Mac offers a way to override functions in a shared library with DYLD_INSERT_LIBRARIES environment variable (which is similar to LD_PRELOAD on Linux). When you make a twin brother of a function that is defined in an existing shared library, put it in you a shared library, and you register your shared library name in DYLD_INSERT_LIBRARIES, your function is used instead of the original one. This is my simple test. Here I’ve replaced f() in mysharedlib.dylib with f() in openhook.dylib.