Le nonidi 29 ventôse, an CCXI, cashin@cs.uga.edu a écrit :
>     Printf.printf "after reading %d chars: \"%s\", position is %d\n"
>       (UnixLabels.read fd ~buf ~pos:0 ~len:10)
>       buf
>       (UnixLabels.lseek fd 0 ~mode:Unix.SEEK_CUR)

Use strace, and you'll see something like that :

open("main.ml", O_RDONLY|O_LARGEFILE)   = 3
_llseek(3, 0, [0], SEEK_CUR)            = 0
read(3, "  let main", 10)               = 10
write(1, "after reading 10 chars: \"  let m"..., 1066) = 1066

So you can see that the lseek is done before the read. And indeed, your
calls to read and lseek can occur in an unspecified order. I guess that
if you write

  let len = UnixLabels.read ... in
  let pos = UnixLabels.lseek ... in
  Printf.printf ...

you will get the right result.