how to bind two vertex buffers to vertex shader input, as in DirectX

[tonyo_au]

Your vertex positions sizes don't match

glVertexAttribPointer(vertexId, 2, GL_FLOAT, GL_FALSE, sizeof(vertices), 0)

size 2

attribute vec4 vertex;

size 4

[saman_artorious]

I did not find any line in my code that binds the VBO to the shader program, is it possible that it's missing, or the code is complete and correct?

[thokra]

that binds the VBO to the shader program

VBOs aren't bound to a shader. VBOs even need not be bound at all to source data from their data store. You can associate a vertex buffer object with an index in the range [0 .. GL_MAX_VERTEX_ATTRIBS] via glVertexAttribPointer (and other means with GL4.3, but since you're obviously on a lower version you shouldn't bother until you get a hold of the concept. You then bind that index to a corresponding location in your shader, either via glBindAttribLocation or, more preferably, via explicit assignment in the shader. For more details, please read this.

Also, I refer you to the corresponding wiki page.

[saman_artorious]

Thanks for the reference.
The shape I rendered is merely what I wanted. Though, 1/4 of the circle is rendered. (I need to find where that comes from).
Now, let's say I need to change the value of colors in my program. I know that for this I need to use glMapBuffer() & glUnmapBuffer().
This code gives me a segmentation fault. Could you tell me the correct way of accessing and modifying the colors variable please.

    float* colors = (float *) glMapBuffer( GL_ARRAY_BUFFER, GL_WRITE_ONLY );
 
    int k = segmentIndex * RANGE_COUNT * 6;
        for(int i = 0; i < RANGE_COUNT; ++i)
        {
            float rnd = ((rand() + segmentIndex) % 10000) / 10000.0f;
 
            colors[k++] = rnd;
            colors[k++] = rnd;
            colors[k++] = rnd;
            colors[k++] = rnd;
            colors[k++] = rnd;
            colors[k++] = rnd;
        }
    glUnmapBuffer( GL_ARRAY_BUFFER );

I want to copy only a specified portion everytime, so this way I think I should use the write() function as glMapBuffer() copies the whole buffer. could explain this to me with a sample please.

[GClements]

Your vertex positions sizes don't match

glVertexAttribPointer(vertexId, 2, GL_FLOAT, GL_FALSE, sizeof(vertices), 0)

attribute vec4 vertex;

This doesn't matter. Vertex attributes are always 4-element vectors. If you specify fewer than 4 components, y and z default to 0 while w defaults to 1.