前言
在iOS动态链接过程中,_objc_init
起了非常重要的作用,因为_objc_init
向dyld动态库中注册了回调函数。跟随源码看一下_objc_init
都做了哪些事
_objc_init
void _objc_init(void) {
static bool initialized = false;
if (initialized) return;
initialized = true;
// fixme defer initialization until an objc-using image is found?
environ_init();
tls_init();
static_init();
runtime_init();
exception_init();
#if __OBJC2__
cache_t::init();
#endif
_imp_implementationWithBlock_init();
_dyld_objc_notify_register(&map_images, load_images, unmap_image);
// map_images()
// load_images()
#if __OBJC2__
didCallDyldNotifyRegister = true;
#endif
}
大致介绍下流程:
environ_init
读取影响运行时的环境变量,如果需要还可以打印,如:xcode中配置OBJC_PRINT_LOAD_METHODS
tls_init
作用:1、线程局部存储,因为有的一些变量是存储在线程的栈中 2、线程key的绑定,线程的析构函数
static_init
运行C++静态构造函数,在dyld调用我们的静态构造函数之前,libobjc内部如果有自己的构造函数需要做准备工作,则在这里就会调用,而不用等着dyld。
runtime_init
运行环境初始化,创建两张表 unattachedCategories & allocatedClasses
exception_init
初始化objc库的异常处理系统
cache_t::init
缓存条件初始化
_imp_implementationWithBlock_init
启动回调机制,通常不会什么,因为所有的初始化都是惰性的,但是对于某些进程,会迫不及待的加载
_dyld_objc_notify_register
向dyld注册回调通知
environ_init
void environ_init() {
//此处省略部分内容...
if (PrintHelp || PrintOptions) {
if (PrintHelp) {
_objc_inform("Objective-C runtime debugging. Set variable=YES to enable.");
_objc_inform("OBJC_HELP: describe available environment variables");
if (PrintOptions) {
_objc_inform("OBJC_HELP is set");
}
_objc_inform("OBJC_PRINT_OPTIONS: list which options are set");
}
if (PrintOptions) {
_objc_inform("OBJC_PRINT_OPTIONS is set");
}
for (size_t i = 0; i < sizeof(Settings)/sizeof(Settings[0]); i++) {
const option_t *opt = &Settings[i];
if (PrintHelp) _objc_inform("%s: %s", opt->env, opt->help);
if (PrintOptions && *opt->var) _objc_inform("%s is set", opt->env);
}
}
}
当我们在xcode中配置环境变量后,在这里就会打印出配置的内容,如下:
load方法使用的多会加长程序的启动时间,当你需要排出时,在xcode中直接配置环境变量打印,可直接查询
tls_init
void tls_init(void) {
#if SUPPORT_DIRECT_THREAD_KEYS
pthread_key_init_np(TLS_DIRECT_KEY, &_objc_pthread_destroyspecific);
#else
//线程key析构函数
_objc_pthread_key = tls_create(&_objc_pthread_destroyspecific);
#endif
}
static_init
运行C++构造函数,。在dyld
调用我们的静态构造函数之前,lib
会调用_objc_init
先调用自己的C++
构造函数,简单说的就是libobjc
会调用自己的全局的C++
函数,因为比较重要所以比dyld
先调用
static void static_init()
{
size_t count;
auto inits = getLibobjcInitializers(&_mh_dylib_header, &count);
for (size_t i = 0; i < count; i++) {
inits[i]();
}
auto offsets = getLibobjcInitializerOffsets(&_mh_dylib_header, &count);
for (size_t i = 0; i < count; i++) {
UnsignedInitializer init(offsets[i]);
init();
}
}
runtime_init
分类表的初始化 & 类表的初始化,allocated指的是已开辟的
void runtime_init(void)
{
objc::unattachedCategories.init(32);
objc::allocatedClasses.init();
}
exception_init
初始化异常处理系统,当程序出现异常时,用户可以自己去获取异常信息回调,可以上报服务器或其他
void exception_init(void)
{
old_terminate = std::set_terminate(&_objc_terminate);
}
顺着源码去找,会找到如下回调处理
objc_uncaught_exception_handler
objc_setUncaughtExceptionHandler(objc_uncaught_exception_handler fn)
{
objc_uncaught_exception_handler result = uncaught_handler;
uncaught_handler = fn;
return result;
}
对应OC代码处理如下, 这些代码可以封装在一个类里,程序启动之后,就可以调用:
+ (void)installUncaughtSignalExceptionHandler {
// objc_setUncaughtExceptionHandler()
NSSetUncaughtExceptionHandler(&exceptionHandlers);
}
// Exception 在这里可以做对应的处理,这就好比是block的回调
void exceptionHandlers(NSException *exception) {
NSLog(@"%s",__func__);
int32_t exceptionCount = atomic_fetch_add_explicit(&LGUncaughtExceptionCount,1,memory_order_relaxed);
if (exceptionCount > LGUncaughtExceptionMaximum) {
