diff --git a/examples/callback_api/main.cc b/examples/callback_api/main.cc
index a7a7422..1911501 100644
--- a/examples/callback_api/main.cc
+++ b/examples/callback_api/main.cc
@@ -129,6 +129,7 @@ int main(int argc, char **argv) {
   cb.object_cb = object_cb;
 
   MyMesh mesh;
+  std::string warn;
   std::string err;
   std::string filename = "../../models/cornell_box.obj";
   if (argc > 1) {
@@ -143,7 +144,11 @@ int main(int argc, char **argv) {
 
   tinyobj::MaterialFileReader mtlReader("../../models/");
 
-  bool ret = tinyobj::LoadObjWithCallback(ifs, cb, &mesh, &mtlReader, &err);
+  bool ret = tinyobj::LoadObjWithCallback(ifs, cb, &mesh, &mtlReader, &warn, &err);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
     std::cerr << err << std::endl;
diff --git a/examples/obj_sticher/obj_sticher.cc b/examples/obj_sticher/obj_sticher.cc
index 47c06eb..779f303 100644
--- a/examples/obj_sticher/obj_sticher.cc
+++ b/examples/obj_sticher/obj_sticher.cc
@@ -1,11 +1,10 @@
 //
 // Stiches multiple .obj files into one .obj.
 //
-
-#define TINYOBJLOADER_IMPLEMENTATION
-#include "../../tiny_obj_loader.h"
 #include "obj_writer.h"
 
+#include "../../tiny_obj_loader.h"
+
 #include <cassert>
 #include <iostream>
 #include <cstdlib>
@@ -134,8 +133,9 @@ int main(int argc, char **argv)
   for (int i = 0; i < num_objfiles; i++) {
     std::cout << "Loading " << argv[i+1] << " ... " << std::flush;
     
+    std::string warn;
     std::string err;
-    bool ret = tinyobj::LoadObj(&attributes[i], &shapes[i], &materials[i], &err, argv[i+1]);
+    bool ret = tinyobj::LoadObj(&attributes[i], &shapes[i], &materials[i], &warn, &err, argv[i+1]);
     if (!err.empty()) {
       std::cerr << err << std::endl;
     }
diff --git a/examples/viewer/viewer.cc b/examples/viewer/viewer.cc
index b5de2e6..9ab1142 100644
--- a/examples/viewer/viewer.cc
+++ b/examples/viewer/viewer.cc
@@ -308,9 +308,13 @@ static bool LoadObjAndConvert(float bmin[3], float bmax[3],
   base_dir += "/";
 #endif
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, filename,
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err, filename,
                               base_dir.c_str());
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
   if (!err.empty()) {
     std::cerr << err << std::endl;
   }
diff --git a/examples/voxelize/main.cc b/examples/voxelize/main.cc
index 035e3a1..6f11939 100644
--- a/examples/voxelize/main.cc
+++ b/examples/voxelize/main.cc
@@ -9,8 +9,9 @@ bool Voxelize(const char* filename, float voxelsizex, float voxelsizey, float vo
     tinyobj::attrib_t attrib;
     std::vector<tinyobj::shape_t> shapes;
     std::vector<tinyobj::material_t> materials;
+    std::string warn;
     std::string err;
-    bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, filename);
+    bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err, filename);
 
     if (!err.empty()) {
       printf("err: %s\n", err.c_str());
diff --git a/loader_example.cc b/loader_example.cc
index 595ae3a..82de6b3 100644
--- a/loader_example.cc
+++ b/loader_example.cc
@@ -285,14 +285,19 @@ static bool TestLoadObj(const char* filename, const char* basepath = NULL,
 
   timerutil t;
   t.start();
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, filename,
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err, filename,
                               basepath, triangulate);
   t.end();
   printf("Parsing time: %lu [msecs]\n", t.msec());
 
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
+
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   if (!ret) {
@@ -376,16 +381,12 @@ static bool TestStreamLoadObj() {
     virtual bool operator()(const std::string& matId,
                             std::vector<material_t>* materials,
                             std::map<std::string, int>* matMap,
+                            std::string* warn,
                             std::string* err) {
+      (void)err;
       (void)matId;
-      std::string warning;
-      LoadMtl(matMap, materials, &m_matSStream, &warning);
+      LoadMtl(matMap, materials, &m_matSStream, warn);
 
-      if (!warning.empty()) {
-        if (err) {
-          (*err) += warning;
-        }
-      }
       return true;
     }
 
@@ -397,8 +398,9 @@ static bool TestStreamLoadObj() {
   tinyobj::attrib_t attrib;
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, &objStream,
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err, &objStream,
                               &matSSReader);
 
   if (!err.empty()) {
diff --git a/tests/tester.cc b/tests/tester.cc
index e24d1e2..094fd5f 100644
--- a/tests/tester.cc
+++ b/tests/tester.cc
@@ -1,87 +1,94 @@
 #define TINYOBJLOADER_IMPLEMENTATION
 #include "../tiny_obj_loader.h"
 
-#define CATCH_CONFIG_MAIN  // This tells Catch to provide a main() - only do this in one cpp file
+#define CATCH_CONFIG_MAIN  // This tells Catch to provide a main() - only do
+                           // this in one cpp file
 #include "catch.hpp"
 
+#include <cassert>
 #include <cstdio>
 #include <cstdlib>
-#include <cassert>
+#include <fstream>
 #include <iostream>
 #include <sstream>
-#include <fstream>
 
-static void PrintInfo(const tinyobj::attrib_t &attrib, const std::vector<tinyobj::shape_t>& shapes, const std::vector<tinyobj::material_t>& materials, bool triangulate = true)
-{
+static void PrintInfo(const tinyobj::attrib_t& attrib,
+                      const std::vector<tinyobj::shape_t>& shapes,
+                      const std::vector<tinyobj::material_t>& materials,
+                      bool triangulate = true) {
   std::cout << "# of vertices  : " << (attrib.vertices.size() / 3) << std::endl;
   std::cout << "# of normals   : " << (attrib.normals.size() / 3) << std::endl;
-  std::cout << "# of texcoords : " << (attrib.texcoords.size() / 2) << std::endl;
+  std::cout << "# of texcoords : " << (attrib.texcoords.size() / 2)
+            << std::endl;
 
   std::cout << "# of shapes    : " << shapes.size() << std::endl;
   std::cout << "# of materials : " << materials.size() << std::endl;
 
   for (size_t v = 0; v < attrib.vertices.size() / 3; v++) {
     printf("  v[%ld] = (%f, %f, %f)\n", v,
-      static_cast<const double>(attrib.vertices[3*v+0]),
-      static_cast<const double>(attrib.vertices[3*v+1]),
-      static_cast<const double>(attrib.vertices[3*v+2]));
+           static_cast<const double>(attrib.vertices[3 * v + 0]),
+           static_cast<const double>(attrib.vertices[3 * v + 1]),
+           static_cast<const double>(attrib.vertices[3 * v + 2]));
   }
 
   for (size_t v = 0; v < attrib.normals.size() / 3; v++) {
     printf("  n[%ld] = (%f, %f, %f)\n", v,
-      static_cast<const double>(attrib.normals[3*v+0]),
-      static_cast<const double>(attrib.normals[3*v+1]),
-      static_cast<const double>(attrib.normals[3*v+2]));
+           static_cast<const double>(attrib.normals[3 * v + 0]),
+           static_cast<const double>(attrib.normals[3 * v + 1]),
+           static_cast<const double>(attrib.normals[3 * v + 2]));
   }
 
   for (size_t v = 0; v < attrib.texcoords.size() / 2; v++) {
     printf("  uv[%ld] = (%f, %f)\n", v,
-      static_cast<const double>(attrib.texcoords[2*v+0]),
-      static_cast<const double>(attrib.texcoords[2*v+1]));
+           static_cast<const double>(attrib.texcoords[2 * v + 0]),
+           static_cast<const double>(attrib.texcoords[2 * v + 1]));
   }
 
   for (size_t i = 0; i < shapes.size(); i++) {
     printf("shape[%ld].name = %s\n", i, shapes[i].name.c_str());
-    printf("Size of shape[%ld].indices: %ld\n", i, shapes[i].mesh.indices.size());
+    printf("Size of shape[%ld].indices: %ld\n", i,
+           shapes[i].mesh.indices.size());
 
