gistfile1.txt

1> {ok, Fd} = file:open("foobar.foobar", [read,write,raw,binary]).
{ok,{file_descriptor,prim_file,{#Port<0.494>,7}}}
2> Data = <<"foobar">>.
<<"foobar">>
3> Size = iolist_size(Data).
6
4> {ok, Start} = file:position(Fd, cur).
{ok,0}
5> file:write(Fd, Data).
ok
6> {ok, End} = file:position(Fd, cur).
{ok,6}
7> End = Start + Size.
6
8>