1. A method, comprising: modifying, prior to run time, an executable file of an application to cause an operating system loader to load additional code by injecting a dynamically-linked library having the additional code into the application; wherein modifying the executable file comprises: determining whether the executable file includes sufficient unused space to accommodate a load command; responsive to the executable file including sufficient unused space, shifting, in the executable file, an existing load command which does not contain dependency information to make space for the load command; and adding the load command to the executable file; and, wherein the additional code, when executed by a processor, causes the processor to change a pointer in a table that indicates an address of an imported function implementing a system call so that the pointer indicates an address of a customized function.