> Why is it that exiting at the end of main() requires a system call? Wouldn't a `ret` instruction go "back" to somplace where the OS itself will do cleanup wor

> Why is it that exiting at the end of main() requires a system call? Wouldn't a `ret` instruction go "back" to somplace where the OS itself will do cleanup work? The only way for execution to cross the barrier between "user space" and "kernel space" is through a system call or an interrupt (we won't speak of call gates). Even if the OS had put an address on the stack, so that the "ret" would go there after returning from main(), the code there would still need to do a system call to go back to the OS. While nowadays Linux has a shared page of code mapped on every process (the vDSO), that wasn't the case in the past; all code on the "user space" side had to come from either the executable itself, or a library it loaded. Given that, it's natural that it was left to the executable to call the "exit" system call at the end.