-    if (triangulate)
-    {
-        printf("Size of shape[%ld].material_ids: %ld\n", i, shapes[i].mesh.material_ids.size());
-        assert((shapes[i].mesh.indices.size() % 3) == 0);
-        for (size_t f = 0; f < shapes[i].mesh.indices.size() / 3; f++) {
-          tinyobj::index_t i0 = shapes[i].mesh.indices[3*f+0];
-          tinyobj::index_t i1 = shapes[i].mesh.indices[3*f+1];
-          tinyobj::index_t i2 = shapes[i].mesh.indices[3*f+2];
-          printf("  idx[%ld] = %d/%d/%d, %d/%d/%d, %d/%d/%d. mat_id = %d\n", f,
-            i0.vertex_index, i0.normal_index, i0.texcoord_index,
-            i1.vertex_index, i1.normal_index, i1.texcoord_index,
-            i2.vertex_index, i2.normal_index, i2.texcoord_index,
-            shapes[i].mesh.material_ids[f]);
-        }
+    if (triangulate) {
+      printf("Size of shape[%ld].material_ids: %ld\n", i,
+             shapes[i].mesh.material_ids.size());
+      assert((shapes[i].mesh.indices.size() % 3) == 0);
+      for (size_t f = 0; f < shapes[i].mesh.indices.size() / 3; f++) {
+        tinyobj::index_t i0 = shapes[i].mesh.indices[3 * f + 0];
+        tinyobj::index_t i1 = shapes[i].mesh.indices[3 * f + 1];
+        tinyobj::index_t i2 = shapes[i].mesh.indices[3 * f + 2];
+        printf("  idx[%ld] = %d/%d/%d, %d/%d/%d, %d/%d/%d. mat_id = %d\n", f,
+               i0.vertex_index, i0.normal_index, i0.texcoord_index,
+               i1.vertex_index, i1.normal_index, i1.texcoord_index,
+               i2.vertex_index, i2.normal_index, i2.texcoord_index,
+               shapes[i].mesh.material_ids[f]);
+      }
     } else {
       for (size_t f = 0; f < shapes[i].mesh.indices.size(); f++) {
         tinyobj::index_t idx = shapes[i].mesh.indices[f];
-        printf("  idx[%ld] = %d/%d/%d\n", f, idx.vertex_index, idx.normal_index, idx.texcoord_index);
+        printf("  idx[%ld] = %d/%d/%d\n", f, idx.vertex_index, idx.normal_index,
+               idx.texcoord_index);
       }
 
-      printf("Size of shape[%ld].material_ids: %ld\n", i, shapes[i].mesh.material_ids.size());
-      assert(shapes[i].mesh.material_ids.size() == shapes[i].mesh.num_face_vertices.size());
+      printf("Size of shape[%ld].material_ids: %ld\n", i,
+             shapes[i].mesh.material_ids.size());
+      assert(shapes[i].mesh.material_ids.size() ==
+             shapes[i].mesh.num_face_vertices.size());
       for (size_t m = 0; m < shapes[i].mesh.material_ids.size(); m++) {
-        printf("  material_id[%ld] = %d\n", m,
-          shapes[i].mesh.material_ids[m]);
+        printf("  material_id[%ld] = %d\n", m, shapes[i].mesh.material_ids[m]);
       }
-
     }
 
-    printf("shape[%ld].num_faces: %ld\n", i, shapes[i].mesh.num_face_vertices.size());
+    printf("shape[%ld].num_faces: %ld\n", i,
+           shapes[i].mesh.num_face_vertices.size());
     for (size_t v = 0; v < shapes[i].mesh.num_face_vertices.size(); v++) {
       printf("  num_vertices[%ld] = %ld\n", v,
-        static_cast<long>(shapes[i].mesh.num_face_vertices[v]));
+             static_cast<long>(shapes[i].mesh.num_face_vertices[v]));
     }
 
-    //printf("shape[%ld].vertices: %ld\n", i, shapes[i].mesh.positions.size());
-    //assert((shapes[i].mesh.positions.size() % 3) == 0);
-    //for (size_t v = 0; v < shapes[i].mesh.positions.size() / 3; v++) {
+    // printf("shape[%ld].vertices: %ld\n", i, shapes[i].mesh.positions.size());
+    // assert((shapes[i].mesh.positions.size() % 3) == 0);
+    // for (size_t v = 0; v < shapes[i].mesh.positions.size() / 3; v++) {
     //  printf("  v[%ld] = (%f, %f, %f)\n", v,
     //    static_cast<const double>(shapes[i].mesh.positions[3*v+0]),
     //    static_cast<const double>(shapes[i].mesh.positions[3*v+1]),
@@ -92,35 +99,30 @@ static void PrintInfo(const tinyobj::attrib_t &attrib, const std::vector<tinyobj
     for (size_t t = 0; t < shapes[i].mesh.tags.size(); t++) {
       printf("  tag[%ld] = %s ", t, shapes[i].mesh.tags[t].name.c_str());
       printf(" ints: [");
-      for (size_t j = 0; j < shapes[i].mesh.tags[t].intValues.size(); ++j)
-      {
-          printf("%ld", static_cast<long>(shapes[i].mesh.tags[t].intValues[j]));
-          if (j < (shapes[i].mesh.tags[t].intValues.size()-1))
-          {
-              printf(", ");
-          }
+      for (size_t j = 0; j < shapes[i].mesh.tags[t].intValues.size(); ++j) {
+        printf("%ld", static_cast<long>(shapes[i].mesh.tags[t].intValues[j]));
+        if (j < (shapes[i].mesh.tags[t].intValues.size() - 1)) {
+          printf(", ");
+        }
       }
       printf("]");
 
       printf(" floats: [");
-      for (size_t j = 0; j < shapes[i].mesh.tags[t].floatValues.size(); ++j)
-      {
-          printf("%f", static_cast<const double>(shapes[i].mesh.tags[t].floatValues[j]));
-          if (j < (shapes[i].mesh.tags[t].floatValues.size()-1))
-          {
-              printf(", ");
-          }
+      for (size_t j = 0; j < shapes[i].mesh.tags[t].floatValues.size(); ++j) {
+        printf("%f", static_cast<const double>(
+                         shapes[i].mesh.tags[t].floatValues[j]));
+        if (j < (shapes[i].mesh.tags[t].floatValues.size() - 1)) {
+          printf(", ");
+        }
       }
       printf("]");
 
