SFML Segmentation Fault when using VBOs?

Question

I'm trying to follow along with the gltut tutorials and for some reason when I call GLDrawArrays my program segmentation faults. I've been looking at the state of my application with the Mac OpenGL Profiler and Googling and I can't seem to figure out why it isn't working. I may have missed setting some OpenGL state somewhere.

My code is here, it segfaults on line 32. If I don't call glEnableClientState(GL_VERTEX_ARRAY); and the respective glDisableClientState(GL_VERTEX_ARRAY); it compiles and runs, but I get no rendering, which is what I would expect given my limited OpenGL knowledge.

Answer 1

Well, there are a couple of things that are... odd about this code.

First:

glDrawArrays(GL_TRIANGLES, 0, 2);

That will never actually draw anything. Triangles are made of 3 vertices and you told it to read 2.

I doubt that's causing the seg-fault, but it might be. Just change the 2 to 3.

Next:

const float vertexPositions[] = {
  0.75f, 0.75f, 0.0f, 0.2f,
  0.75f, -0.75f, 0.0f, 0.2f,
  -0.75f, -0.75f, 0.0f, 0.2f,
};

Again I'm not sure how you copied that wrong, but the "0.2f" for the W component should be "1.0f".

The source of your segfault:

glEnableClientState(GL_VERTEX_ARRAY);

Why do you call this? OK, I know why you're calling it. You've somehow taken a tutorial that's supposed to be showing you how to do modern, shader-based OpenGL, and are adapting it to fixed-function stuff.

If you are, then you need to adapt it properly.

glVertexAttribPointer is for setting up generic vertex attributes. You cannot use generic vertex attributes with fixed-function rendering. If you're going to do things in fixed-function, you need to actually do them in fixed function:

glBindBuffer(GL_ARRAY_BUFFER, positionBufferObject);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(4, GL_FLOAT, 0, 0);

glDrawArrays(GL_TRIANGLES, 0, 3);

glDisableClientState(GL_VERTEX_ARRAY);
glBindBuffer(GL_ARRAY_BUFFER, 0);

Answer 2

My guess: You have 12 floats for your positions. You are telling to OpenGL that each vertex should be 4 floats by second parameter in here: glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);, so you have 3 vertices for these.

Then you tell OpenGL to draw 2 triangles in glDrawArrays command. But this would need 6 vertices - 3 vertices per triangle. So this could cause segmentation fault. Try to draw just one triangle or add more vertices.