Here are some steps to hook functions in an Android app using a main.cpp file

Here are some steps to hook functions in an Android app using a main.cpp file:

1. Identify the target library and function you want to hook. For example:

#define targetLibName "libexample.so"
void (*functionToHook)(int, char*);

2. Get the address of the original function. You can use dlsym for this:

functionToHook = (void (*)(int, char*)) dlsym(RTLD_DEFAULT, "functionName");

3. Write your custom hook function that will replace the original. It should match the original function prototype:

void myHook(int arg1, char* arg2) {
  // custom code here 
  
  // call original function
  functionToHook(arg1, arg2); 
}


4. Redirect execution to your hook function. You can use inline hooking for this:

MemoryPatch patch(targetLibName, functionToHook, myHook); 
patch.hook();

5. Add logic in your hook function to control execution flow, log values, etc.

6. Compile the main.cpp into a shared library.

7. Load the shared library into your Android app, e.g. using loadLibrary() in Java.

This will hook the target function and redirect it to your custom logic. 
You can hook multiple functions by repeating these steps. 
The key things are identifying the target, redirecting to a hook function, and loading the .so file.