Tweak-Tutorial

How Do You Create a Tweak?

%hookf can be confusing at first, but it is pretty easy to understand once you get into it.

What is %hookf used for?

While %hook is used to hook Objective-C classes, %hookf is used to hook C functions. Its syntax is also different from %hook.

Syntax

%hookf(return_type, symbol_name, arguments...) {...}

• return_type - The return type of the function.
• symbol_name - This is the name of the function being hooked.
• arguments - These are the arguments that are passed into the function.

Example

Let’s say we want to hook CGFontRef CGFontCreateWithFontName(CFStringRef name);. This would be done like so:

%hookf(CGFontRef, CGFontCreateWithFontName, CFStringRef name) {
  // code
  return %orig;
}

Below is the Substrate version of the above code, if needed.

static CGFontRef (*CGFontCreateWithFontName_orig)(CFStringRef) = NULL;
static CGFontRef CGFontCreateWithFontName_hook(CFStringRef name) {
  return CGFontCreateWithFontName_orig(name);
}

__attribute__((constructor)) static void initialize() {
  MSHookFunction(dlsym(RTLD_DEFAULT, "CGFontCreateWithFontName"), (void *)CGFontCreateWithFontName_hook, (void **)&CGFontCreateWithFontName_orig);
}

If we are not able to directly link with the binary, we can dynamically look up the symbol. Logos has syntax for doing this:

%hookf(CGFontRef, CGFontCreateWithFontName, CFStringRef name) {
  // code
  return %orig;
}

%ctor {
  %init(CGFontCreateWithFontName=dlsym(RTLD_DEFAULT, "CGFontCreateWithFontName"));
}

For non-exported symbols, we can use MSFindSymbol instead of dlsym. Do note that we will need an extra _ at the beginning of the symbol if we’re using MSFindSymbol.

For further information about %hookf, please go here.

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