return;
}
// 获取堆栈信息 - model 编程思想
NSArray *callStack = [LGUncaughtExceptionHandle lg_backtrace];
NSMutableDictionary *userInfo = [NSMutableDictionary dictionaryWithDictionary:[exception userInfo]];
[userInfo setObject:exception.name forKey:LGUncaughtExceptionHandlerSignalExceptionName];
[userInfo setObject:exception.reason forKey:LGUncaughtExceptionHandlerSignalExceptionReason];
[userInfo setObject:callStack forKey:LGUncaughtExceptionHandlerAddressesKey];
}
cache_t::init
void cache_t::init()
{
#if HAVE_TASK_RESTARTABLE_RANGES
mach_msg_type_number_t count = 0;
kern_return_t kr;
while (objc_restartableRanges[count].location) {
count++;
}
//开启缓存
kr = task_restartable_ranges_register(mach_task_self(),
objc_restartableRanges, count);
if (kr == KERN_SUCCESS) return;
_objc_fatal("task_restartable_ranges_register failed (result 0x%x: %s)",
kr, mach_error_string(kr));
#endif // HAVE_TASK_RESTARTABLE_RANGES
}
_imp_implementationWithBlock_init
向dyld
的注册回调,_dyld_objc_notify_register
仅供objc
运行时调用且方法的实现在dyld
源码中
// _dyld_objc_notify_register
void _dyld_objc_notify_register(_dyld_objc_notify_mapped mapped,
_dyld_objc_notify_init init,
_dyld_objc_notify_unmapped unmapped)
{
dyld::registerObjCNotifiers(mapped, init, unmapped);
}
回调句柄赋值
// _dyld_objc_notify_init
void registerObjCNotifiers(_dyld_objc_notify_mapped mapped, _dyld_objc_notify_init
init, _dyld_objc_notify_unmapped unmapped)
{
// record functions to call
sNotifyObjCMapped = mapped;
sNotifyObjCInit = init;
sNotifyObjCUnmapped = unmapped;
// call 'mapped' function with all images mapped so far
try {
notifyBatchPartial(dyld_image_state_bound, true, NULL, false, true);
}
catch (const char* msg) {
// ignore request to abort during registration
}
// call 'init' function on all images already init'ed (below libSystem)
for (std::vector<ImageLoader*>::iterator it=sAllImages.begin(); it !=
sAllImages.end(); it++) {
ImageLoader* image = *it;
if ( (image->getState() == dyld_image_state_initialized) && image->notifyObjC() ) {
dyld3::ScopedTimer timer(DBG_DYLD_TIMING_OBJC_INIT, (uint64_t)image->machHeader(), 0, 0);
(*sNotifyObjCInit)(image->getRealPath(), image->machHeader());
}
}
}
//调用时机:xcode打打点符号map_images,运行就可发现调用栈,mapImages是在notifyBatchPartial
方法中,而notifyBatchPartial
方法是在registerObjCNotifiers
调用,在objc
初始化注册通知时就调用了,所以是调用map_images
后调用load_images
map_images
打来源码libobjc,查询mag_images,看到如下代码
void
map_images(unsigned count, const char * const paths[],
const struct mach_header * const mhdrs[])
{
mutex_locker_t lock(runtimeLock);
return map_images_nolock(count, paths, mhdrs);
}
进入map_images_nolock
,代码较多
大致流程:计算class数量,调整各种表的大小,初始化sel方法表,重点则为:_read_images
,直接看_read_images方法
_read_images
void _read_images(header_info **hList, uint32_t hCount, int
totalClasses, int
unoptimizedTotalClasses)
{
... //表示省略部分代码
#define EACH_HEADER \
hIndex = 0; \
hIndex < hCount && (hi = hList[hIndex]); \
hIndex++
// 条件控制进行一次的加载
if (!doneOnce) { ... }
// 修复预编译阶段的`@selector`的混乱的问题
// 就是不同类中有相同的方法 但是相同的方法地址是不一样的
// Fix up @selector references
static size_t UnfixedSelectors;
{ ... }
ts.log("IMAGE TIMES: fix up selector references");
// 错误混乱的类处理
// Discover classes. Fix up unresolved future classes. Mark bundle classes.
bool hasDyldRoots = dyld_shared_cache_some_image_overridden();
for (EACH_HEADER) { ... }
ts.log("IMAGE TIMES: discover classes");
// 修复重映射一些没有被镜像文件加载进来的类
// Fix up remapped classes
// Class list and nonlazy class list remain unremapped.
