C++ shared libraries

Question

I am trying to get my head around the way shared libraries work in the c++ unix environment. I understand we only need header files and no shared libraries specification when compiling code. But if I want to create an executable or shared library from my compiled files, do I need to specify shared library dependencies (those are dynamic)? And do the paths of shared libraries need to match the path at runtime loading?

I am using Linux 2.6.18-164.11.1.el5 #1 SMP x86_64 GNU/Linux

I am having a problem where my code is not able to pick up a library at runtime. I have tried setting LD_LIBRARY_PATH and PATH. But at runtime when I run the executable, I get the following error: Error: librc.so: cannot open shared object file: No such file or directory

Sam

Answer 1

The headers are only for the compile phase. At link time, you usually have to specify which shared libs you are going to link to. You might see -L options to set locations to where shared libraries reside, and/or -l to specify which libraries to link. There is usually also a switch on the command line to alert the linker as to whether you are using thread-safe versions of the libs or the 'regular' ones, and another switch to specify dynamic linking.

At run time, whether you are starting the program that uses the libs, or running ldd to find out what it needs, the OS has a system for locating .so files, and this can vary from one unix version to another. The LD_LIBRARY_PATH variable specifies where to look for .so files, but may not be the full story, depending on the exact unix version in question. Also, you probably don't want to fiddle around with modifying LD_LIBRARY_PATH except from a throw-away shell, since it has system wide effects. A better option is to check it the 'missing' .so files are or are not on the existing path set by LD_LIBRARY_PATH, and if not, try putting copies of them somewhere on that path.

Answer 2

At run time, dynamic libraries are searched:

• in a path recorded in the executable (under linux with -rpath at link time, under Solaris with -R, using $ORIGIN in a directory name allows to specify a directory relative to the directory containing the executable)

• in the LD_LIBRARY_PATH (or equivalent, there are sometimes 64/32 bits variant). If a path has been recorded in the executable, LD_LIBRARY_PATH may not searched (under Linux it is searched after the recorded path if the executable has been linked with the option --enable-new-dtags; I don't remember Solaris behavior for now)

• in a set of system dependant directories (Linux allows to specify them in /etc/ld.so.conf and has a cache, see ldconfig)