It is not a runtime in the ways that matter. After it has transferred control to the user program, it is gone completely; it never inserts itself into the prog

It is not a runtime in the ways that matter. After it has transferred control to the user program, it is gone completely; it never inserts itself into the program's execution. It is more like the loader or dynamic linker of the OS than like the runtime of other languages. If you have your own mechanism for transferring control to the C program you don't need the main() wrapper at all.