       printf(" strings: [");
-      for (size_t j = 0; j < shapes[i].mesh.tags[t].stringValues.size(); ++j)
-      {
-          printf("%s", shapes[i].mesh.tags[t].stringValues[j].c_str());
-          if (j < (shapes[i].mesh.tags[t].stringValues.size()-1))
-          {
-              printf(", ");
-          }
+      for (size_t j = 0; j < shapes[i].mesh.tags[t].stringValues.size(); ++j) {
+        printf("%s", shapes[i].mesh.tags[t].stringValues[j].c_str());
+        if (j < (shapes[i].mesh.tags[t].stringValues.size() - 1)) {
+          printf(", ");
+        }
       }
       printf("]");
       printf("\n");
@@ -129,25 +131,45 @@ static void PrintInfo(const tinyobj::attrib_t &attrib, const std::vector<tinyobj
 
   for (size_t i = 0; i < materials.size(); i++) {
     printf("material[%ld].name = %s\n", i, materials[i].name.c_str());
-    printf("  material.Ka = (%f, %f ,%f)\n", static_cast<const double>(materials[i].ambient[0]),       static_cast<const double>(materials[i].ambient[1]),       static_cast<const double>(materials[i].ambient[2]));
-    printf("  material.Kd = (%f, %f ,%f)\n", static_cast<const double>(materials[i].diffuse[0]),       static_cast<const double>(materials[i].diffuse[1]),       static_cast<const double>(materials[i].diffuse[2]));
-    printf("  material.Ks = (%f, %f ,%f)\n", static_cast<const double>(materials[i].specular[0]),      static_cast<const double>(materials[i].specular[1]),      static_cast<const double>(materials[i].specular[2]));
-    printf("  material.Tr = (%f, %f ,%f)\n", static_cast<const double>(materials[i].transmittance[0]), static_cast<const double>(materials[i].transmittance[1]), static_cast<const double>(materials[i].transmittance[2]));
-    printf("  material.Ke = (%f, %f ,%f)\n", static_cast<const double>(materials[i].emission[0]),      static_cast<const double>(materials[i].emission[1]),      static_cast<const double>(materials[i].emission[2]));
-    printf("  material.Ns = %f\n", static_cast<const double>(materials[i].shininess));
+    printf("  material.Ka = (%f, %f ,%f)\n",
+           static_cast<const double>(materials[i].ambient[0]),
+           static_cast<const double>(materials[i].ambient[1]),
+           static_cast<const double>(materials[i].ambient[2]));
+    printf("  material.Kd = (%f, %f ,%f)\n",
+           static_cast<const double>(materials[i].diffuse[0]),
+           static_cast<const double>(materials[i].diffuse[1]),
+           static_cast<const double>(materials[i].diffuse[2]));
+    printf("  material.Ks = (%f, %f ,%f)\n",
+           static_cast<const double>(materials[i].specular[0]),
+           static_cast<const double>(materials[i].specular[1]),
+           static_cast<const double>(materials[i].specular[2]));
+    printf("  material.Tr = (%f, %f ,%f)\n",
+           static_cast<const double>(materials[i].transmittance[0]),
+           static_cast<const double>(materials[i].transmittance[1]),
+           static_cast<const double>(materials[i].transmittance[2]));
+    printf("  material.Ke = (%f, %f ,%f)\n",
+           static_cast<const double>(materials[i].emission[0]),
+           static_cast<const double>(materials[i].emission[1]),
+           static_cast<const double>(materials[i].emission[2]));
+    printf("  material.Ns = %f\n",
+           static_cast<const double>(materials[i].shininess));
     printf("  material.Ni = %f\n", static_cast<const double>(materials[i].ior));
-    printf("  material.dissolve = %f\n", static_cast<const double>(materials[i].dissolve));
-    printf("  material.illum = %d\n",  materials[i].illum);
+    printf("  material.dissolve = %f\n",
+           static_cast<const double>(materials[i].dissolve));
+    printf("  material.illum = %d\n", materials[i].illum);
     printf("  material.map_Ka = %s\n", materials[i].ambient_texname.c_str());
     printf("  material.map_Kd = %s\n", materials[i].diffuse_texname.c_str());
     printf("  material.map_Ks = %s\n", materials[i].specular_texname.c_str());
-    printf("  material.map_Ns = %s\n", materials[i].specular_highlight_texname.c_str());
+    printf("  material.map_Ns = %s\n",
+           materials[i].specular_highlight_texname.c_str());
     printf("  material.map_bump = %s\n", materials[i].bump_texname.c_str());
     printf("  material.map_d = %s\n", materials[i].alpha_texname.c_str());
     printf("  material.disp = %s\n", materials[i].displacement_texname.c_str());
     printf("  material.refl = %s\n", materials[i].reflection_texname.c_str());
-    std::map<std::string, std::string>::const_iterator it(materials[i].unknown_parameter.begin());
-    std::map<std::string, std::string>::const_iterator itEnd(materials[i].unknown_parameter.end());
+    std::map<std::string, std::string>::const_iterator it(
+        materials[i].unknown_parameter.begin());
+    std::map<std::string, std::string>::const_iterator itEnd(
+        materials[i].unknown_parameter.end());
 
     for (; it != itEnd; it++) {
       printf("  material.%s = %s\n", it->first.c_str(), it->second.c_str());
@@ -156,23 +178,25 @@ static void PrintInfo(const tinyobj::attrib_t &attrib, const std::vector<tinyobj
   }
 }
 
-static bool
-TestLoadObj(
-  const char* filename,
-  const char* basepath = NULL,
-  bool triangulate = true)
-{
+static bool TestLoadObj(const char* filename, const char* basepath = NULL,
+                        bool triangulate = true) {
   std::cout << "Loading " << filename << std::endl;
 
   tinyobj::attrib_t attrib;
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, filename, basepath, triangulate);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              filename, basepath, triangulate);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   if (!ret) {
@@ -185,13 +209,10 @@ TestLoadObj(
   return true;
 }
 
-static bool
-TestLoadObjFromPreopenedFile(
-  const char* filename,
-  const char* basepath = NULL,
-  bool readMaterials = true,
-  bool triangulate = true)
-{
+static bool TestLoadObjFromPreopenedFile(const char* filename,
+                                         const char* basepath = NULL,
+                                         bool readMaterials = true,
+                                         bool triangulate = true) {
   std::string fullFilename = std::string(basepath) + filename;
   std::cout << "Loading " << fullFilename << std::endl;
 
@@ -203,19 +224,24 @@ TestLoadObjFromPreopenedFile(
   }
 
   tinyobj::MaterialStreamReader materialStreamReader(fileStream);
-  tinyobj::MaterialStreamReader* materialReader = readMaterials
-    ? &materialStreamReader
-    : NULL;
+  tinyobj::MaterialStreamReader* materialReader =
+      readMaterials ? &materialStreamReader : NULL;
 
   tinyobj::attrib_t attrib;
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, &fileStream, materialReader);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              &fileStream, materialReader);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   if (!ret) {
@@ -228,104 +254,106 @@ TestLoadObjFromPreopenedFile(
   return true;
 }
 
