Function pid_is_alive 

pub fn pid_is_alive(pid: i32) -> bool

Return whether pid names a live, runnable process.

This intentionally treats zombies as not alive. kill(pid, 0) reports success for zombies because the PID still exists. This is not a resource-teardown barrier: on Linux a zombie leader can still have another thread releasing the shared file table, including disk locks. Lifecycle callers must fence those resources separately.