Man page 9 : do_splice_direct

do_splice_direct - splices data directly between two files

long do_splice_direct(struct file *  in , loff_t *  ppos , struct file *  out , loff_t *  opos , size_t  len , unsigned int  flags );

in
    file to splice from

ppos
    input file offset

out
    file to splice to

opos
    output file offset

len
    number of bytes to splice

flags
    splice modifier flags

For use by do_sendfile. splice can easily emulate sendfile, but doing it in the application would incur an extra system call (splice in + splice out, as compared to just sendfile). So this helper can splice directly through a process-private pipe.