Ios – Class loading process (_objc_init implementation principle)

Ios – Class loading process (DyLD loading process)

Preface:

The previous article analyzed the app startup process and mentioned that _objc_init is called during dyLD loading. In this article we’ll take a closer look at what the _objc_init method does.

Explore _objc_init

Open the objC source global search _objc_init method

_objc_init() init() init();

environ_init()

Such as:

Remove lines 426-430 and delete the useless judgment, as shown below

We’ve used some of these environments before, so let’s look at themOBJC_DISABLE_NONPOINTER_ISA, we do as follows

Let’s run the code and print the Person

Remove the set

In common Settings for example:

, etc.

tls_init()

Thread key binding

static_init()

Mainly runs system-level C++ static constructors indyldBefore calling our static constructor,libccall_objc_initMethods, i.e.,System-level C++ constructors run before custom C++ constructors

runtime_init()

mainlyThe runtimeInitialization is divided into two parts:Class initialization,Class to initialize the table

exception_init()

It is mainly to initialize libobJC’s exception processing system, register the callback of the exception processing, so as to monitor the exception processing, the source code is as follows

Look at the **_objc_terminate** function

Enter the **uncaught_handler** method

Uncaught_handler is an instance of objC_uncaught_Exception_handler. The default value is _objC_default_uncaught_Exception_handler

Search ** objc_uncaught_EXCEPtion_handler ** function implementation

Exceptions fall into three categories

• Mach exceptions: These are the lowest level kernel-level exceptions. User developers can catch Mach exceptions by setting exception ports for Thread, task, and host directly through the Mach API.

• Unix signals: Also known as BSD signals, if the developer does not catch a Mach exception, the exception is converted to the corresponding Unix signal by the host layer method ux_Exception () and sent to the error thread via the threadSignal () method. Single can be caught using the method Signal (x, SignalHandler).

• NSException Application-level exception: It is an uncaught Objective-C exception that causes the program to send itself a SIGABRT signal and crash. For an uncaught Objective-C exception, you can catch it with a try catch. To capture or through NSSetUncaughtExceptionHandler () mechanism.

The concrete implementation of this method is inarticleThe source code implementation is described in detaildyldIn the source code, the following_dyld_objc_notify_registerMethod declaration

Description:

• Used by objC runtime only

• Registers handlers to be called when objC images are mapped, unmapped, and initialized

• Dyld will call the mapped function through an array containing objC-image-info’s image file

The meanings of the three parameters in the method are as follows:

• Map_images: This function is triggered when dyld loads an image into memory

• Load_image: this function is triggered when dyld initializes the image

• Unmap_image: Triggered when dyld removes the image

The specific analysis process has been explained in detail in the previous article

Enter the map_images_NOLock function

Go to the _read_images method (core method, remember this method carefully)

The method is long, and the next article will examine it in detail (lazy-loaded and non-lazy-loaded classes).