// Class refs and super refs are remapped for message dispatching.
if (!noClassesRemapped()) { ... }
ts.log("IMAGE TIMES: remap classes");
#if SUPPORT_FIXUP
// 修复一些消息
// Fix up old objc_msgSend_fixup call sites
for (EACH_HEADER) { ... }
ts.log("IMAGE TIMES: fix up objc_msgSend_fixup");
#endif
// 当类中有协议时:`readProtocol`
// Discover protocols. Fix up protocol refs.
for (EACH_HEADER) { ... }
ts.log("IMAGE TIMES: discover protocols");
// 修复没有被加载的协议
// Fix up @protocol references
// Preoptimized images may have the right
// answer already but we don't know for sure.
for (EACH_HEADER) { ... }
ts.log("IMAGE TIMES: fix up @protocol references");
// 分类的处理
// Discover categories. Only do this after the initial category
// attachment has been done. For categories present at startup,
// discovery is deferred until the first load_images call after
// the call to _dyld_objc_notify_register completes.
if (didInitialAttachCategories) { ... }
ts.log("IMAGE TIMES: discover categories");
// 类的加载处理
// Category discovery MUST BE Late to avoid potential races
// when other threads call the new category code befor
// this thread finishes its fixups.
// +load handled by prepare_load_methods()
// Realize non-lazy classes (for +load methods and static instances)
for (EACH_HEADER) { ... }
ts.log("IMAGE TIMES: realize non-lazy classes");
// 没有被处理的类,优化那些被侵犯的类
// Realize newly-resolved future classes, in case CF manipulates them
if (resolvedFutureClasses) { ... }
ts.log("IMAGE TIMES: realize future classes");
...
#undef EACH_HEADER
}
主要流程:
- 条件控制运行一次加载
- 读取方法列表,修复预编译阶段的@selector混乱问题,就是不同类中,有相同的方法,但是相同的方法地址不一样,因为类的地址是不一样的
- 错误混乱的类处理
- 修复重映射一些没有北京向文件加载进来的类
- 修复一些休息
- 读取协议 readProtocol
- 分类的处理
- 类的加载处理,非懒加载的类,读取类的信息,加载的类表中(
realizeClassWithoutSwift
),懒加载的类,第一次发送消息之前加到表中
错误混乱的类处理
for (EACH_HEADER) {
if (! mustReadClasses(hi, hasDyldRoots)) {
// Image is sufficiently optimized that we need not call readClass()
continue;
}
//从macho中读取类列表信息
classref_t const *classlist = _getObjc2ClassList(hi, &count);
bool headerIsBundle = hi->isBundle();
bool headerIsPreoptimized = hi->hasPreoptimizedClasses();
for (i = 0; i < count; i++) {
Class cls = (Class)classlist[i];
Class newCls = readClass(cls, headerIsBundle, headerIsPreoptimized);
// 类信息发生混乱,类运行时可能发生移动,但是没有被删除,相当于常说的野指针
if (newCls != cls && newCls) {
// Class was moved but not deleted. Currently this occurs
// only when the new class resolved a future class.
// Non-lazily realize the class below.
resolvedFutureClasses = (Class *)
realloc(resolvedFutureClasses,
(resolvedFutureClassCount+1) * sizeof(Class));
resolvedFutureClasses[resolvedFutureClassCount++] = newCls;
}
}
}
readClass
Class readClass(Class cls, bool headerIsBundle, bool headerIsPreoptimized)
{
// 获取类名
const char *mangledName = cls->nonlazyMangledName();
if (missingWeakSuperclass(cls)) { ... }
cls->fixupBackwardDeployingStableSwift();
Class replacing = nil;
if (mangledName != nullptr) { ... }
if (headerIsPreoptimized && !replacing) {...
