Getting Segmentation fault while loading OBJ file

Question

I am getting 'Segmentation fault(core dumped)' in C++ using gcc after adding this piece of code to my OpenGL program

vector<GLfloat> vecTofloat(vector<glm::vec3> veca){
    vector<GLfloat> fa;
    for (int i = 0; i < veca.size(); i++){
        glm::vec3 vec = veca[i];
        fa.push_back(vec.x);
        fa.push_back(vec.y);
        fa.push_back(vec.z);
    }
    return fa;
}

vector<GLfloat> vecTofloat(vector<glm::vec2> veca){
    vector<GLfloat> fa;
    for (int i = 0; i < veca.size(); i++){
        glm::vec2 vec = veca[i];
        fa.push_back(vec.x);
        fa.push_back(vec.y);
    }
    return fa;
}

MeshInstance loadOBJ(const char* path, const char* texPath, int tw, int th){
    vector<glm::vec3> vert;
    vector<glm::vec3> tvert;
    vector<GLuint> ivert;
    vector<glm::vec3> norm;
    vector<glm::vec3> tnorm;
    vector<GLuint> inorm;
    vector<glm::vec2> tex;
    vector<glm::vec2> ttex;
    vector<GLuint> itex;

    FILE* file = fopen(path, "r");
    if (file == NULL){
        LOG_ERROR("Unable to load OBJ mesh.");
        exit(-1);
    }

    while(true){
        char start[128];

        int res = fscanf(file, "%s", start);

        if (res == EOF){
            break;
        } else if (strncmp(start, "v", strlen("v")) == 0){
            glm::vec3 vertex;
            fscanf(file, "%f %f %f\n", &vertex.x, &vertex.y, &vertex.z);
            tvert.push_back(vertex);
        } else if (strncmp(start, "vt", strlen("vt")) == 0){
            glm::vec2 tex;
            fscanf(file, "%f %f\n", &tex.x, &tex.y);
            ttex.push_back(tex);
        } else if (strncmp(start, "vn", strlen("vn")) == 0){
            glm::vec3 nrm;
            fscanf(file, "%f %f %f\n", &nrm.x, &nrm.y, &nrm.z);
            tnorm.push_back(nrm);
        } else if (strncmp(start, "f", strlen("f")) == 0){
            std::string vertex1, vertex2, vertex3;
            unsigned int vertexIndex[3], uvIndex[3], normalIndex[3];
            fscanf(file, "%d/%d/%d %d/%d/%d %d/%d/%d\n",&vertexIndex[0],&uvIndex[0],&normalIndex[0], &vertexIndex[1], &uvIndex[1], &normalIndex[1], &vertexIndex[2], &uvIndex[2], &normalIndex[2] );

            ivert.push_back(vertexIndex[0]);
            ivert.push_back(vertexIndex[1]);
            ivert.push_back(vertexIndex[2]);
            itex.push_back(uvIndex[0]);
            itex.push_back(uvIndex[1]);
            itex.push_back(uvIndex[2]);
            inorm.push_back(normalIndex[0]);
            inorm.push_back(normalIndex[1]);
            inorm.push_back(normalIndex[2]);
        }
    }

    for (int i = 0; i < ivert.size(); i+=3){
        unsigned int vertex = ivert[i];
        vert.push_back(tvert[vertex - 1]);
    }

    for (int i = 0; i < itex.size(); i+=3){
        unsigned int vertex = itex[i];
        tex.push_back(ttex[vertex - 1]);
    }

    for (int i = 0; i < inorm.size(); i+=3){
        unsigned int vertex = inorm[i];
        norm.push_back(tnorm[vertex - 1]);     
    }

    return LoadToVAO(vecTofloat(vert), ivert, vecTofloat(tex), texPath, tw, th);
}

What it does is simply load an obj file. I know it is due to accessing memory that is already removed but I am not able to figure out where the segmentation fault is occuring in the program.

Edit: I debugged the program with gdb and now I get this:

Starting program: /home/saroj/workspace/Rachaita3D/Rachaita3D step
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff5120700 (LWP 11819)]

Thread 1 "Rachaita3D" received signal SIGSEGV, Segmentation fault.
0x0000555555565f66 in __gnu_cxx::new_allocator<glm::vec<2, float, (glm::qualifier)0> >::construct<glm::vec<2, float, (glm::qualifier)0>, glm::vec<2, float, (glm::qualifier)0> const&> (this=0x7fffffffdca0, __p=0x555555950a50, __args#0=...)                              
    at /usr/include/c++/8/ext/new_allocator.h:136
136             { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }

Edit 2: At tex.push_back(ttex[vertex - 1]); , p ttex[vertex - 1] returns

Cannot access memory at address 0x0

For some reason my loader is not loading texture coordinates and normal vectors.

Answer 1

Right now the issue is in the way you're parsing the Wavefront OBJ commands.

The first check strncmp(start, "v", strlen("v")) does not check if the command was v, it checks if the command starts with "v".

When code encounters vn or vt, the first check will find 'v' at the beginning, and wrong branch is taken. You fill up the tvert array with unrelated data, leaving tnorm and ttex empty, so the code crashes trying to index those.

Either compare strings with strcmp(), or reorder the checks, so the v will be tested last.