0004166: Incorrect module initialisation order with "external" declarations

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0004166

OCaml

OCaml general

public

2006-11-21 19:52

2012-09-14 23:49

Reporter

mottl

Assigned To

Priority

normal

Severity

minor

Reproducibility

always

Status

acknowledged

Resolution

open

Platform

OS Version

Product Version

3.09.3

Target Version

4.01.0+dev

Fixed in Version

Summary

0004166: Incorrect module initialisation order with "external" declarations

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".

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0004374

closed

Failure to evaluate top level expressions in dependant module

Notes
(0003861) doligez (manager) 2006-11-24 15:43	I'm downgrading this to "minor" since there is an easy workaround: always use "val" instead of "external" in your .mli files.

(0005690) ygrek (reporter) 2010-10-26 16:47	and it also bites those lazy folks (like me) who do not write mli interfaces..

(0005753) edwin (reporter) 2010-12-23 16:10	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.

Issue History
Date Modified	Username	Field	Change
2006-11-21 19:52	mottl	New Issue
2006-11-24 15:43	doligez	Note Added: 0003861
2006-11-24 15:43	doligez	Severity	major => minor
2006-11-24 15:43	doligez	Status	new => acknowledged
2007-08-29 02:13	garrigue	Relationship added	related to 0004374
2010-10-26 16:47	ygrek	Note Added: 0005690
2010-12-23 16:10	edwin	Note Added: 0005753
2012-07-11 16:14	doligez	Target Version	=> 4.01.0+dev
2012-07-31 13:37	doligez	Target Version	4.01.0+dev => 4.00.1+dev
2012-09-14 23:49	doligez	Target Version	4.00.1+dev => 4.01.0+dev