} else {
if (mangledName) {
//some Swift generic classes can lazily generate their names
//将类名和地址关联起来
addNamedClass(cls, mangledName, replacing);
} else { ...}
//将关联的类插入到另一张哈希表中
addClassTableEntry(cls);
}
// for future reference: shared cache never contains MH_BUNDLEs
if (headerIsBundle) { ... }
return cls;
}
- 代码中发现了rw的赋值和ro的获取,但是通过调试过程,发现这块并不走。
addNamedClass
将类名和地址关联绑定起来addClassTableEntry
将关联的类插入到allocatedClasses
表中,这张表中都是初始化过的类 以上的代码和下面的MACH-O一一对应
这个可以在readclass中打断点验证,地址和MACH-O文件中的一样
non-lazy classes加载
注释意思很明显,实现了load方法的类,就会走realizeClassWithoutSwift
方法,没有实现的默认跳过,这个是和MACH-O文件相对应的,如下图
iOS底层探索 _objc_init & readImages
前言
在iOS动态链接过程中,_objc_init
起了非常重要的作用,因为_objc_init
向dyld动态库中注册了回调函数。跟随源码看一下_objc_init
都做了哪些事
_objc_init
void _objc_init(void) {
static bool initialized = false;
if (initialized) return;
initialized = true;
// fixme defer initialization until an objc-using image is found?
environ_init();
tls_init();
static_init();
runtime_init();
exception_init();
#if __OBJC2__
cache_t::init();
#endif
_imp_implementationWithBlock_init();
_dyld_objc_notify_register(&map_images, load_images, unmap_image);
// map_images()
// load_images()
#if __OBJC2__
didCallDyldNotifyRegister = true;
#endif
}
大致介绍下流程:
environ_init
读取影响运行时的环境变量,如果需要还可以打印,如:xcode中配置OBJC_PRINT_LOAD_METHODS
tls_init
作用:1、线程局部存储,因为有的一些变量是存储在线程的栈中 2、线程key的绑定,线程的析构函数
static_init
运行C++静态构造函数,在dyld调用我们的静态构造函数之前,libobjc内部如果有自己的构造函数需要做准备工作,则在这里就会调用,而不用等着dyld。
runtime_init
运行环境初始化,创建两张表 unattachedCategories & allocatedClasses
exception_init
初始化objc库的异常处理系统
cache_t::init
缓存条件初始化
_imp_implementationWithBlock_init
启动回调机制,通常不会什么,因为所有的初始化都是惰性的,但是对于某些进程,会迫不及待的加载
_dyld_objc_notify_register
向dyld注册回调通知
environ_init
void environ_init() {
//此处省略部分内容...
if (PrintHelp || PrintOptions) {
if (PrintHelp) {
_objc_inform("Objective-C runtime debugging. Set variable=YES to enable.");
_objc_inform("OBJC_HELP: describe available environment variables");
if (PrintOptions) {
_objc_inform("OBJC_HELP is set");
}
_objc_inform("OBJC_PRINT_OPTIONS: list which options are set");
}
if (PrintOptions) {
_objc_inform("OBJC_PRINT_OPTIONS is set");
}
for (size_t i = 0; i < sizeof(Settings)/sizeof(Settings[0]); i++) {
const option_t *opt = &Settings[i];
if (PrintHelp) _objc_inform("%s: %s", opt->env, opt->help);
if (PrintOptions && *opt->var) _objc_inform("%s is set", opt->env);
}
}
}
当我们在xcode中配置环境变量后,在这里就会打印出配置的内容,如下:
load方法使用的多会加长程序的启动时间,当你需要排出时,在xcode中直接配置环境变量打印,可直接查询
tls_init
void tls_init(void) {
#if SUPPORT_DIRECT_THREAD_KEYS
pthread_key_init_np(TLS_DIRECT_KEY, &_objc_pthread_destroyspecific);
#else
//线程key析构函数
_objc_pthread_key = tls_create(&_objc_pthread_destroyspecific);
#endif
}
static_init
运行C++构造函数,。在dyld
调用我们的静态构造函数之前,lib
会调用_objc_init
先调用自己的C++
构造函数,简单说的就是libobjc
会调用自己的全局的C++
函数,因为比较重要所以比dyld
先调用
static void static_init()
{
size_t count;
auto inits = getLibobjcInitializers(&_mh_dylib_header, &count);
for (size_t i = 0; i < count; i++) {
inits[i]();
}
auto offsets = getLibobjcInitializerOffsets(&_mh_dylib_header, &count);
for (size_t i = 0; i < count; i++) {
UnsignedInitializer init(offsets[i]);
init();
}
}
runtime_init
分类表的初始化 & 类表的初始化,allocated指的是已开辟的
void runtime_init(void)
{
