OC底层探索16 -应用程序加载OC底层探索16 -应用程序加载

uintptr_t
_main(const macho_header* mainExecutableMH, uintptr_t mainExecutableSlide, 
        int argc, const char* argv[], const char* envp[], const char* apple[], 
        uintptr_t* startGlue)
{
    ......

    // 第一步、设置运行环境，可执行文件准备工作
    ......

    // 第二步、 加载共享缓存(已经加载到内存中的动态库无需再次加载，如：UIKit、Founation等)
    //load shared cache   
    mapSharedCache();
    ......

reloadAllImages:

    ......
    // 第三步、 加载可主执行文件并生成一个ImageLoader实例对象
    // instantiate ImageLoader for main executable
    sMainExecutable = instantiateFromLoadedImage(mainExecutableMH, mainExecutableSlide, sExecPath);

    ......

    // 第四步、   加载插入的动态库
    // load any inserted libraries 
    if  ( sEnv.DYLD_INSERT_LIBRARIES != NULL ) {
        for (const char* const* lib = sEnv.DYLD_INSERT_LIBRARIES; *lib != NULL; ++lib) 
            loadInsertedDylib(*lib);
    }
        
    // 第五步、  链接主程序
    // link main executable 
    link(sMainExecutable, sEnv.DYLD_BIND_AT_LAUNCH, true, ImageLoader::RPathChain(NULL, NULL), -1);

    ......
    // 第六步、 链接所有插入的动态库
    // link any inserted libraries 
    if ( sInsertedDylibCount > 0 ) {
        for(unsigned int i=0; i < sInsertedDylibCount; ++i) {
            ImageLoader* image = sAllImages[i+1];
            link(image, sEnv.DYLD_BIND_AT_LAUNCH, true, ImageLoader::RPathChain(NULL, NULL), -1);
            image->setNeverUnloadRecursive();
        }
        if ( gLinkContext.allowInterposing ) {
            for(unsigned int i=0; i < sInsertedDylibCount; ++i) {
                ImageLoader* image = sAllImages[i+1];
                // 注册符号插入
                image->registerInterposing(gLinkContext);
            }
        }
    }

    ......
    // 第七步、 弱符号绑定
    sMainExecutable->weakBind(gLinkContext);
        
    sMainExecutable->recursiveMakeDataReadOnly(gLinkContext);

    ......
    // 第八步、   执行初始化方法
    // run all initializers 
    initializeMainExecutable(); 

    // 第九步、 main()调用
    // enter main()
    notifyMonitoringDyldMain();

    return result;
}

void
load_images(const char *path __unused, const struct mach_header *mh)
{
    // Call +load methods (without runtimeLock - re-entrant)
    call_load_methods();
}

void call_load_methods(void)
{
    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);
}

static void call_class_loads(void)
{
    int i;
    // Call all +loads for the detached list.
    for (i = 0; i < used; i++) {
        Class cls = classes[i].cls;
        load_method_t load_method = (load_method_t)classes[i].method;
        (*load_method)(cls, @selector(load));
    }
}