Re: Empty allow_undefined_flag in libtool.m4 seems to cause undefined symbol errors on Linux

[Ralf Wildenhues]

Hello Ben,

any chance you could get your mailer to word-wrap at 72 chars?  Thanks.

* Ben Roberts wrote on Sat, Jul 03, 2010 at 01:55:51AM CEST:
> I've been trying to adapt a set of libraries and programs to use
> autoconf, automake and libtool instead of being manually configured.
> One of the features of this particular package is that there are some
> symbols that should be satisfied at run time, rather than at
> compile-and-link time.

Is this a feature for this package's shared libraries only, or also for
programs?

> Now, when I try to build on a Linux box with GNU ld 2.17.50.0.6-9.el5,
> I get errors about undefined symbols.

Please show an example link command line that fails, including output.

> That version of the linker can be persuaded to ignore undefined
> symbols using the "--unresolved-symbols=ignore-all" flag.
> 
> When I consulted libtool.m4, I found that there's a variable in there
> that is supposed to contain that flag, or its equivalent on different
> systems: namely, allow_undefined_flag (with a suffix for the compiler
> in certain instances).
> 
> The problem: in libtool.m4, allow_undefined_flag is left as an empty
> string on Linux systems.

Yes, well, GNU/Linux systems allow undefined symbols by default in
shared libraries.

> Is modifying, and perhaps preparing a patch for, libtool.m4 the best
> approach? An alternative is, no doubt, to write some kind of
> Linux-specific workaround in my configure.ac, but as far as I know
> libtool aims to spare me that kind of thing...

Well, a patch is acceptable for behavior we can generalize.  In order to
find that out, we should have a testsuite addition that tries out the
feature that you would like to see work (showing a small example is
fine, we can easily rework that into a testsuite addition).  Then we can
judge it better, and see whether this is good to have generally.
Otherwise, let's then see what you can do instead.

Hope that helps.

Cheers,
Ralf