Function splice 

pub fn splice<FdIn: AsFd, FdOut: AsFd>(
    fd_in: FdIn,
    off_in: Option<&mut u64>,
    fd_out: FdOut,
    off_out: Option<&mut u64>,
    len: usize,
    flags: SpliceFlags,
) -> Result<usize>

splice(fd_in, off_in, fd_out, off_out, len, flags)—Transfer data between a file and a pipe.

This function transfers up to len bytes of data from the file descriptor fd_in to the file descriptor fd_out, where one of the file descriptors must refer to a pipe.

off_* must be None if the corresponding fd refers to a pipe. Otherwise its value points to the starting offset to the file, from which the data is read/written. On success, the number of bytes read/written is added to the offset.

Passing None causes the read/write to start from the file offset, and the file offset is adjusted appropriately.

References

• Linux