Function tee 

pub fn tee<FdIn: AsFd, FdOut: AsFd>(
    fd_in: FdIn,
    fd_out: FdOut,
    len: usize,
    flags: SpliceFlags,
) -> Result<usize>

tee(fd_in, fd_out, len, flags)—Copy data between pipes without consuming it.

This reads up to len bytes from in_fd without consuming them, and writes them to out_fd.

References

• Linux