As a student, one of the biggest lessons I've picked up is that a program which compiles and runs is not the same as a program that is correct. It really hit home while I was building a multithreaded C++ network proxy server — a project that authenticates users with SHA-256, enforces role-based website filtering, and caches HTTP responses using a custom LRU cache.
The happy path worked on pretty early. The real learning started when I went looking for the bugs that don't announce themselves — the ones that only show up under concurrency, fragmented packets, or heavy load. Here are four of the most interesting and challenging issues I ran into and how I fixed them.
1. The Thread Pool That Never Detached
The proxy spawns 20 persistent worker threads at startup. The idea was simple: create the workers, then detach them from the main thread so they run independently.
The bug was an ordering mistake. My detachment loop ran before the threads were created:
