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Subject: Re: [hwloc-devel] hwloc embedding vs libltdl
From: Brice Goglin (Brice.Goglin_at_[hidden])
Date: 2013-05-09 08:55:00


Le 09/05/2013 12:43, Jeff Squyres (jsquyres) a écrit :
> On May 8, 2013, at 4:16 PM, Brice Goglin <Brice.Goglin_at_[hidden]> wrote:
>
>> But this issue is only in the embedders (OMPI), not in the embeddee
>> (hwloc), right?
> Yes.
>
>> I can get plugins to work in tests/embedded by adding 2 lines to its
>> configure.ac (see the attached patch, which also removes your error
>> message and creates a shared lib containing libhwloc_embedded).
>>
>> In short, I don't really see what risk we would be taking on the hwloc
>> side if we keep embedding+plugins possible (and still don't enable
>> plugins by default).
> Ok. It's probably worth documenting, though. In the OMPI case, for example, OMPI cannot be opened in a private namespace (e.g., as a python plugin) and then have hwloc also opened in a private namespace; hwloc must be opened in a public namespace. This has caused unhappiness in certain cases where upper-layer application authors were trying to open all plugins in private namespaces but couldn't. The same will be true with hwloc: those who embed hwloc should be *strongly advised* to not use libltdl (even though it's not the default) because of the private/public namespace issue.
>

Something like this?

--- a/doc/hwloc.doxy
+++ b/doc/hwloc.doxy
@@ -2147,6 +2147,9 @@ is registered to the hwloc core.
 Components are then only enabled if the topology configuration
 requests it, as explained in the previous sections.
 
+Also note that plugins should carefully be enabled and used when
+embedding hwloc in another project, see \ref embed for details.
+
 \section plugins_adding Adding new discovery components and plugins
 
 The types and functions cited below are declared in the hwloc/plugins.h header.
@@ -2338,6 +2341,14 @@ you can directly integrate hwloc's m4 configure macro into your
 configure script. You can then invoke hwloc's configuration tests and
 build setup by calling an m4 macro (see below).
 
+Although hwloc plugins may be used in embedded mode, the embedder
+project will have to manually setup libltdl in its build system so
+that hwloc can load its plugins.
+Also, embedders should avoid using their own plugins and hwloc plugins
+simultaneously because of possible issues with public and private
+namespaces when using libltdl.
+The embedder project is strongly advised not to use libltdl.
+
 \section embedding_m4 Using hwloc's M4 Embedding Capabilities
 
 Every project is different, and there are many different ways of

Brice