ld.so is the UNIX/Linux helper program which loads the
shared library files (.so files) needed by dynamically
linked programs, and does all the fiddly stuff need to get them to
work with the program. Ordinarily you don't have to care at all
about ld.so, except that sometimes you want to tell it where to
look for .so files, in what order to look for them, and
extra libraries that you want to be forced to load.
These are the means of controlling ld.so, in the order that they
are used (first to last):
The libraries listed in the file /etc/ld.so.preload are
loaded first. These libraries can override the functions that
are defined in libraries that are loaded later. This file
affects every dynamically linked program that runs;
ld.so has no affected on statically linked programs.
The environmental variable LD_PRELOAD, which acts
just like the /etc/ld.so.prelaod file, except that it
isn't global. Also, for security reasons, if the program being
affected is SETUID or SETGID, the libraries are only loaded
if they are in /lib or /usr/lib, plus
they have have the proper SETUID/SETGID file
permission bits set; the libraries listed in
/etc/ld.so.preload are always loaded.
The libraries in LD_PRELOAD must be separated by spaces
(whitespace), and not by colons (":").
Libraries are searched searched for in /lib, then
Libraries are searched for in the the directories listed in the
colon separated environmental variables
LD_LIBRARY_PATH. This is ignored if the program is
SETUID or SETGID, for security reasons.
Libraries are searched for in the directories listed in the file
/etc/ld.so.conf. Actually, the libraries are searched
for in the index file /etc/ld.so.cache, which is
created when the program ldconfig looks through all the
directories listed in /etc/ld.so.conf. If you add any
libraries to directories other than /lib or
/usr/lib, you will need to run ldconfig.
The program ldd can be used to list which shared
libraries a program depends on.