ld.so loads the shared libraries needed by a program, prepares the program to run, and then runs it. Unless explicitly specified via the -static option to ld during compilation, all Linux programs are incomplete and require further linking at run time.
The necessary shared libraries needed by the program are searched for in the following order
Using the environment variable LD_LIBRARY_PATH (LD_AOUT_LIBRARY_PATH for a.out programs). Except if the executable is a setuid/setgid binary, in which case it is ignored.
From the cache file /etc/ld.so.cache which contains a compiled list of candidate libraries previously found in the augmented library path.
A colon-separated list of directories in which to search for ELF libraries at execution-time. Similar to the PATH environment variable.
A whitespace-separated list of additional, user-specified, ELF shared libraries to be loaded before all others. This can be used to selectively override functions in other shared libraries. For setuid/setgid ELF binaries, only libraries in the standard search directories that are also setgid will be loaded.
If present, causes the program to list its dynamic library dependencies, as if run by ldd, instead of running normally.
If present, causes the dynamic linker to resolve all symbols at program startup instead of when they are first referenced.
A colon-separated list of directories in which to search for a.out libraries at execution-time. Similar to the PATH environment variable.
The name of an additional, user-specified, a.out shared library to be loaded after all others. This can be used to selectively override functions in other shared libraries.
Suppress warnings about a.out libraries with incompatible minor version numbers.
/lib/ld.so a.out dynamic linker/loader
ELF dynamic linker/loader
File containing a compiled list of directories in which to search for libraries and an ordered list of candidate libraries.
File containing a whitespace separated list of ELF shared libraries to be loaded before the program. libraries and an ordered list of candidate libraries.
Currently ld.so has no means of unloading and searching for compatible or newer version of libraries.