Re: GCC-provided runtime libraries.

[Kai Ruottu <karuottu at mbnet dot fi>]

Christian BÃhme kirjoitti:
> I am currently trying to install the 4.2.0 version of GCC on a
> Linux system that has not seen much administration work over the past
> years.
> The problem here is that this new compiler with its updated/
> improved/bug-less runtime libraries (such as libgcc_s.so,
> libstdc++.so, libgfortran.so) does not explicitly tell the linker
> to link against them (or set DT_RUNPATH in the resulting executables
> accordingly) but to use what is setup by the sysadmin (via 
> /etc/ld.so.conf
> and friends).
The GNU linker knows the '-rpath-link <libpath>' and '-rpath <libpath>' 
options
to force the pointed shared libs being searched at 'linktime' when 
producing the
executable and at 'runtime' when running that executable. So the latter 
would be
what you will need...  You could use it in every compile/link command or 
put it
into the 'specs' file of the new gcc-4.2.0, into the same 'spec' where that
'--dynamic-linker /lib/ld-linux.so.2' is...

>   Consequently, I reverted back to configuring with static
> runtime libraries which even more surprisingly yielded the same result.
> It appears that g++ only passes a lone -lstdc++ to the linker
> but not the path where GCC supposedly installed its own sparkly
> new libraries (either shared or static).

Surprisingly GCC DOESN'T as default install its $target/$gcc-version
specific link-time libraries into the expected $target/$gcc-version 
directory:

  $prefix/lib/gcc/$target/$gcc-version

where it searches these libraries first!  But puts them into the 'native 
search
place', '/usr/lib' or into some other totally unexpected place :-(  So 
people
like me have learned to NOT use the 'make install' and use their own local
install template scripts for putting things where they should go 
following the
GCC builder's opinions...  OR use the :

  --enable-version-specific-runtime-libs

GCC configure option (as you did!)  in order to put the produced libs 
into that '$prefix/lib/gcc/$target/$gcc-version'!  In any case the goal 
could be to use the $target/$gcc-version specific install directory for 
the stuff required at link-time,
it will be used automatically and first!  Just check with the 'g++ 
-print-search-dirs'.

For the run-time issue using the '-rpath <libdir>' marks every produced 
executable
to search the shared  libraries first from the '<libdir>' before going 
to the native
'/lib' and '/usr/lib', '/usr/local/lib' etc.   What this "alternative 
search  place" could be
is then totally dependent on your tastes...   But  '/opt/lib' could 
sound nice for me...

The '-rpath <libdir>' can be given on the GCC 'compile&link' command like:

  g++ -Wl,-rpath,<libdir> -O2 -o hello hello.cpp

using the '-Wl,<linker-options>' GCC option for giving extra options to 
the linker.

That there really is a new built-in/hard-wired RPATH stamped into the 
executable
can be seen via 'objdump' like:

 objdump -p  hello