The source code for the core of Putty is an astonishing construction, a sort of extreme case of Duff's device spanning many, many pages. At each point where an operation might block, there is a macro that expands to a break and a switch/case clause that enables returning to that point.

A colleague said "I love it, and hate myself for loving it".

The new "async/await" features appearing in numerous languages encode this pattern -- i.e., result in substantially the same object code -- in a more robust way, but Putty works fine.