Question

I’m having problems with LD_LIBRARY_PATH at link time (this question has nothing to do with run time).

The link line looks like this when I run make (this is a Linux system using g++ version 4.1.x):

g++ a.o b.o c.o -o myapp \
 -L/long/path/to/libs/ \
 -L/another/long/path/ \
 -labc -ldef -lghi

The -l options reference shared libraries (e.g., libabc.so) which exist in the directories specified by the -L options. Those directories also appear in LD_LIBRARY_PATH. With that configuration, the link is successful, and I can run the application.

If I remove the directories from LD_LIBRARY_PATH, then I get a single error line such as:

/usr/bin/ld: cannot find -labc

On the other hand, if I remove the directories from the list of -L options, then I get many warnings such as:

/usr/bin/ld: warning: libabc.so, needed by /long/path/to/libs/libxyz.so,
    not found (try using -rpath or -rpath-link)

and then many more errors, such as:

/long/path/to/libs/libdef.so: undefined reference to `Foo::Bar<Baz>::junk(Fred*)'

Can someone explain the difference between LD_LIBRARY_PATH and -L? I would like to understand this stuff in depth, so references are greatly appreciated!

Also, what do I have to add to the link line to avoid using LD_LIBRARY_PATH?

EDIT: When directories were missing from -L, the compiler suggested to “try using -rpath or -rpath-link”. I don’t think I’ve ever seen those options in a makefile before. Have you? Not sure if that would help the LD_LIBRARY_PATH problem though.

Answer 1

The settings of LD_LIBRARY_PATH has the highest precedence, so when it is set, the set of directories mentioned by LD_LIBRARY_PATH are searched first even before the standard set
of directories. So in your case setting of LD_LIBRARY_PATH is influencing the lookup of
the libraries mentioned with -l option. Without LD_LIBRARY_PATH some of the dependencies
might have been resolved from the standard set of directories.

Though setting of LD_LIBRARY_PATH help with debugging and to try out a newer version of
a library its usage in the general development environment setup and deployment is considered bad.

Also refer this HOWTO from Linux Documentation for more details on Shared Libraries