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user_search_path vs libtool --mode=execute -dlopen

From: Mark H Weaver
Subject: user_search_path vs libtool --mode=execute -dlopen
Date: Sat, 06 Oct 2012 12:09:23 -0400

Hello all,

I'm a Guile developer who is trying to figure out the best way to add a
Guile-specific 'extensions' directory to the library search path, while
retaining the ability of 'libtool --mode=execute -dlopen' to take
precedence over our default extensions directory.

As I understand it, on GNU systems 'libtool --mode=execute -dlopen' has
the effect of prepending a directory to $LD_LIBRARY_PATH.  It also
appears from the libltdl source code that the search order for
'lt_dlopenext' is as follows:

* user_search_path (augmented by 'lt_dladdsearchdir' et al)
* system default search path

I see only two ways to add our extensions directory to the search path,
such that it can be overridden by 'libtool --mode=execute -dlopen':

1. Append it to LD_LIBRARY_PATH.
2. Search in our extensions directory manually if 'lt_dlopenext' fails,
   by calling 'lt_dlopenext' a second time.

The problem with option 1 is that all subprocesses will inherit our
modified search path.  This could be a problem if there are two
different versions of Guile on the system, and one is used by a
subprocess of the other, because the sub-Guile will search in the
parent-Guile's extensions directory first.

Therefore, it seems that option 2 is the right thing, even though it is
quite kludgy.

I can't help but suspect that 'lt_dladdsearchdir' et al were intended
for programs to add their own system-wide extensions directory to the
search path.  Is that right?  If so, what's the recommended way to
override a program's default extensions directory?

Maybe the problem is that programs should not expect 'libtool
--mode=execute -dlopen' to override a program's extension directory.
However, it is being used this way in practice, for example by GNU TLS
(which provides a Guile extension) to ask that the uninstalled extension
in its build directory should take precedence over the one in the
system-wide Guile extensions directory.  See

I'm hoping that the libtool experts here can help me understand how this
is supposed to work.  I'm not subscribed to this list, so please CC
replies to me.


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