-static bool
-TestStreamLoadObj()
-{
+static bool TestStreamLoadObj() {
   std::cout << "Stream Loading " << std::endl;
 
   std::stringstream objStream;
-  objStream
-    << "mtllib cube.mtl\n"
-    "\n"
-    "v 0.000000 2.000000 2.000000\n"
-    "v 0.000000 0.000000 2.000000\n"
-    "v 2.000000 0.000000 2.000000\n"
-    "v 2.000000 2.000000 2.000000\n"
-    "v 0.000000 2.000000 0.000000\n"
-    "v 0.000000 0.000000 0.000000\n"
-    "v 2.000000 0.000000 0.000000\n"
-    "v 2.000000 2.000000 0.000000\n"
-    "# 8 vertices\n"
-    "\n"
-    "g front cube\n"
-    "usemtl white\n"
-    "f 1 2 3 4\n"
-    "g back cube\n"
-    "# expects white material\n"
-    "f 8 7 6 5\n"
-    "g right cube\n"
-    "usemtl red\n"
-    "f 4 3 7 8\n"
-    "g top cube\n"
-    "usemtl white\n"
-    "f 5 1 4 8\n"
-    "g left cube\n"
-    "usemtl green\n"
-    "f 5 6 2 1\n"
-    "g bottom cube\n"
-    "usemtl white\n"
-    "f 2 6 7 3\n"
-    "# 6 elements";
+  objStream << "mtllib cube.mtl\n"
+               "\n"
+               "v 0.000000 2.000000 2.000000\n"
+               "v 0.000000 0.000000 2.000000\n"
+               "v 2.000000 0.000000 2.000000\n"
+               "v 2.000000 2.000000 2.000000\n"
+               "v 0.000000 2.000000 0.000000\n"
+               "v 0.000000 0.000000 0.000000\n"
+               "v 2.000000 0.000000 0.000000\n"
+               "v 2.000000 2.000000 0.000000\n"
+               "# 8 vertices\n"
+               "\n"
+               "g front cube\n"
+               "usemtl white\n"
+               "f 1 2 3 4\n"
+               "g back cube\n"
+               "# expects white material\n"
+               "f 8 7 6 5\n"
+               "g right cube\n"
+               "usemtl red\n"
+               "f 4 3 7 8\n"
+               "g top cube\n"
+               "usemtl white\n"
+               "f 5 1 4 8\n"
+               "g left cube\n"
+               "usemtl green\n"
+               "f 5 6 2 1\n"
+               "g bottom cube\n"
+               "usemtl white\n"
+               "f 2 6 7 3\n"
+               "# 6 elements";
 
-std::string matStream(
-    "newmtl white\n"
-    "Ka 0 0 0\n"
-    "Kd 1 1 1\n"
-    "Ks 0 0 0\n"
-    "\n"
-    "newmtl red\n"
-    "Ka 0 0 0\n"
-    "Kd 1 0 0\n"
-    "Ks 0 0 0\n"
-    "\n"
-    "newmtl green\n"
-    "Ka 0 0 0\n"
-    "Kd 0 1 0\n"
-    "Ks 0 0 0\n"
-    "\n"
-    "newmtl blue\n"
-    "Ka 0 0 0\n"
-    "Kd 0 0 1\n"
-    "Ks 0 0 0\n"
-    "\n"
-    "newmtl light\n"
-    "Ka 20 20 20\n"
-    "Kd 1 1 1\n"
-    "Ks 0 0 0");
+  std::string matStream(
+      "newmtl white\n"
+      "Ka 0 0 0\n"
+      "Kd 1 1 1\n"
+      "Ks 0 0 0\n"
+      "\n"
+      "newmtl red\n"
+      "Ka 0 0 0\n"
+      "Kd 1 0 0\n"
+      "Ks 0 0 0\n"
+      "\n"
+      "newmtl green\n"
+      "Ka 0 0 0\n"
+      "Kd 0 1 0\n"
+      "Ks 0 0 0\n"
+      "\n"
+      "newmtl blue\n"
+      "Ka 0 0 0\n"
+      "Kd 0 0 1\n"
+      "Ks 0 0 0\n"
+      "\n"
+      "newmtl light\n"
+      "Ka 20 20 20\n"
+      "Kd 1 1 1\n"
+      "Ks 0 0 0");
 
-    using namespace tinyobj;
-    class MaterialStringStreamReader:
-        public MaterialReader
-    {
-        public:
-            MaterialStringStreamReader(const std::string& matSStream): m_matSStream(matSStream) {}
-            virtual ~MaterialStringStreamReader() {}
-            virtual bool operator() (
-              const std::string& matId,
-              std::vector<material_t>* materials,
-              std::map<std::string, int>* matMap,
-              std::string* err)
-            {
-                (void)matId;
-                (void)err;
-                std::string warning;
-                LoadMtl(matMap, materials, &m_matSStream, &warning);
-                return true;
-            }
+  using namespace tinyobj;
+  class MaterialStringStreamReader : public MaterialReader {
+   public:
+    MaterialStringStreamReader(const std::string& matSStream)
+        : m_matSStream(matSStream) {}
+    virtual ~MaterialStringStreamReader() {}
+    virtual bool operator()(const std::string& matId,
+                            std::vector<material_t>* materials,
+                            std::map<std::string, int>* matMap,
+                            std::string* warn, std::string* err) {
+      (void)matId;
+      (void)warn;
+      (void)err;
+      std::string warning;
+      std::string error_msg;
+      LoadMtl(matMap, materials, &m_matSStream, &warning, &error_msg);
+      return true;
+    }
 
-        private:
-            std::stringstream m_matSStream;
-    };
+   private:
+    std::stringstream m_matSStream;
+  };
 
   MaterialStringStreamReader matSSReader(matStream);
   tinyobj::attrib_t attrib;
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, &objStream, &matSSReader);    
-  
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              &objStream, &matSSReader);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
+
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   if (!ret) {
@@ -333,28 +361,33 @@ std::string matStream(
   }
 
   PrintInfo(attrib, shapes, materials);
-    
+
   return true;
 }
 
 const char* gMtlBasePath = "../models/";
 
 TEST_CASE("cornell_box", "[Loader]") {
-
-    REQUIRE(true == TestLoadObj("../models/cornell_box.obj", gMtlBasePath));
+  REQUIRE(true == TestLoadObj("../models/cornell_box.obj", gMtlBasePath));
 }
 
 TEST_CASE("catmark_torus_creases0", "[Loader]") {
-
   tinyobj::attrib_t attrib;
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/catmark_torus_creases0.obj", gMtlBasePath, /*triangulate*/false);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/catmark_torus_creases0.obj",
+                              gMtlBasePath, /*triangulate*/ false);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   REQUIRE(true == ret);
@@ -364,16 +397,22 @@ TEST_CASE("catmark_torus_creases0", "[Loader]") {
 }
 
 TEST_CASE("pbr", "[Loader]") {
-
   tinyobj::attrib_t attrib;
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/pbr-mat-ext.obj", gMtlBasePath, /*triangulate*/false);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/pbr-mat-ext.obj", gMtlBasePath,
+                              /*triangulate*/ false);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
   REQUIRE(true == ret);
   REQUIRE(1 == materials.size());
@@ -391,18 +430,18 @@ TEST_CASE("pbr", "[Loader]") {
   REQUIRE(0 == materials[0].normal_texname.compare("normalmap.tex"));
 }
 
-TEST_CASE("stream_load", "[Stream]") {
-    REQUIRE(true == TestStreamLoadObj());
-}
+TEST_CASE("stream_load", "[Stream]") { REQUIRE(true == TestStreamLoadObj()); }
 
 TEST_CASE("stream_load_from_file_skipping_materials", "[Stream]") {
   REQUIRE(true == TestLoadObjFromPreopenedFile(
-    "../models/pbr-mat-ext.obj", gMtlBasePath, /*readMaterials*/false, /*triangulate*/false));
+                      "../models/pbr-mat-ext.obj", gMtlBasePath,
+                      /*readMaterials*/ false, /*triangulate*/ false));
 }
 
 TEST_CASE("stream_load_from_file_with_materials", "[Stream]") {
   REQUIRE(true == TestLoadObjFromPreopenedFile(
-    "../models/pbr-mat-ext.obj", gMtlBasePath, /*readMaterials*/true, /*triangulate*/false));
+                      "../models/pbr-mat-ext.obj", gMtlBasePath,
+                      /*readMaterials*/ true, /*triangulate*/ false));
 }
 
