Call out to a shell? Pass structs between C-based programs using sockets? Use runtime marshalling? Make your "API" be just different arbitrary JSON objects mapp

Call out to a shell? Pass structs between C-based programs using sockets? Use runtime marshalling? Make your "API" be just different arbitrary JSON objects mapping to maps? Hide the entire API behind a lazy cache with undocumented side effects when cache misses occur? I wouldn't consider any of these "API"s although they are interfaces.