[plt-scheme] loading libraries into memory with ffi-lib on osx

At Tue, 29 Aug 2006 06:40:38 0800, Matthew Flatt wrote:
> > This works fine in linux and windows but on OSX it doesnt seem to work.
> > [...]
> 
> I don't think it's so different from Linux.

I wasn't thinking very well. Linking in OS X is certainly different
than in Linux.

Roughly, inter-dylib references in OS X are like `(lib ...)' references
in MzScheme modules. If your dylib links with -llibpng, then it doesn't
matter whether some "libpng" is already loaded if that "libpng" isn't
the one found through the lib search path. (And that's why things work
when you put libpng in /usr/lib.)

In MzScheme, when you don't want to use `lib', you can use a relative
path. In OS X 10.4 and later, you can use a @loader_path prefix in a
similar way. For now, I recommend against depending on 10.4 features.

Another way around the problem is to add

  -flat_namespace -undefined suppress

to the link command, and drop the "-llibpng". The "-flat_namespace"
part is roughly analogous to using the Scheme top-level instead of a
module. Dropping "-llibpng" means that you don't have something like a
top-level `(require (lib "png"))', so whatever bindings are already
available at load time will be used. The "-undefined suppress" says
that you intentionally didn't specify a specific source for some
bindings. Finally, use `(ffi-lib "libpng")' before `(ffi-lib
"liballeg")'.

Matthew