[c++] How to specify preference of library path?



Answers

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?

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 en` variable
  • adding /my/dir/libfoo.so.0 as an argument to g++



This is an old question, but no one seems to have mentioned this.

You were getting lucky that the thing was linking at all.

You needed to change

g++ -g -Wall -o my_binary -L/my/dir -lfoo bar.cpp

to this:

g++ -g -Wall -o my_binary -L/my/dir bar.cpp -lfoo

Your linker keeps track of symbols it needs to resolve. If it reads the library first, it doesn't have any needed symbols, so it ignores the symbols in it. Specify the libraries after the things that need to link to them so that your linker has symbols to find in them.

Also, -lfoo makes it search specifically for a file named libfoo.a or libfoo.so as needed. Not libfoo.so.0. So either ln the name or rename the library as appopriate.

To quote the gcc man page:

-l library
   ...
   It makes a difference where in the command you 
   write this option; the linker searches and processes 
   libraries and object files in the order they are 
   specified.  Thus, foo.o -lz bar.o searches library z 
   after file foo.o but before bar.o.  If bar.o refers 
   to functions in z, those functions may not be loaded.

Adding the file directly to g++'s command line should have worked, unless of course, you put it prior to bar.cpp, causing the linker to ignore it for lacking any needed symbols, because no symbols were needed yet.






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Tags

c++ c++   linker   g++