Question

I have a written a small c++ project on a linux box. I am able to compile and link the code properly with the -I and _L,-l commands.

I read on the net that while running the code, we need to have the path to the .so files in the LD_LIBRARY_PATH variable. Now the box that I am using is 64 bit and there is another application that uses 32 bit version of the library. So the LD_LIBRARY_PATH has paths to both versions of the library.

When I try to run my application eg. ./xyz arg1 arg2, I get the following error

./DaemonCpp: error while loading shared libraries: libldap.so: wrong ELF class: ELFCLASS32

I think this is because it is trying to load 32 bit version instead of 64 bit version. I tried changing the order in which the paths are mentioned in LD_LIBRARY_PATH such that the path to 64 bit comes first but still I get this error. Is there a way I could direct the app to use the 64 bit version of the library.

As a secondary question to my primary problem, when we are linking the c++ code, we do specify the library path using the -L and library name using -l. Then it should have known which library to use when I run the executable and I shouldn’t have got this error in the first place

Answer 1

In answer to your second question, you only used -L to specify the link time path. You need to also add -rpath=<path> for each path that you specify with -L. to get it to look at run-time as well.

If you’re using the gcc/g++ driver for linking, then you should use -Wl,-rpath=<path>

e.g. library libfoo.so in /opt/libs/lib64:

gcc -o test test.o -L/opt/libs/lib64 -Wl,-rpath=/opt/libs/lib64 -lfoo

In answer to your first question, if LD_LIBRARY_PATH is causing issues, then your first port of call is ldd which shows the paths to the libraries that are being linked at run-time. Start with an empty string and work up from there.

Thirdly, if the reason that LD_LIBRARY_PATH needs to be set is because there are libraries in /opt/lib, etc… then you should be adding these paths to your /etc/ld.so.conf and using ldconfig to update the search map for these libraries. This will typically save you from needing the LD_LIBRARY_PATH variable at all.