Using struct for indexed buffer object in OpenGL results in segfault

Question

I've been using std::vector's for storing vertex attributes and everything was working just fine, rendering all kinds of different meshes. But after refactoring so that my vertex attributes are stored in a struct I can't get the simplest thing to render. Here is the struct (simplified):

struct Vertex{
  GLfloat x, y, z;        //Vertex
  GLfloat r, g, b, a;     //Color
};

I have two std::vector's, one for storing the vertex attributes and one for the index:

std::vector indices;
std::vector vertices;

In the initialization function I fill these vectors with a simple green triangle:

struct Vertex vertex1;
vertex1.x=1.0;
vertex1.y=0.0;
vertex1.z=0.0;
vertex1.r=0.0;
vertex1.g=1.0;
vertex1.b=0.0;
vertex1.a=1.0;
vertices.push_back(vertex1);
struct Vertex vertex2;
vertex2.x=0.0;
vertex2.y=1.0;
vertex2.z=0.0;
vertex2.r=0.0;
vertex2.g=1.0;
vertex2.b=0.0;
vertex2.a=1.0;
vertices.push_back(vertex2);
struct Vertex vertex3;
vertex3.x=1.0;
vertex3.y=1.0;
vertex3.z=0.0;
vertex3.r=0.0;
vertex3.g=1.0;
vertex3.b=0.0;
vertex3.a=1.0;
vertices.push_back(vertex3);

indices.push_back(1);
indices.push_back(2);
indices.push_back(3);

Then I bind the buffers:

glGenBuffers(1, &ibo_elements);
glBindBuffer(GL_ARRAY_BUFFER, ibo_elements);
glBufferData(GL_ARRAY_BUFFER, vertices.size() * sizeof(struct Vertex), &vertices[0], GL_STATIC_DRAW);

glGenBuffers(1, &elementbuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, elementbuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size() * sizeof(GLushort), &indices[0], GL_STATIC_DRAW);

Then after setting up the shader program and binding attribute names I use glutDisplayFunc to run this callback:

#define BUFFER_OFFSET(i) ((char *)NULL + (i))

void onDisplay()
{
  glClearColor(0.0, 0.0, 0.0, 1.0);
  glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);

  glUseProgram(program);

  glBindBuffer(GL_ARRAY_BUFFER, ibo_elements);
  glVertexAttribPointer(
                        attribute_v_coord,
                        3,
                        GL_FLOAT,
                        GL_FALSE,
                        sizeof(struct Vertex),
                        BUFFER_OFFSET(0)
                        );
  glEnableVertexAttribArray(attribute_v_coord);

  glBindBuffer(GL_ARRAY_BUFFER, colorbuffer);
  glVertexAttribPointer(
                        attribute_v_color,
                        4,
                        GL_FLOAT,
                        GL_FALSE,
                        sizeof(struct Vertex),
                        BUFFER_OFFSET(sizeof(GLfloat)*3)
                        );
  glEnableVertexAttribArray(attribute_v_color);

  glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, elementbuffer);
  int size; glGetBufferParameteriv(GL_ELEMENT_ARRAY_BUFFER, GL_BUFFER_SIZE, &size);
  glDrawElements(GL_TRIANGLES, size/sizeof(GLushort), GL_UNSIGNED_SHORT, 0);

  glDisableVertexAttribArray(attribute_v_coord);
  glDisableVertexAttribArray(attribute_v_color);
  glutSwapBuffers();
}

Everything is very similar to what I had working before. So I'm guessing it has something to do with the change of data structure. Valgrind shows this error:

==5382== Invalid read of size 4
==5382==    at 0x404EF6A: ??? (in /tmp/glR69wrn (deleted))
==5382==    by 0x870E8A9: ??? (in /usr/lib/libnvidia-glcore.so.325.15)
==5382==    by 0x200000003: ???
==5382==    by 0x404EEBF: ??? (in /tmp/glR69wrn (deleted))
==5382==    by 0x2: ???
==5382==    by 0xAFFC09F: ???
==5382==    by 0x41314D3: ???
==5382==    by 0x40E6FFF: ??? (in /dev/nvidia0)
==5382==    by 0xFFFFFFFE: ???
==5382==  Address 0x28 is not stack'd, malloc'd or (recently) free'd

Am I not defining the vertex attribute pointers correctly? It looks like OpenGL is trying to read a float that was never set properly.

Andon M. Coleman · Accepted Answer

You should use a single VBO for every vertex attrib pointer in this situation. You are supplying a single datastore, with interleaved data. All you need to do is alter the calls that setup the vertex attrib pointers so that they have the correct stride and base offset address. So this "colorbuffer" VBO (which is a poor choice of names, since OpenGL already has something called a colorbuffer) is more than likely a source of your problems.

Another problem, as mentioned elsewhere, is that your element indices are starting at 1. You have 3 vertices in this example, the element buffer should be populated with some combination of 0,1,2. Having 3 in your element buffer is going to lead to undefined behavior at draw-time. A lot of times drivers will not crash if you use an invalid index, and the GL unfortunately does not have an error state for index out of bounds. Often you only know something is wrong in this situation because garbage appears on your screen.

I am concerned that you don't even know how many elements are in your IBO without querying that information from the GL state machine. That is poor application design, sorry to say. You should definitely know how many elements you want to draw before hand. VBOs should be wrapped in a data structure or class anyway (that includes at the very least the number of elements allocated), you don't want to simply toss around buffer object handles with no idea what they represent.

Also it can be wasteful to use floating-point values for vertex color, you almost never need them (GLubyte and 0-255 usually works well enough). 1 GLfloat takes as much memory as 4 GLubytes, and you are using 4 GLfloats... Using 4 GLubytes can also help with alignment if you opt to use xyz instead of xyzw for vertex position.

On older hardware 4x GLubyte colors were a "fast path" for hardware T&L. They still take less memory on newer hardware so they're a win in virtually all situations :)

Using struct for indexed buffer object in OpenGL results in segfault

Answers (2)

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