What happens when rendering an OpenGL buffer that has been padded with NULL (or another value)?

Question

I have the following setup:

• S - the scene that includes terrain, some houses and other man-made objects
• T - a set of tree configurations that I would like to iterate through and place in S.

Every iteration results in a configuration C(i) = S + T(i) that is rendered onto the screen.

Since S is static and does not change between iterations I decided to employ glBufferSubData() with a large buffer split into two regions, where the first region of my buffer will store S, while the second region will contain the T(i). With every new iteration a new C(i) will be the state of the big buffer.

This will allow me to pre-allocate the big buffer, except for one issue I am facing, namely that T(i) may have slight variations in size. In order to not have to expand my buffer (e.g. through buffer orphaning aka buffer re-specification) I would like to pre-calculate the largest T(i) resulting in a T_max, which I can then use to calculate the total size of my big buffer as C_max = S + T_max.

I know that glBufferSubData allows a partial update of the data. The problem is that after my second region is updated with some T(i) I may have old data remaining from T(i-j) if the size of T(i) is LESS THAN the size of T(i-j).

In this case I will have C(i,j) = S + T(i) + partial(T(i-j)).

I was thinking that in this case I can calculate the offset of the partial data from T(i-j) and use a second glBufferSubData() that will set that chunk of memory to NULL or actual 0 (see bullet points below). So I will have

[                 S               ][         T(i)       ][   NULLs/0s   ]

I read that for

• VBOs this is not really an issue
• IBOs this may lead to
  • Ignore the index and continue processing (most likely scenario)
  • Restart the primitive being drawn (potentially causing rendering artifacts)
  • In rare cases, it could lead to a program crash
• UBOs (which I am not using here, at least yet) this may lead to
  • For numeric data types (float, int, etc.), NULL values will likely be interpreted as zeros
  • For data types like vec3 or mat4 (representing vectors or matrices), NULL might be treated as all zeros, leading to unexpected behavior in the shader.

I am curious what exactly happens under the hood and how to avoid crashes. My hope is that this is not yet another "it depends on the driver implementation" scenario. :D