glVertexAttribPointer() working only with the first stream - opengl

Question

I am trying to use glVertexAttribPointer() to give some data to my vertex shader. The thing is that it's working only with the FIRST attribute...
Here is my OpenGL code:
struct Flag_vertex
{
GLfloat position_1[ 8 ];
GLfloat position_2[ 8 ];
};
Flag_vertex flag_vertex;
... // fill some data to flag_vertex
GLuint vertexbuffer_id;
glGenBuffers( 1, &vertexbuffer_id );
glBindBuffer( GL_ARRAY_BUFFER, vertexbuffer_id );
glBufferData( GL_ARRAY_BUFFER, sizeof(flag_vertex), &flag_vertex, GL_STATIC_DRAW );
glEnableVertexAttribArray( 0 );
glEnableVertexAttribArray( 1 );
glBindBuffer( GL_ARRAY_BUFFER, vertexbuffer_id );
glVertexAttribPointer( 0, 2, GL_FLOAT, GL_FALSE, 0, (void*)offsetof(Flag_vertex, position_1) );
glVertexAttribPointer( 1, 2, GL_FLOAT, GL_FALSE, 0, (void*)offsetof(Flag_vertex, position_2) );
and my shader is something like:
#version 420 core
layout(location = 0) in vec2 in_position_1;
layout(location = 1) in vec2 in_position_2;
out vec2 texcoord;
void main()
{
gl_Position = vec4(in_position_X, 0.0, 1.0);
texcoord = in_position_X * vec2(0.5) + vec2(0.5);
}
If I use "in_position_1" my texture RENDERS PERFECTLY, but if I use in_position_2 nothing happens...
Tip: before link my shaders I am doing:
glBindAttribLocation( programID, 0, "in_position_1");
glBindAttribLocation( programID, 1, "in_position_2");
Why it works only with the first stream? I need more data going to my vertex... I need to send color, etc... any hint?

Answer 1

glVertexAttribPointer( 0, 2, GL_FLOAT, GL_FALSE, 0, (void*)offsetof(Flag_vertex, position_1) );
glVertexAttribPointer( 1, 2, GL_FLOAT, GL_FALSE, 0, (void*)offsetof(Flag_vertex, position_2) );
These lines don't make sense. At least, not with how Flag_vertex is defined. If Flag_vertex is really supposed to be a vertex (and not a quad), then it makes no sense for it to have 8 floats. If each Flag_vertex defines a full quad, then you named it wrong; it's not a vertex at all, it's Flag_quad.
So it's hard to know what you're even trying to accomplish here.
Also:
If I use "in_position_1" my texture RENDERS PERFECTLY, but if I use in_position_2 nothing happens...
Of course it does. Your position data is in attribute 0. Your position data is therefore not in attribute 1. If you pretend attribute 1 has your position data when it clearly doesn't, you will not get reasonable results.
Your problem is that you're always using attribute 0 when you should be using both of them. You shouldn't be picking one or the other. Use in_position_1 for the position and in_position_2 for the texture coordinate. And try to name them reasonably, based on what they do (like position and texture_coord or something). Don't use numbers for them.
Tip: before link my shaders I am doing:
That is the exact same thing as the layout(location=#) setting in the shader. If you want it in the shader, then put it in the shader. If you want it in your OpenGL code, then put it in your OpenGL code. Don't put it in both places.