 TEST_CASE("trailing_whitespace_in_mtl", "[Issue92]") {
@@ -410,11 +449,17 @@ TEST_CASE("trailing_whitespace_in_mtl", "[Issue92]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/issue-92.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/issue-92.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
   REQUIRE(true == ret);
   REQUIRE(1 == materials.size());
@@ -426,11 +471,17 @@ TEST_CASE("transmittance_filter", "[Issue95]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/issue-95.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/issue-95.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
   REQUIRE(true == ret);
   REQUIRE(1 == materials.size());
@@ -444,11 +495,17 @@ TEST_CASE("transmittance_filter_Tf", "[Issue95-Tf]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/issue-95-2.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/issue-95-2.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
   REQUIRE(true == ret);
   REQUIRE(1 == materials.size());
@@ -462,11 +519,17 @@ TEST_CASE("transmittance_filter_Kt", "[Issue95-Kt]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/issue-95.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/issue-95.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
   REQUIRE(true == ret);
   REQUIRE(1 == materials.size());
@@ -480,11 +543,17 @@ TEST_CASE("usemtl_at_last_line", "[Issue104]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/usemtl-issue-104.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/usemtl-issue-104.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
   REQUIRE(true == ret);
   REQUIRE(1 == shapes.size());
@@ -495,11 +564,18 @@ TEST_CASE("texture_opts", "[Issue85]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/texture-options-issue-85.obj", gMtlBasePath);
+  bool ret =
+      tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                       "../models/texture-options-issue-85.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
   REQUIRE(true == ret);
   REQUIRE(1 == shapes.size());
@@ -525,11 +601,13 @@ TEST_CASE("texture_opts", "[Issue85]") {
 
   REQUIRE(tinyobj::TEXTURE_TYPE_SPHERE == materials[2].diffuse_texopt.type);
   REQUIRE(tinyobj::TEXTURE_TYPE_CUBE_TOP == materials[2].specular_texopt.type);
-  REQUIRE(tinyobj::TEXTURE_TYPE_CUBE_BOTTOM == materials[2].specular_highlight_texopt.type);
+  REQUIRE(tinyobj::TEXTURE_TYPE_CUBE_BOTTOM ==
+          materials[2].specular_highlight_texopt.type);
   REQUIRE(tinyobj::TEXTURE_TYPE_CUBE_LEFT == materials[2].ambient_texopt.type);
   REQUIRE(tinyobj::TEXTURE_TYPE_CUBE_RIGHT == materials[2].alpha_texopt.type);
   REQUIRE(tinyobj::TEXTURE_TYPE_CUBE_FRONT == materials[2].bump_texopt.type);
-  REQUIRE(tinyobj::TEXTURE_TYPE_CUBE_BACK == materials[2].displacement_texopt.type);
+  REQUIRE(tinyobj::TEXTURE_TYPE_CUBE_BACK ==
+          materials[2].displacement_texopt.type);
 }
 
 TEST_CASE("mtllib_multiple_filenames", "[Issue112]") {
@@ -537,11 +615,18 @@ TEST_CASE("mtllib_multiple_filenames", "[Issue112]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/mtllib-multiple-files-issue-112.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/mtllib-multiple-files-issue-112.obj",
+                              gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
   REQUIRE(true == ret);
   REQUIRE(1 == materials.size());
@@ -552,11 +637,17 @@ TEST_CASE("tr_and_d", "[Issue43]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/tr-and-d-issue-43.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/tr-and-d-issue-43.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
   REQUIRE(true == ret);
   REQUIRE(2 == materials.size());
@@ -570,11 +661,17 @@ TEST_CASE("refl", "[refl]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/refl.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/refl.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   PrintInfo(attrib, shapes, materials);
@@ -590,11 +687,17 @@ TEST_CASE("map_Bump", "[bump]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/map-bump.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/map-bump.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   PrintInfo(attrib, shapes, materials);
@@ -610,11 +713,17 @@ TEST_CASE("g_ignored", "[Issue138]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/issue-138.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/issue-138.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   PrintInfo(attrib, shapes, materials);
@@ -622,7 +731,6 @@ TEST_CASE("g_ignored", "[Issue138]") {
   REQUIRE(true == ret);
   REQUIRE(2 == shapes.size());
   REQUIRE(2 == materials.size());
-
 }
 
 TEST_CASE("vertex-col-ext", "[Issue144]") {
@@ -630,15 +738,21 @@ TEST_CASE("vertex-col-ext", "[Issue144]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
 
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/cube-vertexcol.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/cube-vertexcol.obj", gMtlBasePath);
 
-  if (!err.empty()) {
-    std::cerr << err << std::endl;
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
   }
 
-  //PrintInfo(attrib, shapes, materials);
+  if (!err.empty()) {
+    std::cerr << "ERR: " << err << std::endl;
+  }
+
+  // PrintInfo(attrib, shapes, materials);
 
   REQUIRE(true == ret);
   REQUIRE((8 * 3) == attrib.colors.size());
@@ -663,18 +777,23 @@ TEST_CASE("norm_texopts", "[norm]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/norm-texopt.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/norm-texopt.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   REQUIRE(true == ret);
   REQUIRE(1 == shapes.size());
   REQUIRE(1 == materials.size());
   REQUIRE(3.0 == Approx(materials[0].normal_texopt.bump_multiplier));
-
 }
 
 TEST_CASE("zero-face-idx-value", "[Issue140]") {
@@ -682,16 +801,21 @@ TEST_CASE("zero-face-idx-value", "[Issue140]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/issue-140-zero-face-idx.obj", gMtlBasePath);
+  bool ret =
+      tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                       "../models/issue-140-zero-face-idx.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
-  
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
   REQUIRE(false == ret);
   REQUIRE(!err.empty());
-
 }
 
 TEST_CASE("texture-name-whitespace", "[Issue145]") {
@@ -699,12 +823,18 @@ TEST_CASE("texture-name-whitespace", "[Issue145]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/texture-filename-with-whitespace.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/texture-filename-with-whitespace.obj",
+                              gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
-  
   if (!err.empty()) {
-    std::cerr << "[Issue145] " << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   REQUIRE(true == ret);
@@ -714,7 +844,6 @@ TEST_CASE("texture-name-whitespace", "[Issue145]") {
   REQUIRE(0 == materials[0].diffuse_texname.compare("texture 01.png"));
   REQUIRE(0 == materials[1].bump_texname.compare("bump 01.png"));
   REQUIRE(2 == Approx(materials[1].bump_texopt.bump_multiplier));
-
 }
 
 TEST_CASE("smoothing-group", "[Issue162]") {
@@ -722,12 +851,18 @@ TEST_CASE("smoothing-group", "[Issue162]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/issue-162-smoothing-group.obj", gMtlBasePath);
+  bool ret =
+      tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                       "../models/issue-162-smoothing-group.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
-  
   if (!err.empty()) {
-    std::cerr << "[Issue162] " << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   REQUIRE(true == ret);
@@ -748,7 +883,6 @@ TEST_CASE("smoothing-group", "[Issue162]") {
   REQUIRE(0 == shapes[1].mesh.smoothing_group_ids[7]);
   REQUIRE(6 == shapes[1].mesh.smoothing_group_ids[8]);
   REQUIRE(6 == shapes[1].mesh.smoothing_group_ids[9]);
-
 }
 
