The last iteration of the rewrite uncovered a dirty, ugly bug that should remain secret, unless you dare to diff the recent versions. But now it's fixed and the system is more stable. I didn't even encounter the problem in real life but it sure would have been easy to understand because all system calls have explicit error messages.
TL;DR : don't use versions of the code before HTTaP_src.20200502.2.tgz !
Now I start to work on a second example code that switches dynamically between polling mode and blocking mode, under control of both the workload and the user. This implies giving orders in HTTaP form from the user interface, hence more JS and C code.
The C code is not really developed on the HTTaP side, I have something ambitious in mind but I prefer to develop it later. But I need HTTaP now. So I have found a solution : split the HTTaP request parsing code, away from the server. This will be a separate file. And I can provide a preliminary/quickanddirty version for the iteration that I start now, through a HTTaP_parse() function.