Function waitpid 

pub fn waitpid(
    pid: isize,
    status: Option<&mut i32>,
    options: usize,
) -> Result<usize, Error>

Wait for a child process (raw, single attempt).

pid supports POSIX values (-1 = any child). status receives encoded wait status (same layout as Linux waitpid).

Prefer waitpid_blocking for blocking waits — it automatically retries on EINTR.