weird behavior of glVertexAttribPointer

Question

I'm creating default VAO and one VBO, and bind them. I'm loading model data to the array of structs vertex_data_t

    glBufferData(GL_ARRAY_BUFFER, nvertices * sizeof(vertex_data_t), vertices, GL_STATIC_DRAW);

Then in draw function i do:

    glBindVertexArray(vao);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(vertex_data_t), (const GLvoid *)offsetof(vertex_data_t, position));
    glEnableVertexAttribArray(1);
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_TRUE, sizeof(vertex_data_t), (const GLvoid *)offsetof(vertex_data_t, position));
    glBindVertexArray(0);
    glDrawArrays(GL_TRIANGLES, 0, nvertices);

I'm getting nice, shaded Suzanne:

https://i.sstatic.net/uRjpv.png

However, this is wrong! Last argument of glVertexAttribPointer for normal attribute should be 12 aka (const GLvoid *)offsetof(vertex_data_t, normal), but when I do so my Suzanne is broken:

https://i.sstatic.net/zBjTS.png

How is it possible? How does shader know an offset to the normal?

Vertex shader:

     attribute vec3 vertex;
     attribute vec3 normal;

     uniform vec4 ambient_color;
     uniform vec4 diffuse_color;
     uniform vec3 light_position;
     uniform mat3 normal_matrix;
     uniform mat4 model_view_matrix;
     uniform mat4 model_view_projection_matrix;

     varying vec4 varying_color;

     void main(void) {
              vec4 vertex4 = vec4(vertex.x, vertex.y, vertex.z, 1.0);
              vec3 eye_normal = normal_matrix * normal;
              vec4 position4 = model_view_matrix * vertex4;
              vec3 position3 = position4.xyz / position4.w;
              vec3 light_direction = normalize(light_position - position3);
              float diffuse = max(0.0, dot(eye_normal, light_direction));
              varying_color.rgb = diffuse * diffuse_color.rgb;
              varying_color.a = diffuse_color.a;
              varying_color += ambient_color;
              gl_Position = model_view_projection_matrix * vertex4;
    }

Answer 1

I think you miss something like:

glBindAttribLocation(progId, 0, "vertex");
glBindAttribLocation(progId, 1, "normal");