Explain how to load/link libraries in the tutorial

[yallop]

This comes up rather often (e.g. #41, #49, #70, #71), and the current tutorial doesn't cover it at all.

Chapter 19 of Real World OCaml has some helpful remarks.

[yawnt]

👍 , I was having trouble loading a static library on ArchLinux. Converting the very same .a to a .so fixed it. I'd be curious as to how to link against static libraries (-force_load doesn't seem to exist, I'm using Oasis to compile, if that helps)

Thanks for the great work :)

[XVilka]

I would like to know how to link with static libraries as well.

[Ilazki]

This issue is still relevant; I just spent far too long trying to figure out how to load and link in a library to use with ctypes.foreign, largely because the tutorial only uses toy examples that interface with already-loaded libraries via the toplevel.

I eventually found what I needed to use dune to include a C library, thanks to a single, unexplained line of code in Real World OCaml's FFI section (using the flags stanza to pass -cclib -lname to ocamlc/ocamlopt), but it was pretty inconvenient getting there, with far too long wasted trying to find a way to load .so files at the toplevel because of the tutorial's use of only toplevel-capable toy examples. At this point I'm still not sure if there's a way to load C libraries into a toplevel directly for experimentation or not, but it's not a big deal since I have what I need for the moment.

This is a cool project that seems to really improve the convenience of making library bindings, but the documentation is still severely lacking despite there being a 6+ year old bug report on the documentation shortcomings. :(

[yallop]

@Ilazki:

At this point I'm still not sure if there's a way to load C libraries into a toplevel directly for experimentation or not, but it's not a big deal since I have what I need for the moment.

Is the following the sort of thing you're looking for?

# #use "topfind";;
- : unit = ()
Findlib has been successfully loaded. Additional directives:
[...]
- : unit = ()
# #require "ctypes.foreign";;
[...]
/home/jeremy/.opam/4.09.0/lib/ctypes/ctypes-foreign-unthreaded.cma: loaded
# let libclingo = Dl.dlopen ~filename:"/usr/lib/libclingo.so" ~flags:Dl.[RTLD_NOW];;
val libclingo : Dl.library = <abstr>
# let clingo_version = Foreign.foreign ~from:libclingo "clingo_version"
      Ctypes.(ptr int @-> ptr int @-> ptr int @-> returning void);;
val clingo_version :
  int Ctypes_static.ptr ->
  int Ctypes_static.ptr -> int Ctypes_static.ptr -> unit = <fun>
# let major, minor, revision = Ctypes.(allocate int 0, allocate int 0, allocate int 0);;
val major : int Ctypes.ptr = (int*) 0x55a608bbff00
val minor : int Ctypes.ptr = (int*) 0x55a608bbfee0
val revision : int Ctypes.ptr = (int*) 0x55a608bbfec0
# clingo_version major minor revision;;
- : unit = ()
# Ctypes.(Printf.printf "Clingo version: %d.%d.%d\n" !@major !@minor !@revision);;
Clingo version: 5.3.0
- : unit = ()

[Ilazki]

Yes, that seems to be precisely what I was originally after, thank you very much. I like to experiment in a REPL first, sort of work out a rough draft of what I'm after in it, but after failing to find any information on doing something like that, I was going to just give up on the idea and work with a save/compile/test cycle using what info could find.

I didn't even expect an answer, I just wrote my comment to let you know there are still new people finding this library, thinking it's pretty awesome, but getting confounded by the lack of info online. :)

Anyhow, thanks again, that's pretty convenient and will make things a lot easier for me.