 TEST_CASE("invalid-face-definition", "[face]") {
@@ -756,11 +890,18 @@ TEST_CASE("invalid-face-definition", "[face]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/invalid-face-definition.obj", gMtlBasePath);
+  bool ret =
+      tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                       "../models/invalid-face-definition.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << "[face] " << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   REQUIRE(true == ret);
@@ -768,23 +909,29 @@ TEST_CASE("invalid-face-definition", "[face]") {
   REQUIRE(0 == shapes[0].mesh.indices.size());
 }
 
-TEST_CASE("Empty mtl basedir", "[Issue177]") {  
+TEST_CASE("Empty mtl basedir", "[Issue177]") {
   tinyobj::attrib_t attrib;
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
-    
+
+  std::string warn;
   std::string err;
-  
+
   // A case where the user explicitly provides an empty string
   // Win32 specific?
-  const char * userBaseDir = "";
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "issue-177.obj", userBaseDir);
-  
-  // if mtl loading fails, we get an error message (WARN) here
-  ret &= err.empty();
+  const char* userBaseDir = "";
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "issue-177.obj", userBaseDir);
+
+  // if mtl loading fails, we get an warning message here
+  ret &= warn.empty();
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << "[Issue177] " << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   REQUIRE(true == ret);
@@ -795,11 +942,17 @@ TEST_CASE("line-primitive", "[line]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/line-prim.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/line-prim.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << "[line] " << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   REQUIRE(true == ret);
@@ -812,17 +965,25 @@ TEST_CASE("multiple-group-names", "[group]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/cube.obj", gMtlBasePath);
+  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                              "../models/cube.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << "[group] " << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
 
   REQUIRE(true == ret);
   REQUIRE(6 == shapes.size());
   REQUIRE(0 == shapes[0].name.compare("front cube"));
-  REQUIRE(0 == shapes[1].name.compare("back cube")); // multiple whitespaces are aggregated as single white space.
+  REQUIRE(0 == shapes[1].name.compare("back cube"));  // multiple whitespaces
+                                                      // are aggregated as
+                                                      // single white space.
 }
 
 TEST_CASE("colorspace", "[Issue184]") {
@@ -830,12 +991,20 @@ TEST_CASE("colorspace", "[Issue184]") {
   std::vector<tinyobj::shape_t> shapes;
   std::vector<tinyobj::material_t> materials;
 
+  std::string warn;
   std::string err;
-  bool ret = tinyobj::LoadObj(&attrib, &shapes, &materials, &err, "../models/colorspace-issue-184.obj", gMtlBasePath);
+  bool ret =
+      tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err,
+                       "../models/colorspace-issue-184.obj", gMtlBasePath);
+
+  if (!warn.empty()) {
+    std::cout << "WARN: " << warn << std::endl;
+  }
 
   if (!err.empty()) {
-    std::cerr << err << std::endl;
+    std::cerr << "ERR: " << err << std::endl;
   }
+
   REQUIRE(true == ret);
   REQUIRE(1 == shapes.size());
   REQUIRE(1 == materials.size());
@@ -846,7 +1015,8 @@ TEST_CASE("colorspace", "[Issue184]") {
 
 // Fuzzer test.
 // Just check if it does not crash.
-// Disable by default since Windows filesystem can't create filename of afl testdata
+// Disable by default since Windows filesystem can't create filename of afl
+// testdata
 #if 0 
 
 TEST_CASE("afl000000", "[AFL]") {
diff --git a/tiny_obj_loader.h b/tiny_obj_loader.h
index f627e94..8ef44a7 100644
--- a/tiny_obj_loader.h
+++ b/tiny_obj_loader.h
@@ -23,6 +23,7 @@ THE SOFTWARE.
 */
 
 //
+// version 1.3.0 : Separate warning and error message(breaking API of LoadObj)
 // version 1.2.3 : Added color space extension('-colorspace') to tex opts.
 // version 1.2.2 : Parse multiple group names.
 // version 1.2.1 : Added initial support for line('l') primitive(PR #178)
@@ -117,8 +118,9 @@ namespace tinyobj {
 //
 // TinyObjLoader extension.
 //
-//   -colorspace SPACE                      # Color space of the texture. e.g.  'sRGB` or 'linear' 
-// 
+//   -colorspace SPACE                      # Color space of the texture. e.g.
+//   'sRGB` or 'linear'
+//
 
 #ifdef TINYOBJLOADER_USE_DOUBLE
 //#pragma message "using double"
@@ -153,9 +155,10 @@ typedef struct {
   bool blendu;   // -blendu (default on)
   bool blendv;   // -blendv (default on)
   real_t bump_multiplier;  // -bm (for bump maps only, default 1.0)
-  
+
   // extension
-  std::string colorspace;   // Explicitly specify color space of stored value. Usually `sRGB` or `linear` (default empty).
+  std::string colorspace;  // Explicitly specify color space of stored value.
+                           // Usually `sRGB` or `linear` (default empty).
 } texture_option_t;
 
 typedef struct {
@@ -307,7 +310,7 @@ class MaterialReader {
 
   virtual bool operator()(const std::string &matId,
                           std::vector<material_t> *materials,
-                          std::map<std::string, int> *matMap,
+                          std::map<std::string, int> *matMap, std::string *warn,
                           std::string *err) = 0;
 };
 
@@ -318,7 +321,8 @@ class MaterialFileReader : public MaterialReader {
   virtual ~MaterialFileReader() {}
   virtual bool operator()(const std::string &matId,
                           std::vector<material_t> *materials,
-                          std::map<std::string, int> *matMap, std::string *err);
+                          std::map<std::string, int> *matMap, std::string *warn,
+                          std::string *err);
 
  private:
   std::string m_mtlBaseDir;
@@ -331,7 +335,8 @@ class MaterialStreamReader : public MaterialReader {
   virtual ~MaterialStreamReader() {}
   virtual bool operator()(const std::string &matId,
                           std::vector<material_t> *materials,
-                          std::map<std::string, int> *matMap, std::string *err);
+                          std::map<std::string, int> *matMap, std::string *warn,
+                          std::string *err);
 
  private:
   std::istream &m_inStream;
@@ -341,7 +346,7 @@ class MaterialStreamReader : public MaterialReader {
 /// 'attrib', 'shapes' and 'materials' will be filled with parsed shape data
 /// 'shapes' will be filled with parsed shape data
 /// Returns true when loading .obj become success.
-/// Returns warning and error message into `err`
+/// Returns warning message into `warn`, and error message into `err`
 /// 'mtl_basedir' is optional, and used for base directory for .mtl file.
 /// In default(`NULL'), .mtl file is searched from an application's working
 /// directory.
@@ -350,34 +355,36 @@ class MaterialStreamReader : public MaterialReader {
 /// Option 'default_vcols_fallback' specifies whether vertex colors should
 /// always be defined, even if no colors are given (fallback to white).
 bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
-             std::vector<material_t> *materials, std::string *err,
-             const char *filename, const char *mtl_basedir = NULL,
-             bool triangulate = true, bool default_vcols_fallback = true);
+             std::vector<material_t> *materials, std::string *warn,
+             std::string *err, const char *filename,
+             const char *mtl_basedir = NULL, bool triangulate = true,
+             bool default_vcols_fallback = true);
 
 /// Loads .obj from a file with custom user callback.
 /// .mtl is loaded as usual and parsed material_t data will be passed to
 /// `callback.mtllib_cb`.
 /// Returns true when loading .obj/.mtl become success.
-/// Returns warning and error message into `err`
+/// Returns warning message into `warn`, and error message into `err`
 /// See `examples/callback_api/` for how to use this function.
 bool LoadObjWithCallback(std::istream &inStream, const callback_t &callback,
                          void *user_data = NULL,
                          MaterialReader *readMatFn = NULL,
-                         std::string *err = NULL);
+                         std::string *warn = NULL, std::string *err = NULL);
 
