In this post, the author uses a simple example program to demonstrate how to modify the behavior of applications in Linux without touching their source code or recompiling them. By creating a custom library and using the LD_PRELOAD environmental variable, the author is able to influence the output of the example program. The author also provides an example of how to inject code into an application in a transparent manner using the dynamic linker interface. The author suggests several possible uses for this technique, including cheating in games, investigating unknown applications, monitoring file and internet access, and implementing new features or fixing bugs in applications.
https://rafalcieslak.wordpress.com/2013/04/02/dynamic-linker-tricks-using-ld_preload-to-cheat-inject-features-and-investigate-programs/