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[Caml-list] Big executables from ocamlopt; dynamic libraries again
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Date: 2002-03-18 (01:12)
From: Jacques Garrigue <garrigue@k...>
Subject: Re: [Caml-list] Big executables from ocamlopt; dynamic libraries again
From: (Tim Freeman)

> When I compile a simple "hello world" app using lablgtk, the resulting
> executable exceeds 800KB.  With a few apps like this, my package will
> take offensively long to download, even over DSL.  The exectuable
> seems to be including most of the lablgtk library in the executable,
> which makes sense because lablgtk is statically linked in the
> executable.
> This isn't a problem with lablgtk.  A native code app that just prints
> "hello world" on standard output takes 5K bytes if written in C but
> 95K in ocaml.  

This is indeed a problem (in my opininion small, but yet).
But you're wrong in assuming that static linking means including the
whole library. Actually this is quite the opposite: dynamic linking
includes the whole library (but not in the executable), while static
linking only includes needed parts. In particular dynamic linking
without code sharing (using patched code) use more memory at runtime
than static linking.
You can see it as lablgtk.a is 1MB, while your app is only 800K
(include the ocaml runtime, etc...)
The unfortunate thing is that the structure of the LablGTK library
creates lots of spurious dependencies. For instance, using a button
links code for all kind of buttons, or using a label links code for
the calendar widget... My decision was to privilege meaning (put
related things together) over size (hack to be smaller).
Yet, lablGTK stubs are tuned to produce small code on x86, but here
your problem is with the size of the native code produced by ocamlopt,
which is harder to optimize.

> The GTK executable would be smaller if lablgtk were dynamically linked
> into it.  I hear that a real shared library isn't an option because
> the present ocaml compiler can't generate position independent code.
> However, one can do dynamic linking on many machines without position
> independent code or a shared library; the dynamic linker just
> relocates the library at run time.

You're right, but there is a bit more about dynamic linking.
A major problem with using dynamic linking with ocaml (in particular
with native code), is that your program come cut into small pieces,
and you must be sure that they are all compatible. Somebody posted
recently about problems when upgrading ocaml, and part of it is caused
by incompatibilities in the binary format between versions. Just
imagine the reaction of your user when, after having loaded various
packages required for your program, he only gets an error message or a
segmentation fault when trying to run it.

Static linking considerably improves that, since your program will no
longer depend on ocaml being installed, and the versions of its
different components. By the way, static linking here only concerns
the ocaml specific part of the code, libgtk itself is dynamically
linked, since one can expect to find a compatible implementation on
each platform.

The situation is a little bit better with bytecode: this time one only
depends on the version of ocaml, no longer the platform.
This is only in the CVS version (next release), but you can now use
the ocaml toplevel as a dynamic loader:
Compile your application as a .cmo or .cma
        ocamlc -a -o myapp.cma  tools.cmo main.cmo
(you don't need to include required libraries here)
Load it with ocaml
        ocaml lablgtk.cma myapp.cma

If speed is not a major problem, I think this can be nice in practice.

> Is there any reason there's no support for writing dynamically
> linkable libraries in OCAML?
> Hmm, if you memorized the MD5 checksum of the library at compile time,
> and checked it at run time, it could even be type safe.  Or you could
> just memorize the MD5 of the signature of the library, in some sense;
> this would allow patches like the recent zlib double-free.  If the
> library knows it's checksum, and the code loading it knows the
> expected checksum, then you can do this checking without computing a
> checksum at run time.

One point is that, on one side, you really want the checking, and on
the other side, with the current MD5 approach, the checking is too
version dependent. That is, even if you've changed nothing in your
code, and it would be perfectly safe to run it, you have good chances
the loader will refuse to do it. And this nullifies the zlib example:
the newly compiled version would not be compatible with existing
Of course, this could be improved, but this needs some research and/or
engineering work.


        Jacques Garrigue
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