 /// Loads object from a std::istream, uses GetMtlIStreamFn to retrieve
 /// std::istream for materials.
 /// Returns true when loading .obj become success.
 /// Returns warning and error message into `err`
 bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
-             std::vector<material_t> *materials, std::string *err,
-             std::istream *inStream, MaterialReader *readMatFn = NULL,
-             bool triangulate = true, bool default_vcols_fallback = true);
+             std::vector<material_t> *materials, std::string *warn,
+             std::string *err, std::istream *inStream,
+             MaterialReader *readMatFn = NULL, bool triangulate = true,
+             bool default_vcols_fallback = true);
 
 /// Loads materials into std::map
 void LoadMtl(std::map<std::string, int> *material_map,
              std::vector<material_t> *materials, std::istream *inStream,
-             std::string *warning);
+             std::string *warning, std::string *err);
 
 }  // namespace tinyobj
 
@@ -721,12 +728,13 @@ static inline bool parseVertexWithColor(real_t *x, real_t *y, real_t *z,
   (*y) = parseReal(token, default_y);
   (*z) = parseReal(token, default_z);
 
-  const bool found_color = parseReal(token, r) && parseReal(token, g) && parseReal(token, b);
-  
+  const bool found_color =
+      parseReal(token, r) && parseReal(token, g) && parseReal(token, b);
+
   if (!found_color) {
     (*r) = (*g) = (*b) = 1.0;
   }
-  
+
   return found_color;
 }
 
@@ -959,11 +967,12 @@ static bool ParseTextureNameAndOption(std::string *texname,
     } else if ((0 == strncmp(token, "-mm", 3)) && IS_SPACE((token[3]))) {
       token += 4;
       parseReal2(&(texopt->brightness), &(texopt->contrast), &token, 0.0, 1.0);
-    } else if ((0 == strncmp(token, "-colorspace", 11)) && IS_SPACE((token[11]))) {
+    } else if ((0 == strncmp(token, "-colorspace", 11)) &&
+               IS_SPACE((token[11]))) {
       token += 12;
       texopt->colorspace = parseString(&token);
     } else {
-      // Assume texture filename
+// Assume texture filename
 #if 0
       size_t len = strcspn(token, " \t\r");  // untile next space
       texture_name = std::string(token, token + len);
@@ -1303,7 +1312,9 @@ static void SplitString(const std::string &s, char delim,
 
 void LoadMtl(std::map<std::string, int> *material_map,
              std::vector<material_t> *materials, std::istream *inStream,
-             std::string *warning) {
+             std::string *warning, std::string *err) {
+  (void)err;
+
   // Create a default material anyway.
   material_t material;
   InitMaterial(&material);
@@ -1312,7 +1323,7 @@ void LoadMtl(std::map<std::string, int> *material_map,
   bool has_d = false;
   bool has_tr = false;
 
-  std::stringstream ss;
+  std::stringstream warn_ss;
 
   std::string linebuf;
   while (inStream->peek() != -1) {
@@ -1455,9 +1466,9 @@ void LoadMtl(std::map<std::string, int> *material_map,
       material.dissolve = parseReal(&token);
 
       if (has_tr) {
-        ss << "WARN: Both `d` and `Tr` parameters defined for \""
-           << material.name << "\". Use the value of `d` for dissolve."
-           << std::endl;
+        warn_ss << "Both `d` and `Tr` parameters defined for \""
+                << material.name << "\". Use the value of `d` for dissolve."
+                << std::endl;
       }
       has_d = true;
       continue;
@@ -1466,9 +1477,9 @@ void LoadMtl(std::map<std::string, int> *material_map,
       token += 2;
       if (has_d) {
         // `d` wins. Ignore `Tr` value.
-        ss << "WARN: Both `d` and `Tr` parameters defined for \""
-           << material.name << "\". Use the value of `d` for dissolve."
-           << std::endl;
+        warn_ss << "Both `d` and `Tr` parameters defined for \""
+                << material.name << "\". Use the value of `d` for dissolve."
+                << std::endl;
       } else {
         // We invert value of Tr(assume Tr is in range [0, 1])
         // NOTE: Interpretation of Tr is application(exporter) dependent. For
@@ -1683,14 +1694,14 @@ void LoadMtl(std::map<std::string, int> *material_map,
   materials->push_back(material);
 
   if (warning) {
-    (*warning) = ss.str();
+    (*warning) = warn_ss.str();
   }
 }
 
 bool MaterialFileReader::operator()(const std::string &matId,
                                     std::vector<material_t> *materials,
                                     std::map<std::string, int> *matMap,
-                                    std::string *err) {
+                                    std::string *warn, std::string *err) {
   std::string filepath;
 
   if (!m_mtlBaseDir.empty()) {
@@ -1702,21 +1713,14 @@ bool MaterialFileReader::operator()(const std::string &matId,
   std::ifstream matIStream(filepath.c_str());
   if (!matIStream) {
     std::stringstream ss;
-    ss << "WARN: Material file [ " << filepath << " ] not found." << std::endl;
-    if (err) {
-      (*err) += ss.str();
+    ss << "Material file [ " << filepath << " ] not found." << std::endl;
+    if (warn) {
+      (*warn) += ss.str();
     }
     return false;
   }
 
-  std::string warning;
-  LoadMtl(matMap, materials, &matIStream, &warning);
-
-  if (!warning.empty()) {
-    if (err) {
-      (*err) += warning;
-    }
-  }
+  LoadMtl(matMap, materials, &matIStream, warn, err);
 
   return true;
 }
@@ -1724,32 +1728,26 @@ bool MaterialFileReader::operator()(const std::string &matId,
 bool MaterialStreamReader::operator()(const std::string &matId,
                                       std::vector<material_t> *materials,
                                       std::map<std::string, int> *matMap,
-                                      std::string *err) {
+                                      std::string *warn, std::string *err) {
+  (void)err;
   (void)matId;
   if (!m_inStream) {
     std::stringstream ss;
-    ss << "WARN: Material stream in error state. " << std::endl;
-    if (err) {
-      (*err) += ss.str();
+    ss << "Material stream in error state. " << std::endl;
+    if (warn) {
+      (*warn) += ss.str();
     }
     return false;
   }
 
-  std::string warning;
-  LoadMtl(matMap, materials, &m_inStream, &warning);
-
-  if (!warning.empty()) {
-    if (err) {
-      (*err) += warning;
-    }
-  }
+  LoadMtl(matMap, materials, &m_inStream, warn, err);
 
   return true;
 }
 
 bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
-             std::vector<material_t> *materials, std::string *err,
-             const char *filename, const char *mtl_basedir,
+             std::vector<material_t> *materials, std::string *warn,
+             std::string *err, const char *filename, const char *mtl_basedir,
              bool trianglulate, bool default_vcols_fallback) {
   attrib->vertices.clear();
   attrib->normals.clear();
@@ -1779,14 +1777,15 @@ bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
   }
   MaterialFileReader matFileReader(baseDir);
 
