42.5. Preloading Shared Libraries

For testing purposes, it can sometimes be useful to selectively override functions (and other symbols) that would normally be found by the dynamic linker using the rules described in Section 41.11. To do this, we can define the environment variable LD_PRELOAD as a string consisting of space-separated or colon-separated names of shared libraries that should be loaded before any other shared libraries. Since these libraries are loaded first, any functions they define will automatically be used if required by the executable, thus overriding any other functions of the same name that the dynamic linker would otherwise have searched for. For example, suppose that we have a program that calls functions x1() and x2(), defined in our libdemo library. When we run this program, we see the following output:

$ ./prog
Called mod1-x1 DEMO
Called mod2-x2 DEMO