rpath - runtime search path

RPath is a very interesting topic which can instruct the binary to find the right location of libraries and dependencies.

GNU ld.so has some rules in regarding how to use the rpath to find the right dependencies (basically it has attribute or environment which direct the runtime to look for directories for things within).

 

You may find some explaination from this wikipedia link: http://en.wikipedia.org/wiki/Rpath 

GNU ld.so

The dynamic linker of the GNU C Library and its derivative Embedded GLIBC implement a rather complicated algorithm for searching for shared libraries. The basic search order is:^[1]

1. The (colon-separated) paths in the DT_RPATH dynamic section attribute of the binary if present and DT_RUNPATH attribute does not exist.
2. The (colon-separated) paths in the environment variable LD_LIBRARY_PATH, unless the executable is a setuid/setgidbinary, in which case it is ignored. LD_LIBRARY_PATH can be overridden by calling the dynamic linker with the option --library-path (e.g. /lib/ld-linux.so.2 --library-path $HOME/mylibs myprogram).
3. The (colon-separated) paths in the DT_RUNPATH dynamic section attribute of the binary if present.
4. Lookup based on the ldconfig cache file (often located at /etc/ld.so.cache) which contains a compiled list of candidate libraries previously found in the augmented library path (set by /etc/ld.so.conf). If, however, the binary was linked with the-z nodeflib linker option, libraries in the default library paths are skipped.
5. In the trusted default path /lib, and then /usr/lib. If the binary was linked with the -z nodeflib linker option, this step is skipped.

Notes:

• The option --inhibit-rpath LIST of the dynamic linker instructs it to ignore DT_RPATH and DT_RUNPATH attributes of the object names in LIST.
• Libraries specified by the environment variable LD_PRELOAD and then those listed in /etc/ld.so.preload are loaded before the search begins. A preload can thus be used to replace some (or all) of the requested library's normal functionalities, or it can simply be used to supply a library that would otherwise not be found.
• Static libraries are searched and linked into the ELF file at link time and are not linked at run time.

[edit]The role of GNU ld

The GNU Linker (GNU ld) implements a feature which it calls "new-dtags": ^[2]

If the new-dtags feature is enabled in the linker (at run time using --enable-new-dtags), GNU ld, besides setting the DT_RPATHattribute, also sets the DT_RUNPATH attribute to the same string. At run time, if the dynamic linker finds a DT_RUNPATH attribute, it ignores the value of the DT_RPATH attribute, with the effect that LD_LIBRARY_PATH is checked next and the paths in theDT_RUNPATH attribute are only searched after it.

This means that in such configurations, the paths in LD_LIBRARY_PATH are searched before those given at link time using -rpath if--enable-new-dtags was active.

Instead of specifying the -rpath to the linker, the environment variable LD_RUN_PATH can be set to the same effect.