Just learned that there is no way for a GLSL shader to send data to the CPU side program. I think, at least. No way to emit debugging information at all, not even a puts.

At least if I understand, "Uniforms are so named because they do not change from one shader invocation to the next within a particular rendering call thus their value is uniform among all invocations."

Thought I could define a uniform, then set that to some debugging value in the shader.