I am very new to OpenGL so I am using GLTools to learn the basics.

For the start I would like to create a circle with adjustable tesselation (e.g. the number of triangles which form the circle).

I have a toolbar with a variable tesselation:

TwBar *bar;
void InitGUI() {
    bar = TwNewBar("TweakBar");
    TwDefine(" TweakBar size='200 400'");
    TwAddVarRW(bar, "Tesselation", TW_TYPE_UINT32, &tesselation, "");
}

Then I have these two functions and global variables which calculate the vertices for the circle:

GLBatch circle;

unsigned int tesselation = 8;

void topAndBottomGeometry(M3DVector3f *vertices, M3DVector4f *colors, const int vertOffset) {
	m3dLoadVector3(vertices[0], 0, 0, vertOffset);
	m3dLoadVector4(colors[0], 0, 0, 1, 1);

	int i = 1;
	int color = 1;
	for (int step = 0; step <= tesselation; step++) {
		const double angle = step * ((2.0 * GL_PI) / tesselation);
		const double x = width * sin(angle);
		const double y = width * cos(angle);

		if ((color % 2) == 0) {
			m3dLoadVector4(colors[i], 0, 0, 1, 1);
		}
		else {
			m3dLoadVector4(colors[i], 0, 1, 0, 1);
		}

		m3dLoadVector3(vertices[i], x, y, vertOffset);

		color++;
		i++;
	}
}


void CreateGeometry() {
	const int amount = tesselation + 2;
	M3DVector3f *vertices = (M3DVector3f*)malloc(sizeof(float) * 3 * amount);
	M3DVector4f *colors = (M3DVector4f*)malloc(sizeof(float) * 4 * amount);

	topAndBottomGeometry(vertices, colors, 0);

	circle.Begin(GL_TRIANGLE_FAN, amount);
	circle.CopyVertexData3f(vertices);
	circle.CopyColorData4f(colors);
	circle.End();

	free(vertices);
	free(colors);
}

Now, everything works fine in my GUI if the tesselation value is below or equal to the initial value (in this case 8). But if the value is any higher than 8 (doesn't matter which value), the GUI "freezes" and the circle stays the way it was like when tesselation is set to 8:

Tesselation at 8
Tesselation at 6
https://i.stack.imgur.com/2lzvi.png