iOS: de-caching dyld shared cache and hidden gem

Since iOS3 all system dylib/frameworks are combined into big cache file for performance reason. I needed these files to obtain export symbols from them for my future work on Windows/Linux support for RoboVM. There are several ways to get these.
There is lot of information about caches in iPhone Development Wiki
First of all cache files are located in /System/Library/Caches/com.apple.dyld/. There are two files: dyld_shared_cache_armv7s.dms and dyld_shared_cache_arm64.dms.

Retrieving cache

There are several way to extract these files listed in iPhone Development Wiki, but lets go RoboVM way and use all power of java/maven repos. This gives us simple project to run. Once it is started there will be file web server on device, navigate your desktop browser to http://x.x.x.x:8081 (x.x.x.x is IP address of your iOS device) and files are available there to download.

// app delegate class
@Override
public boolean didFinishLaunching(UIApplication application, UIApplicationLaunchOptions launchOptions) {
    ...
    new Thread(() -> {
        try {
            SimpleWebServer server = new SimpleWebServer(null, 8081, new File("/System/Library/Caches/com.apple.dyld/"), false);
            server.start();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }).start();

    return true;
}

// build.gradle
dependencies {
    ...
    // https://mvnrepository.com/artifact/org.nanohttpd/nanohttpd-webserver
    compile group: 'org.nanohttpd', name: 'nanohttpd-webserver', version: '2.3.1'
}

Dealing with cache file

There is bunch of tools listed in iPhone Development Wiki:

• dyld_decache
• DySlim
• decache
• dsc_extractor (from apple)
• and other

All of them are kind of broken, limited and don’t produce reliable result. I started fixing dyld_decache but ended in writing own cache parser in java. But suddenly found hidden gems…

Hidden gems

check dsc_extractor (from apple), look at the end of dsc_extractor.cpp there will be commented out code:

#if 0
typedef int (*extractor_proc)(const char* shared_cache_file_path, const char* extraction_root_path,
													void (^progress)(unsigned current, unsigned total));
int main(int argc, const char* argv[])
{
	if ( argc != 3 ) {
		fprintf(stderr, "usage: dsc_extractor <path-to-cache-file> <path-to-device-dir>\n");
		return 1;
	}

	void* handle = dlopen("/Developer/Platforms/iPhoneOS.platform/usr/lib/dsc_extractor.bundle", RTLD_LAZY);
	if ( handle == NULL ) {
		fprintf(stderr, "dsc_extractor.bundle could not be loaded\n");
		return 1;
	}

	extractor_proc proc = (extractor_proc)dlsym(handle, "dyld_shared_cache_extract_dylibs_progress");
	if ( proc == NULL ) {
		fprintf(stderr, "dsc_extractor.bundle did not have dyld_shared_cache_extract_dylibs_progress symbol\n");
		return 1;
	}

	int result = (*proc)(argv[1], argv[2], ^(unsigned c, unsigned total) { printf("%d/%d\n", c, total); } );
	fprintf(stderr, "dyld_shared_cache_extract_dylibs_progress() => %d\n", result);
	return 0;
}
#endif

Just paste snippet into .c file, compile it and (if you have installed xcode) it will give you native apple cache extractor.

Hidden gems 2

It is listed at iPhone Development Wiki but it should be listed with huge text size. Once device is connected xcode is enabling device for development. Same time it extracts all dylib with dsc_extractor and puts them to ~/Library/Developer/Xcode/iOS DeviceSupport/. So just connect device and xcode will extracts everything without any tool. But there will just one arch, e.g. ARM7