Dynamic/runtime loading with GHC

[Duncan Coutts]

On Sun, 22 Sep 2002 00:19:11 +1000
Andre Pang <ozone@algorithm.com.au> wrote:

> It's a little example stdin text filter program, that loads in .o
> and .so "plugins" at runtime.  New in version 1.1 is the ability
> to load .so files as plugins :).  I think that with a bit more
> polish, we may even be able to rename this to a dynamic loader
> one day.

Indeed great. But I thought ghc code wasn;t supposed to work when inside
a .so file!? What's going on?

In fact it doesn't seem to be working entirely, I get an error on doing
$ ./src/TextFilter ./plugins/ROT13.so < README
[snip ROT13d README text]
TextFilter: fatal error: scavenge_stack: weird activation record found on stack: 0

But still, it works mostly. Perhaps this success should motivate us to
ask the ghc guys nicely what the remaining issues are since we seem
quite close.

As we all know using shared libraries would solve a some problems with
the current dynamic linking scheme. In particular, dependancies.
Dependancies of plugins on other plugins, external .so libraries and
base packages. At the moment if a plugin depends on an external library
you have to explicitly load the library before trying to load and
resolve references in the plugin. The same goes for dependancies on
other plugin modules and the base packages. If a plugin depends on a
base package which you have not linked into the executable then that
needs to be loaded before loading the plugin. This makes plugin
management a bit of a pain.

Duncan