Re: inter-library linking

[Ralf Wildenhues]

Hello Joachim,

* Joachim Worringen wrote on Fri, Oct 12, 2007 at 02:39:16PM CEST:
>
> "libshared" needs to be a shared library, as it will be used with 
> LD_RELOAD. It builds from its own sources, but also depends on two other 
> libraries that are built within the same source three: "libnoinst" and 
> "libfoo". libnoinst is a static convenience library, while libfoo is just 
> another shared library.

First a general question: why do you use (need to use?) LD_PRELOAD,
as oppposed to, say, just linking your programs against the libraries?
(I can imagine two prime use cases: dealing with programs one cannot
recompile like binary-only software, or symbol interpose "hacks" for
programs like valgrind or other malloc debug stuff.)

Another general remark: convenience libraries don't enter the picture
here much; think of them as a short-hand notation for a collection of
objects that will all be copied into the final library.

Now to the meat: the library libfoo that libshared depends on:

> While the symbols from libnoinst are placed into libshared, the references 
> to libfoo remain unresolved. This lets the linking fail when I set 
> LD_PRELOAD=libshared.so: symbols from libfoo.so can not be resolved 
> (although it is in the LD path, same as where libshared is placed).

There are two ways this can be made to work:

1) Explicit linkage of libshared against libfoo should cause the runtime
linker to pull it in:
   lib_LTLIBRARIES = libfoo.la libshared.la
   libshared_la_LIBADD = libfoo.la
   libfoo_la_SOURCES = ...
   ...

If that's what you already did, but it's producing errors anyway, then
please post a build log that contains the mode=link and mode=install
commands for both of these libraries plus all output they generate,
and the error output of the `LD_PRELOAD=...' command.

(The key here is that the *_LIBADD line will cause a DT_NEEDED entry
for libfoo.so.X to be put in libshared.so, which in turn will cause
the runtime glibc linker to pull in libfoo.so.X.  If libfoo lives in a
nonstandard directory, then a DT_RPATH entry will be added, too.
For ELF runtime linkers other than the GNU/Linux one, I don't know
offhand whether LD_PRELOAD has the same semantics.)


2) Just list both to-be-preloaded libraries in LD_PRELOAD:

  LD_PRELOAD=libfoo.so:libshared.so ...

But actually you should be listing the inter-library dependencies in
*_LIBADD anyway, and your build order should be such that libfoo is
build before libshared.

> Explicitely linking libshared.so to libfoo.a fails as its object files are 
> not compiled as PIC (and I don't know how tell automake to do this 
> anyway...).

Well you can make libfoo a convenience archive.  But then its code will
end up completely in libshared; and it's not a good idea to install
convenience archives, which is why it's not done by Automake.  A good
rule of thumb is that code should live in at most one installed library,
everything else can cause ugly sooner or later.

Hope that helps.

Cheers,
Ralf