preface
How does the code load into memory let’s explore
The code for
- Libsystem – 1292.120.1
- Libdispatch – 1271.120.2
- dyld-852
- Objc4-818.2 –
1. Application loading principle
Libraries: executable binaries -> can be loaded into memory by the operating system -> static and dynamic libraries
The build process
What is an executable
Open project Command +b into the following operation and the final result is the executable file
Verify that the executable executes the code in main
The process of loading executable files into memory through dynamic linker dyld is shown in the following figure
Runtime to register the callback function is
We print out the image list to see which dyld’s were loaded. We can find the path to the CoreFoundation library: dyLD source dyLD DYLD_ROOT_PATH
The main function is called by executing map_images and Load_images (which will be examined in the next section)
2.dyldWhich leads to the
How do we find dyld by first opening the project breakpoint main function
0 main 1 start 在mainI left this earlier1 start 1 startWhat did he do
Welcome tolibdyld.dylibAdd system breakpoints to the worldstartWalk a wave and find no break but[ViewController load] 在mainIf we print the breakpoint here and see if we find something like this
Dyld ‘ ‘_dyLD_start | | | | | | | | | | | | | |
The application loading process is as follows
- dyld`_dyld_start ->
- dyld`dyldbootstrap::start ->
- dyld`dyld::_main ->
- dyld`dyld::useSimulatorDyld ->
- dyld_sim`dyld::_main ->
- dyld_sim`dyld::initializeMainExecutable() ->
- dyld_sim`ImageLoader::runInitializers ->
- dyld_sim`ImageLoader::processInitializers ->
- dyld_sim`ImageLoader::recursiveInitialization ->
- dyld_sim`dyld::notifySingle ->
- libobjc.A.dylib`load_images
3.dyldIn the process
Search for _dyLD_start through the dyld source code and we enter the assembly
Call dyLDbootstrap ::start(app_mh, argc, argv, dyLD_MH, &startglue
dyld::_main((macho_header*)appsMachHeader, appsSlide, argc, argv, envp, apple, startGlue);Let’s go in and see
4.dyldIn the processmainMain flow of functions
Because the main function has a long code and the main function is going to return result and it’s going to be executing sMainExecutable
Found this function sMainExecutable = instantiateFromLoadedImage instantiation of the main program
Use this function to add the image file
Add a format to machO using this function.
Add specific formats such as load_command with the sniffLoadCommands function
The next step is to load the inserted dynamic library
The next step links to the main program
Next link inserts the dynamic library
The next step is weakBind weak reference binding
The next step is to initialize initializeMainExecutable and run
The next step is to tell dyld that it can enter main()
5. Dyld process – Main program run
We mainly analyze initializeMainExecutable images to start initialization
Enter runInitializers -> processInitializers
Enter processInitializers->recursiveInitialization
Enter recursiveInitialization->context.notifySingle single notification injection
-> doInitialization Call the init method -> context.notifysingle to notify that initialization is complete
Go to notifySingle-> and find sNotifyObjCInit
Through the sNotifyObjCInit – > registerObjCNotifiers
Through the registerObjCNotifiers – > _dyld_objc_notify_register
_dyLD_OBJC_NOTIFy_register is the Runtime register callback
6.dyldProcess –imagesInitialization process
We pass objC4-818.2 source breakpoint _objc_init
libdispatch.dylib``_os_object_init
_OS_object_init -> Calls _objc_init
libdispatch.dylib``libdispatch_init->_os_object_init
libSystem.B.dylib``libSystem_initializer->libdispatch_init
Dyld ` ` ImageLoaderMachO: : doModInitFunctions: – > initialize libSystem_initializer (through STRCMP (installPath, libSystemPath (context))
ImageLoaderMachO::doInitialization->doModInitFunctions
Return to recursion recursiveInitialization-> doInitialization
