Do you literally mean 1) C++, 2) just a lower level language with control over those or 3) reading the relevant libraries or system code implementing IPC etc. (isn't it in C not C++?)?
Well, I (and also my current employer and the whole team) think that C++ is the best proxy for general programming ability and understanding of CS. A person that understands the intricacies of C++ and all of its aspects, including the STL, template metaprogramming, design patterns, policy based design, concepts, concurrency, etc. is a person demonstrably capable of reasoning about complex systems, rulesets, different intersecting approaches and such, not to mention an intuitive understanding of the meat and potatoes of resource management and code generation. Even if you're hiring for something else and adjacent, like systems C or C# for finance, you'll be hard pressed to find a better measure of general programming ability and understanding of CS concepts. If you're really familiar with C++, you're at peace with best practice deprecation, handling legacy code, compromises of language design, etc. You're a grownup.
Another great measure is knowledge of (Linux) systems programming, this also tends to weed out people with inferior understanding of, you know, programming and deploying software on actual computers. The interfaces here are obviously in C.
There are many things that aren't congruent with C++ ability, such as general functional programming ability, or hiring for JS frontend or Erlang roles and such, but for most industry jobs, it's a useful determinator.
I don't know any good C++ programmers that aren't great and all-rounded overall developers. I can't say the same for any other language or technology.
Do you literally mean 1) C++, 2) just a lower level language with control over those or 3) reading the relevant libraries or system code implementing IPC etc. (isn't it in C not C++?)?