objc::unattachedCategories.init(32);
objc::allocatedClasses.init();
}
exception_init
初始化异常处理系统,当程序出现异常时,用户可以自己去获取异常信息回调,可以上报服务器或其他
void exception_init(void)
{
old_terminate = std::set_terminate(&_objc_terminate);
}
顺着源码去找,会找到如下回调处理
objc_uncaught_exception_handler
objc_setUncaughtExceptionHandler(objc_uncaught_exception_handler fn)
{
objc_uncaught_exception_handler result = uncaught_handler;
uncaught_handler = fn;
return result;
}
对应OC代码处理如下, 这些代码可以封装在一个类里,程序启动之后,就可以调用:
+ (void)installUncaughtSignalExceptionHandler {
// objc_setUncaughtExceptionHandler()
NSSetUncaughtExceptionHandler(&exceptionHandlers);
}
// Exception 在这里可以做对应的处理,这就好比是block的回调
void exceptionHandlers(NSException *exception) {
NSLog(@"%s",__func__);
int32_t exceptionCount = atomic_fetch_add_explicit(&LGUncaughtExceptionCount,1,memory_order_relaxed);
if (exceptionCount > LGUncaughtExceptionMaximum) {
return;
}
// 获取堆栈信息 - model 编程思想
NSArray *callStack = [LGUncaughtExceptionHandle lg_backtrace];
NSMutableDictionary *userInfo = [NSMutableDictionary dictionaryWithDictionary:[exception userInfo]];
[userInfo setObject:exception.name forKey:LGUncaughtExceptionHandlerSignalExceptionName];
[userInfo setObject:exception.reason forKey:LGUncaughtExceptionHandlerSignalExceptionReason];
[userInfo setObject:callStack forKey:LGUncaughtExceptionHandlerAddressesKey];
}
cache_t::init
void cache_t::init()
{
#if HAVE_TASK_RESTARTABLE_RANGES
mach_msg_type_number_t count = 0;
kern_return_t kr;
while (objc_restartableRanges[count].location) {
count++;
}
//开启缓存
kr = task_restartable_ranges_register(mach_task_self(),
objc_restartableRanges, count);
if (kr == KERN_SUCCESS) return;
_objc_fatal("task_restartable_ranges_register failed (result 0x%x: %s)",
kr, mach_error_string(kr));
#endif // HAVE_TASK_RESTARTABLE_RANGES
}
_imp_implementationWithBlock_init
向dyld
的注册回调,_dyld_objc_notify_register
仅供objc
运行时调用且方法的实现在dyld
源码中
// _dyld_objc_notify_register
void _dyld_objc_notify_register(_dyld_objc_notify_mapped mapped,
_dyld_objc_notify_init init,
_dyld_objc_notify_unmapped unmapped)
{
dyld::registerObjCNotifiers(mapped, init, unmapped);
}
回调句柄赋值
// _dyld_objc_notify_init
void registerObjCNotifiers(_dyld_objc_notify_mapped mapped, _dyld_objc_notify_init
init, _dyld_objc_notify_unmapped unmapped)
{
// record functions to call
sNotifyObjCMapped = mapped;
sNotifyObjCInit = init;
sNotifyObjCUnmapped = unmapped;
// call 'mapped' function with all images mapped so far
try {
notifyBatchPartial(dyld_image_state_bound, true, NULL, false, true);
}
catch (const char* msg) {
// ignore request to abort during registration
}
// call 'init' function on all images already init'ed (below libSystem)
for (std::vector<ImageLoader*>::iterator it=sAllImages.begin(); it !=
sAllImages.end(); it++) {
ImageLoader* image = *it;
if ( (image->getState() == dyld_image_state_initialized) && image->notifyObjC() ) {
dyld3::ScopedTimer timer(DBG_DYLD_TIMING_OBJC_INIT, (uint64_t)image->machHeader(), 0, 0);
(*sNotifyObjCInit)(image->getRealPath(), image->machHeader());
}
}
}
//调用时机:xcode打打点符号map_images,运行就可发现调用栈,mapImages是在notifyBatchPartial
方法中,而notifyBatchPartial
方法是在registerObjCNotifiers
调用,在objc
初始化注册通知时就调用了,所以是调用map_images
后调用load_images