Modify g++ library path

Question

I recently installed gcc 4.9.2 and found a problem when linking with libs.

The output for search path:

install: /usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/
programs: =/usr/libexec/gcc/x86_64-unknown-linux-gnu/4.9.2/:/usr/libexec/gcc/x86_64-unknown-linux-gnu/4.9.2/:/usr/libexec/gcc/x86_64-unknown-linux-gnu/:/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/:/usr/lib/gcc/x86_64-unknown-linux-gnu/:/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../x86_64-unknown-linux-gnu/bin/x86_64-unknown-linux-gnu/4.9.2/:/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../x86_64-unknown-linux-gnu/bin/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../x86_64-unknown-linux-gnu/bin/
libraries: =/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/:/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../x86_64-unknown-linux-gnu/lib/x86_64-unknown-linux-gnu/4.9.2/:/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../x86_64-unknown-linux-gnu/lib/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../x86_64-unknown-linux-gnu/lib/../lib64/:/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../x86_64-unknown-linux-gnu/4.9.2/:/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../lib64/:/lib/x86_64-unknown-linux-gnu/4.9.2/:/lib/x86_64-linux-gnu/:/lib/../lib64/:/usr/lib/x86_64-unknown-linux-gnu/4.9.2/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib64/:/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../x86_64-unknown-linux-gnu/lib/:/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../:/lib/:/usr/lib/

Really, the problematic thing is this:

/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib64/

I need to change the order of these two libs(so that lib64 has higher search priority), because both dirs have the libstdc++.so.6, and I need to use the one in lib64 folder.

How do I do it?

Please don't:

1, suggest that I change LD_LIBRARY_PATH to explicitly include libstdc++.so.6 as I'm sure there are other things in x86_64-linux-gnu folder that are older versions of that in the lib64 folder - I upgraded g++ from an older version.

2, suggest that I explicitly include that lib64/libstdc++.so.6 in the g++ -L option.

Thanks a lot.

/************************EDIT FOR MORE INFO************************/

Upon the request below, here're some more details about configuration and installation(I downloaded 4.9.2 source from gcc.gnu.org, extracted it and started in the 4.9.2 top level folder):

mkdir ../gcc-build                                   &&
cd    ../gcc-build                                   &&

../gcc-4.9.2/configure                               \
    --prefix=/usr                                    \
    --libdir=/usr/lib                                \
    --enable-shared                                  \
    --enable-threads=posix                           \
    --enable-__cxa_atexit                            \
    --disable-multilib                               \
    --with-system-zlib                               \
    --enable-languages=c,c++ &&
make

Answer 1

You could have a new (or modify the existing) GCC specs file, documentation is here.

AFAIK, the specs file is in your "install" dir, so for you would be in
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/specs (which you could create if it does not exist).

AFAIK there is some built-in default, but you could configure your system to have an explicit one.

Read also about the debugging options of GCC. You may want to use -dumpspecs to get the built-in default spec.

Details may be highly specific to your system, especially if you compiled GCC from its source code.

^{I am not familiar enough with specs files to give a reliable solution for your particular issue. You might ask on gcc-help@gcc.gnu.org for details.}

---

NB: I would tend to believe that configuring a gcc with --prefix=/usr (and not a non-system prefix like the default --prefix=/usr/local/ or some --prefix=/opt/ etc...) is a mistake (or at least use also --program-suffix=-4.9). You are likely to mix up your gcc with the system gcc; If you want to replace your system gcc (which is probably dangerous) you should configure your new gcc with the same arguments as your system gcc had. Notice that /usr/bin/gcc -v tells you how was your system gcc configured (to be done before overwriting it).

When compiling a recent GCC 4.9 on some older system I generally would recommend to configure it with --prefix=/usr/local/ and --program-suffix=-4.9 then add /usr/local/bin/ to your $PATH, and use make CC=gcc-4.9 CXX=g++-4.9 for building programs with it.