g++: how to specify preference of library path?

Question

I'm compiling a c++ program using g++ and ld. I have a .so library I want to be used during linking. However, a library of the same name exists in /usr/local/lib, and ld is choosing that library over the one I'm directly specifying. How can I fix this?

For the examples below, my library file is /my/dir/libfoo.so.0. Things I've tried that don't work:

• my g++ command is "g++ -g -Wall -o my_binary -L/my/dir -lfoo bar.cpp"
• adding /my/dir to the beginning or end of my $PATH env variable
• adding /my/dir/libfoo.so.0 as an argument to g++

Thanks.

Answer 1

Add the path to where your new library is to LD_LIBRARY_PATH (it has slightly different name on Mac ...)

Your solution should work with using the -L/my/dir -lfoo options, at runtime use LD_LIBRARY_PATH to point to the location of your library.

OR

Use the rpath option via gcc to linker - runtime library search path, will be used instead of looking in standard dir (gcc option):

-Wl,-rpath,$(DEFAULT_LIB_INSTALL_PATH)

This is good for a temporary solution. Linker first searches the LD_LIBRARY_PATH for libraries before looking into standard directories.

If you don't want to permanently update LD_LIBRARY_PATH you can do it on the fly on command line:

LD_LIBRARY_PATH=/some/custom/dir ./fooo

You can check what libraries linker knows about using (example):

/sbin/ldconfig -p | grep libpthread
        libpthread.so.0 (libc6, OS ABI: Linux 2.6.4) => /lib/libpthread.so.0

And you can check which library your application is using:

ldd foo
        linux-gate.so.1 =>  (0xffffe000)
        libpthread.so.0 => /lib/libpthread.so.0 (0xb7f9e000)
        libxml2.so.2 => /usr/lib/libxml2.so.2 (0xb7e6e000)
        librt.so.1 => /lib/librt.so.1 (0xb7e65000)
        libstdc++.so.6 => /home/bimbo/local/linux/lib/libstdc++.so.6 (0xb7d81000)
        libm.so.6 => /lib/libm.so.6 (0xb7d5b000)
        libgcc_s.so.1 => /home/bimbo/local/linux/lib/libgcc_s.so.1 (0xb7d50000)
        libc.so.6 => /lib/libc.so.6 (0xb7c2e000)
        /lib/ld-linux.so.2 (0xb7fc7000)
        libdl.so.2 => /lib/libdl.so.2 (0xb7c2a000)
        libz.so.1 => /lib/libz.so.1 (0xb7c18000)

Answer 2

Specifying the absolute path to the library should work fine:

g++ /my/dir/libfoo.so.0  ...

Did you remember to remove the -lfoo once you added the absolute path?

Answer 3

As an alternative, you can use the environment variables $LIBRARY_PATH and $CPLUS_INCLUDE_PATH, which respectively indicate where to look for libraries and where to look for headers ($CPATH will also do the job), without specifying the -L and -I options.