从 dyld 到 runtime

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dyld中c++部分:
    
    // In dyld we have to do this manually.
    start(){
        // others work...
        return _main();//而_main()返回的是主程序main()的地址
    }
dyld汇编中部分：
    __dyld_start: 会jumps 到start()返回的地址

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// 
// Entry point for dyld. The kernel loads dyld and jumps to __dyld_start which
// sets up some registers and call this function.
//
// Returns address of main() in target program which __dyld_start jumps to
//

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1,// instantiate ImageLoader for main executable
// load shared cache
// Now that shared cache is loaded,setup an versioned dylib overrIDes
2,// load any inserted librarIEs
3,// link main executable
4,// link any inserted librarIEs -- do this after linking main executable so that any dylibs pulled in by inserted // dylibs (e.g. libSystem) will not be in front of dylibs the program uses
5,// run all initializers
6,// find entry point for main executable
7,// Returns address of main() in target program

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//
// ImageLoader is an abstract base class. To support loading a particular executable
// file format,you make a concrete subclass of ImageLoader.
//
// 每个.o文件都会生成一个ImageLoader子类实例
// For each executable file (dynamic shared object) in use,an ImageLoader is instantiated.
//
// The ImageLoader base class does the work of linking together images,but it kNows nothing
// about any particular file format.
//
//
class ImageLoader {...}

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// instantiate ImageLoader for main executable
sMainExecutable = instantiateFromloadedImage(mainExecutableMH,mainExecutableSlIDe,sExecPath);

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其中instantiateFromloadedImage()的主要实现是：
    检查.o是否合法
    加载.o实例化为 image：
        根据mach-o文件segments的规则将数据加载到内存中
        根据mach-o文件的头部信息,将segments的具体信息构建到image的实例中
    addImage():把image添加到images容器中，并更新内存分布

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// The kernel maps in main executable before dyld gets control. 
// We need to make an ImageLoader* for the already mapped in main executable.
static ImageLoader* instantiateFromloadedImage(...){
    if ( isCompatibleMachO((const uint8_t*)mh,path) ) { //检查.o是否合法
       //instantiateMainExecutable()：得到主程序的image。省略不贴code了(影响美观，??)
        ImageLoader* image = ImageLoaderMachO::instantiateMainExecutable(mh,slIDe,path,glinkContext);
        addImage(image);
        return image;
    }
}

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static voID addImage(ImageLoader* image){
    //add to master List
    // update mapped ranges
    // other work…
    if ( sEnv.DYLD_PRINT_liBRARIES || ...) {
        dyld::log("dyld: loaded: %sn",image->getPath());
   }
}

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// load any inserted librarIEs
if( sEnv.DYLD_INSERT_liBRARIES != NulL) {
    for (const char* const* lib = sEnv.DYLD_INSERT_liBRARIES; *lib != NulL; ++lib)
        loadInsertedDylib(*lib);
}

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voID initializeMainExecutable(){
    // run initialzers for any inserted dylibs
    // run initializers for main executable and everything it brings up
    // register cxa_atexit() handler to run static terminators in all loaded images when this process exits
    // dump info if requested
    if( sEnv.DYLD_PRINT_STATISTICS ){
        ImageLoaderMachO::printStatistics((unsigned int)sAllimages.size(),initializerTimes[0]);
    }

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static voID libSystem_initializer(...){
    // others init...
    libdispatch_init();
}
voID libdispatch_init(voID){
    // others init...
    _os_object_init();
}
voID _os_object_init(voID){
    _objc_init();
    // others init work...
}
/***********************************************************************
* _objc_init
* bootstrap initialization. Registers our image notifIEr with dyld.
* old ABI: called by dyld as a library initializer
* New ABI: called by libSystem BEFORE library initialization time
**********************************************************************/
voID _objc_init(voID){
    // other init work…
    
    // old objc-runtime.mm(??的主要用这种来介绍)
    dyld_register_image_state_change_handler(dyld_image_state_bound,1/*batch*/,&map_2_images);
    dyld_register_image_state_change_handler(dyld_image_state_dependents_initialized,0/*not batch*/,&load_images);
    
    // 在新的 objc-runtime.mm中是这样的，反正都差不多就不计较用法了。
    // _dyld_objc_notify_register(&map_images,load_images,unmap_image);
}
// 关于 dyld_register_image_state_change_handler的介绍,其中第三个参数是callback!!
// Register a handler to be called when any image changes to the requested state.
extern voID dyld_register_image_state_change_handler(enum dyld_image_states state,bool batch,dyld_image_state_change_handler handler);

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调用过程：map_2_images() -> map_images_nolock() -> _read_images()
map_images_nolock注释：
* Process the given images which are being mapped in by dyld.
* All class registration and fixups are performed (or deferred pending discovery of missing superclasses etc),and +load methods are called.
voID _read_images(header_info **hList,uint32_t hCount){
    // 比较重要的部分：
    // Read classes from all images.   
    // Read categorIEs from all images
    // Connect classes from all images.【 Connect the classes in the given image to their superclasses,or register them for later connection if any superclasses are missing.】
    // Fix up class refs,selector refs,and protocol objects from all images.
}

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const char*load_images(enum dyld_image_states state,uint32_t infoCount,const structdyld_image_info infoList[]){
    // other work ...
    // Call +load methods (without runtimeLock - re-entrant)
    call_load_methods();
}
/***********************************************************************
* call_load_methods
* Call all pending class and category +load methods.
* Class +load methods are called superclass-first. 
* category +load methods are not called until after the parent class's +load.
…………
**********************************************************************/
voID call_load_methods(voID){
    // other work...
    do{
        // 1. Repeatedly call class +loads until there aren't any more
        while (loadable_classes_used > 0) {
           call_class_loads();
        }
        // 2. Call category +loads ONCE
        more_categorIEs = call_category_loads();
        // 3. Run more +loads if there are classes OR more untrIEd categorIEs
    }while (loadable_classes_used > 0  ||  more_categorIEs);
}