1. A method comprising: locating a target function to modify on a computing device, the target function compiled to include a place holder instruction added as a non-operative entry point in the target function to enable hooking a hook function to the target function; identifying the hook function to execute when a call is made to the target function the hook function being configured to test printer functionality provided at least in part through a print subsystem of the computing device by at least determining which printer handles are being used by the print subsystem, how long the print handles have been in use, and whether handle leaks related to the target function are suspected; and overwriting the place holder instruction of the target function with instruction to cause execution of the hook function, the place holder instruction being overwritten without preserving and without copying the place holder instruction of the target function.