undefined symbol: readline

[technomancy]

I'm probably missing something obvious, but I couldn't figure out how to use Ctypes to define a function calling out to GNU Readline's readline function:

let read_line = foreign "readline" (string @-> returning (ptr string));;
Exception:
Dl.DL_error
 "/home/phil/.opam/4.01.0dev+trunk/lib/stublibs/dllctypes-foreign_stubs.so: undefined symbol: readline".

full toplevel transcript

This is from ctypes 0.1.1 with OCaml 4.01.0+dev18-2013-06-13 on Debian stable. I've confirmed both libffi5 and libreadline6 are installed. Any idea what's going on?

[yallop]

You need to tell foreign where to find readline by passing it a handle returned by dlopen

  # #require "ctypes.foreign";;
  /home/jeremy/.opam/4.01.0dev+trunk/lib/ocaml/unix.cma: loaded
  /home/jeremy/.opam/4.01.0dev+trunk/lib/ctypes: added to search path
  /home/jeremy/.opam/4.01.0dev+trunk/lib/ctypes/ctypes.cma: loaded
  /home/jeremy/.opam/4.01.0dev+trunk/lib/ctypes/ctypes-foreign.cma: loaded
  # open Ctypes;;
  # open Foreign;;
  # let libreadline = Dl.(dlopen ~filename:"libreadline.so" ~flags:[RTLD_NOW]);;
  val libreadline : Dl.library = <abstr>
  # let read_line = foreign "readline" (string @-> returning string) ~from:libreadline;;
  val read_line : string -> string = <fun>
  # read_line "What's your name? ";;
  What's your name? Jeremy
  - : string = "Jeremy"
  #

(You also have an extra ptr in the return type.)

[technomancy]

Thanks for the clarification. Is this something that should be added to the tutorial? In particular it's not clear to me why some functions (like those from ncurses) work fine without this.

[rgrinberg]

I would also not mind a clarification. I believe my issue #70 can be fixed using the same Dl.dlopen call Jeremy used here.

[yallop]

Yes, this should be covered in the tutorial. I've opened #73 as a reminder.