Rebuilding target files when source files have changed is seems easy, but is not. Commonly used build systems (make, ninja, etc.) are often unable to guarantee both that they rebuild only what needs to be rebuilt and that they do not rebuild what does not need to be rebuilt.

I will show how to reliably encounter common build system failure modes and explain which architectural choices lead to those. Using DJB's “redo” design as an example, I will show how build system architecture determines if failure modes can be addressed at all. Lastly, I will speculate why many developers dismiss such issues before encountering them – and some even do afterwards.