Function sys_waitpid 

pub fn sys_waitpid(
    pid: i64,
    status_ptr: u64,
    options: u32,
) -> Result<u64, SyscallError>

SYS_PROC_WAITPID (310): wait for a child process to exit.

Arguments:

• pid : child task ID to wait for, or -1 (any child).
• status_ptr : userspace *i32 to receive the encoded wait status (W_EXITCODE). Pass 0 to discard.
• options : WNOHANG (1) — return immediately if no child ready.

Returns:

• child task ID on success.
• 0 if WNOHANG and no child has exited yet.

Errors:

• -ECHILD (-10) — no matching children.
• -EINTR (-4) — interrupted by a pending signal.
• -EINVAL (-22) — unknown option bits.