Incorrect module initialisation order with "external" declarations

[vicuna]

Original bug ID: 4166
Reporter: @mmottl
Status: resolved (set by @xavierleroy on 2015-11-22T10:45:03Z)
Resolution: open
Priority: normal
Severity: minor
Version: 3.09.3
Target version: 4.03.0+dev / +beta1
Fixed in version: 4.03.0+dev / +beta1
Category: ~DO NOT USE (was: OCaml general)
Has duplicate: #6956
Related to: #4374 #6063
Monitored by: yziquel mehdi @ygrek @hcarty @dbuenzli @Chris00 @mmottl

Bug description

We have a module (lets call it "Foo") that interfaces C-bindings and exports functions defined as "external" in its .mli-file. This module is packed with others into one module, which again is part of a .cmxa-file.

If we link this library with another module (say, "Bar") that uses only functions defined "external" in "Foo", we observed that toplevel expressions in "Foo" did not get evaluated.

Here is a schematic example:

foo.ml:

external foo : unit -> unit = "foo"
external init : unit -> unit = "init"

let () =
print_endline "init";
init ()

foo.mli:

external foo : unit -> unit

bar.ml:

let () =
print_endline "bar";
Foo.foo ()

Since "Bar" depends on "Foo", you would expect that the toplevel expression in "foo.ml" gets evaluated first, which should print "init" and e.g. initialise the C-bindings. But we see "bar" first, and our application crashes, because the C-bindings are not evaluated.

Interestingly, changing "foo.mli" as follows solves the problem:

foo.mli:

val foo : unit -> unit

This indicates that exclusively calling functions exported as "external" will prevent the linker from figuring out which modules should be initialized first. This seems to be a bug, it shouldn't matter whether we use "val" or "external".

[vicuna]

Comment author: @damiendoligez

I'm downgrading this to "minor" since there is an easy workaround: always use "val" instead of "external" in your .mli files.

[vicuna]

Comment author: @ygrek

and it also bites those lazy folks (like me) who do not write mli interfaces..

[vicuna]

Comment author: @edwintorok

Perhaps ocaml should give a warning if you use 'external' in .mli files.

Due to this bug exceptions are not registered in C code, which causes caml_raise_arg_with to be invoked with a NULL exception, which in turn causes a segfault.

[vicuna]

Comment author: @garrigue

Since a similar problem appears when introducing module aliases, we might want to solve the two simultaneously. A clean approach would require adding dependency information to the cmo format.

[vicuna]

Comment author: @xavierleroy

See #289. Fixed by commit 07b8eb3.