-  return LoadObj(attrib, shapes, materials, err, &ifs, &matFileReader,
+  return LoadObj(attrib, shapes, materials, warn, err, &ifs, &matFileReader,
                  trianglulate, default_vcols_fallback);
 }
 
 bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
-             std::vector<material_t> *materials, std::string *err,
-             std::istream *inStream, MaterialReader *readMatFn /*= NULL*/,
-             bool triangulate, bool default_vcols_fallback) {
+             std::vector<material_t> *materials, std::string *warn,
+             std::string *err, std::istream *inStream,
+             MaterialReader *readMatFn /*= NULL*/, bool triangulate,
+             bool default_vcols_fallback) {
   std::stringstream errss;
 
   std::vector<real_t> v;
@@ -1813,7 +1812,7 @@ bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
   shape_t shape;
 
   bool found_all_colors = true;
-  
+
   size_t line_num = 0;
   std::string linebuf;
   while (inStream->peek() != -1) {
@@ -1850,9 +1849,9 @@ bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
       token += 2;
       real_t x, y, z;
       real_t r, g, b;
-            
+
       found_all_colors &= parseVertexWithColor(&x, &y, &z, &r, &g, &b, &token);
-      
+
       v.push_back(x);
       v.push_back(y);
       v.push_back(z);
@@ -1862,7 +1861,7 @@ bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
         vc.push_back(g);
         vc.push_back(b);
       }
-      
+
       continue;
     }
 
@@ -1936,8 +1935,10 @@ bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
         }
 
         greatest_v_idx = greatest_v_idx > vi.v_idx ? greatest_v_idx : vi.v_idx;
-        greatest_vn_idx = greatest_vn_idx > vi.vn_idx ? greatest_vn_idx : vi.vn_idx;
-        greatest_vt_idx = greatest_vt_idx > vi.vt_idx ? greatest_vt_idx : vi.vt_idx;
+        greatest_vn_idx =
+            greatest_vn_idx > vi.vn_idx ? greatest_vn_idx : vi.vn_idx;
+        greatest_vt_idx =
+            greatest_vt_idx > vi.vt_idx ? greatest_vt_idx : vi.vt_idx;
 
         face.vertex_indices.push_back(vi);
         size_t n = strspn(token, " \t\r");
@@ -1949,7 +1950,7 @@ bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
 
       continue;
     }
-      
+
     // use mtl
     if ((0 == strncmp(token, "usemtl", 6)) && IS_SPACE((token[6]))) {
       token += 7;
@@ -1986,19 +1987,24 @@ bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
         SplitString(std::string(token), ' ', filenames);
 
         if (filenames.empty()) {
-          if (err) {
-            (*err) +=
-                "WARN: Looks like empty filename for mtllib. Use default "
+          if (warn) {
+            (*warn) +=
+                "Looks like empty filename for mtllib. Use default "
                 "material. \n";
           }
         } else {
           bool found = false;
           for (size_t s = 0; s < filenames.size(); s++) {
+            std::string warn_mtl;
             std::string err_mtl;
             bool ok = (*readMatFn)(filenames[s].c_str(), materials,
-                                   &material_map, &err_mtl);
+                                   &material_map, &warn_mtl, &err_mtl);
+            if (warn && (!warn_mtl.empty())) {
+              (*warn) += warn_mtl;
+            }
+
             if (err && (!err_mtl.empty())) {
-              (*err) += err_mtl;  // This should be warn message.
+              (*err) += err_mtl;
             }
 
             if (ok) {
@@ -2008,9 +2014,9 @@ bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
           }
 
           if (!found) {
-            if (err) {
-              (*err) +=
-                  "WARN: Failed to load material file(s). Use default "
+            if (warn) {
+              (*warn) +=
+                  "Failed to load material file(s). Use default "
                   "material.\n";
             }
           }
@@ -2048,14 +2054,13 @@ bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
 
       if (names.size() < 2) {
         // 'g' with empty names
-        if (err) {
+        if (warn) {
           std::stringstream ss;
-          ss << "WARN: Empty group name. line: " << line_num << "\n";
-          (*err) += ss.str();
+          ss << "Empty group name. line: " << line_num << "\n";
+          (*warn) += ss.str();
           name = "";
         }
       } else {
-
         std::stringstream ss;
         ss << names[1];
 
@@ -2068,7 +2073,6 @@ bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
         }
 
         name = ss.str();
-
       }
 
       continue;
@@ -2184,34 +2188,31 @@ bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
 
     // Ignore unknown command.
   }
-  
+
   // not all vertices have colors, no default colors desired? -> clear colors
-  if (!found_all_colors && !default_vcols_fallback) { 
+  if (!found_all_colors && !default_vcols_fallback) {
     vc.clear();
   }
-  
-  if (greatest_v_idx >= static_cast<int>(v.size() / 3))
-  {
-    if (err) {
+
+  if (greatest_v_idx >= static_cast<int>(v.size() / 3)) {
+    if (warn) {
       std::stringstream ss;
-      ss << "WARN: Vertex indices out of bounds.\n" << std::endl;
-      (*err) += ss.str();
+      ss << "Vertex indices out of bounds.\n" << std::endl;
+      (*warn) += ss.str();
     }
   }
-  if (greatest_vn_idx >= static_cast<int>(vn.size() / 3))
-  {
-    if (err) {
+  if (greatest_vn_idx >= static_cast<int>(vn.size() / 3)) {
+    if (warn) {
       std::stringstream ss;
-      ss << "WARN: Vertex normal indices out of bounds.\n" << std::endl;
-      (*err) += ss.str();
+      ss << "Vertex normal indices out of bounds.\n" << std::endl;
+      (*warn) += ss.str();
     }
   }
-  if (greatest_vt_idx >= static_cast<int>(vt.size() / 2))
-  {
-    if (err) {
+  if (greatest_vt_idx >= static_cast<int>(vt.size() / 2)) {
+    if (warn) {
       std::stringstream ss;
-      ss << "WARN: Vertex texcoord indices out of bounds.\n" << std::endl;
-      (*err) += ss.str();
+      ss << "Vertex texcoord indices out of bounds.\n" << std::endl;
+      (*warn) += ss.str();
     }
   }
 
@@ -2241,6 +2242,7 @@ bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
 bool LoadObjWithCallback(std::istream &inStream, const callback_t &callback,
                          void *user_data /*= NULL*/,
                          MaterialReader *readMatFn /*= NULL*/,
+                         std::string *warn, /* = NULL*/
                          std::string *err /*= NULL*/) {
   std::stringstream errss;
 
@@ -2377,19 +2379,25 @@ bool LoadObjWithCallback(std::istream &inStream, const callback_t &callback,
         SplitString(std::string(token), ' ', filenames);
 
         if (filenames.empty()) {
-          if (err) {
-            (*err) +=
-                "WARN: Looks like empty filename for mtllib. Use default "
+          if (warn) {
+            (*warn) +=
+                "Looks like empty filename for mtllib. Use default "
                 "material. \n";
           }
         } else {
           bool found = false;
           for (size_t s = 0; s < filenames.size(); s++) {
+            std::string warn_mtl;
             std::string err_mtl;
             bool ok = (*readMatFn)(filenames[s].c_str(), &materials,
-                                   &material_map, &err_mtl);
+                                   &material_map, &warn_mtl, &err_mtl);
+
+            if (warn && (!warn_mtl.empty())) {
+              (*warn) += warn_mtl;  // This should be warn message.
+            }
+
             if (err && (!err_mtl.empty())) {
-              (*err) += err_mtl;  // This should be warn message.
+              (*err) += err_mtl;
             }
 
             if (ok) {
@@ -2399,9 +2407,9 @@ bool LoadObjWithCallback(std::istream &inStream, const callback_t &callback,
           }
 
           if (!found) {
-            if (err) {
-              (*err) +=
-                  "WARN: Failed to load material file(s). Use default "
+            if (warn) {
+              (*warn) +=
+                  "Failed to load material file(s). Use default "
                   "material.\n";
             }
           } else {