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Re: Linking automatically with dlopen

From: Ralf Wildenhues
Subject: Re: Linking automatically with dlopen
Date: Tue, 17 Apr 2007 20:32:18 +0200
User-agent: Mutt/1.5.13 (2006-08-11)

Hello Reuben,

To add to Bob's answer:

* Reuben Thomas wrote on Mon, Apr 16, 2007 at 02:01:24AM CEST:
> I want to be able to link against a library which may not be present at 
> runtime, be certain that the application starts up (i.e. the dynamic linker 
> doesn't discover that a library is missing and abort), and then be able to 
> find out whether a given library was successfully linked or not (and hence 
> decide whether I can execute code that uses symbols from that library).

If you can find out the set of libraries at 'configure' time, then there
is no need for dlopen.  Otherwise, no, I don't see another way.

For increased portability, you can use libltdl and lt_dlopen the library.

> (I'm not quite clear, but perhaps doing this fully automatically is beyond 
> the scope of libtool and needs linker support?)

Doing this fully automatically does not work on any system I know of.
AIX in some cases allows startup with some symbols not yet defined, but
that's pretty obscure.  I don't think it works with libs that may or may
not be present, though.

> The use I have in mind is linking CODECs with different licenses into an 
> application without needing to change the code so that the CODECs are 
> dlopened. Then, if certain CODECs are not available, the application can 
> simply give an error message if the user attempts to use them, but the 
> application author doesn't need to write (or in this case, re-write) the 
> code to use dlopen.

This is the typical application for dlopen/lt_dlopen.

Hope that helps.


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