Modifying a Shader Storage Buffer Object from within a Vertex Shader - c++

Question

In my base program (C++/OpenGL 4.5) I have copied the content of the Vertex Buffer to an Shader Storage Buffer (SSBO):
float* buffer = (float*) glMapBuffer(GL_ARRAY_BUFFER, GL_READ_ONLY);
glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 3, ssbo[2]);
glBufferData(GL_SHADER_STORAGE_BUFFER, sizeof(GLfloat)*size,buffer, GL_DYNAMIC_DRAW);
glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 3, 0);
glUnmapBuffer(GL_ARRAY_BUFFER);
In the Vertex Shader this data is bound to an array:
#version 430
#extension GL_ARB_shader_storage_buffer_object : require
layout(shared, binding = 3) buffer storage
{
float array[];
}
But when I'm trying to overwrite an array entry in the main function like this:
array[index_in_bounds] = 4.2;
nothing happens.
What am I doing wrong? Can I change the buffer from within the Vertex Shader? Is this only possible in a Geometry Shader? Do I have to do this with Transform Feedback (that I have never used before)?
edit:
I'm mapping the buffers for test purposes in my main program, just to see if the data changes:
float* buffer = (float*) glMapBuffer(GL_ARRAY_BUFFER, GL_READ_ONLY);
float* ssbo = (float*) glMapNamedBuffer(3, GL_READ_ONLY);
for(int i = 0; i < SIZE_OF_BUFFERS; i++)
printf("% 5f | % 5f\n", ssbo[i], buffer[i]);
glUnmapNamedBuffer(3);
glUnmapBuffer(GL_ARRAY_BUFFER);

Answer 1

Okay, I have found the problem using the red book. I have not bound the buffer correctly and binding the buffer base has to happen after buffering the data:
float* buffer = (float*) glMapBuffer(GL_ARRAY_BUFFER, GL_READ_ONLY);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, &ssbo); // bind buffer
// switched these two:
glBufferData(GL_SHADER_STORAGE_BUFFER, sizeof(GLfloat)*size,buffer, GL_DYNAMIC_DRAW);
glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 3, &ssbo);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0); // unbind buffer
glUnmapBuffer(GL_ARRAY_BUFFER);