* elem, mjCGeom* pgeom, mjCDef* def) {
   } else {
     WriteAttr(elem, "density", 1, &pgeom->density, &def->geom.density);
   }
-  if (pgeom->material != def->geom.material) {
-    WriteAttrTxt(elem, "material", pgeom->material);
+  if (pgeom->get_material() != def->geom.get_material()) {
+    WriteAttrTxt(elem, "material", pgeom->get_material());
   }
   WriteAttr(elem, "rgba", 4, pgeom->rgba, def->geom.rgba);
 
@@ -444,16 +444,16 @@ void mjXWriter::OneSite(XMLElement* elem, mjCSite* psite, mjCDef* def) {
   // defaults and regular
   WriteAttrInt(elem, "group", psite->group, def->site.group);
   WriteAttrKey(elem, "type", geom_map, mjNGEOMTYPES, psite->type, def->site.type);
-  if (psite->material != def->site.material) {
-    WriteAttrTxt(elem, "material", psite->material);
+  if (psite->get_material() != def->site.get_material()) {
+    WriteAttrTxt(elem, "material", psite->get_material());
   }
   WriteAttr(elem, "rgba", 4, psite->rgba, def->site.rgba);
 
   // userdata
   if (writingdefaults) {
-    WriteVector(elem, "user", psite->userdata);
+    WriteVector(elem, "user", psite->get_userdata());
   } else {
-    WriteVector(elem, "user", psite->userdata, def->site.userdata);
+    WriteVector(elem, "user", psite->get_userdata(), def->site.get_userdata());
   }
 }
 
@@ -636,8 +636,8 @@ void mjXWriter::OneTendon(XMLElement* elem, mjCTendon* pten, mjCDef* def) {
   }
   // spatial only
   if (!fixed) {
-    if (pten->material!=def->tendon.material) {
-      WriteAttrTxt(elem, "material", pten->material);
+    if (pten->get_material()!=def->tendon.get_material()) {
+      WriteAttrTxt(elem, "material", pten->get_material());
     }
     WriteAttr(elem, "width", 1, &pten->width, &def->tendon.width);
     WriteAttr(elem, "rgba", 4, pten->rgba, def->tendon.rgba);