Re: Interface to a dynamic lib which loads other dynamiclibs [SEC=UNCLASSIFIED]

> From: Georg Holzmann [mailto:[email protected]]

> I try to use a dynamic library (slv2) in python, which is actually a
> host for plugins (lv2 audio plugins, http://lv2plug.in/).
> That means slv2 loads other dynamic libraries (= plugins) at runtime
and
> I want to be able to call functions from those plugins.
> 
> The basic interface to slv2 works as it should, but as soon as I want
to
> instanciate some plugins, I get the following problem with ctypes:
> 
>    AttributeError: /usr/lib/libslv2.so.9: undefined symbol:
> slv2_instance_activate
> 
> because the symbol slv2_instance_activate is not in the slv2 library.
> 
> I also tried to additionally load the plugin with ctypes:
>    ctypes.CDLL("/usr/lib/lv2/mypluginname.so",
mode=ctypes.RTLD_GLOBAL)
> but that also did not work.
> 
> So my question:
> Is it possible to interface such complicated libraries with ctypes ?
(or
> should I use something else)
> How should this be done ?

I was intrigued by your query, and had a look at the SLV2 source - the
authors have made a decision that assumes that all users of SLV2 will
have some compiled interface to the library, as a number of documented
API functions only get compiled as part of the host interface
(slv2_instance_activate() among them).  See the "ifndef LIBSLV2_SOURCE"
section of slv2/plugininstance.h in the SLV2 source...

You have several choices that I can see:
- build a private version of SLV2 modified to includes all the symbols
in the library (and doesn't expect them to be built them into a host
interface);
- use the SWIG interface supplied in the SLV2 source (if you can build
it or find a pre-built version to match your SLV2 library version);
- write your own Python interface to SLV2 in C;
- write a wrapper library that includes the "host only" functions and
forwards the rest to the real SLV2 library, so its all ctypes loadable
from one library;
- become familiar enough with ctypes and the SLV2 headers to implement
the "missing" functions with ctypes - you will need to understand struct
& pointer manipulation and how to create function pointers.

-------------------------> "These thoughts are mine alone!" <---------
Andrew MacIntyre           Operations Branch
tel:   +61 2 6219 5356     Communications Infrastructure Division
fax:   +61 2 6253 3277     Australian Communications & Media Authority
email: [email protected]            http://www.acma.gov.au/


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