程序真实第一步在哪？
我们都知道，网上普遍流行的一种说法就是，一个iOS程序的入口就是我们的main函数，那么在main函数真的就没别的了吗？答案是否的，下面一个小案例探究一下：
1.1程序入口探究
在程序
main.m文件中写下一个C方法kcFunc()，在main函数打下断点，然后在ViewController.m文件中写下一个+load()：
1.2程序入口探究
运行程序，来到断点处，点击左侧的堆栈信息查看，在
main之前还有两个start，点击查看发现是汇编 libdyld.dylib start ：
1.3查看程序运行堆栈信息
或者利用
lldb 命令bt 也可查看程序的堆栈信息：
1.4查看程序运行堆栈信息
两种方式查看堆栈信息可知，在main函数调用之前libdyld.dylib 中start就已经被调用，所以说libdyld.dylib的start 才是程序的第一步，那么libdyld.dylib 又是啥呢？
dylib 源码分析
dyld简介
dyld (the dynamic link editor) 是苹果的动态链接器，是苹果操作系统一个重要组成部分，在系统内核做好程序准备工作之后，交由dyld负责余下的工作。而且它是开源的，任何人可以通过苹果官网下载它的源码来阅读理解它的运作方式，了解系统加载动态库的细节。
dyld源码下载地址 笔者下载的是750.6版本
在已知程序入口函数main()之前的libdyld.dylib start之后，我们打开源码在dyldStartup.s文件中找到__dyld_start:
__dyld_start:
popl %edx # edx = mh of app
pushl $0 # push a zero for debugger end of frames marker
movl %esp,%ebp # pointer to base of kernel frame
andl $-16,%esp # force SSE alignment
subl $32,%esp # room for locals and outgoing parameters
call L__dyld_start_picbase
L__dyld_start_picbase:
popl %ebx # set %ebx to runtime value of picbase
// app_mh MachOLoaded* appsMachHeader
// argc 和 argv main函数带进来的
// dyld_mh MachOLoaded* dyldsMachHeader
# call dyldbootstrap::start(app_mh, argc, argv, dyld_mh, &startGlue)
subl $L__dyld_start_picbase-__dyld_start, %ebx # ebx = &__dyld_start
subl $0x1000, %ebx # ebx = load address of dyld
movl %edx,(%esp) # param1 = app_mh
movl 4(%ebp),%eax
movl %eax,4(%esp) # param2 = argc
lea 8(%ebp),%eax
movl %eax,8(%esp) # param3 = argv
movl %ebx,12(%esp) # param4 = dyld load address
lea 28(%esp),%eax
movl %eax,16(%esp) # param5 = &startGlue
call __ZN13dyldbootstrap5startEPKN5dyld311MachOLoadedEiPPKcS3_Pm
movl 28(%esp),%edx
cmpl $0,%edx
jne Lnew
# clean up stack and jump to "start" in main executable
movl %ebp,%esp # restore the unaligned stack pointer
addl $4,%esp # remove debugger end frame marker
movl $0,%ebp # restore ebp back to zero
jmp *%eax # jump to the entry point
# LC_MAIN case, set up stack for call to main()
Lnew: movl 4(%ebp),%ebx
movl %ebx,(%esp) # main param1 = argc
leal 8(%ebp),%ecx
movl %ecx,4(%esp) # main param2 = argv
leal 0x4(%ecx,%ebx,4),%ebx
movl %ebx,8(%esp) # main param3 = env
Lapple: movl (%ebx),%ecx # look for NULL ending env[] array
add $4,%ebx
testl %ecx,%ecx
jne Lapple # once found, next pointer is "apple" parameter now in %ebx
movl %ebx,12(%esp) # main param4 = apple
pushl %edx # simulate return address into _start in libdyld
jmp *%eax # jump to main(argc,argv,env,apple) with return address set to _start
能看到注释里面有call dyldbootstrap::start方法(我们可以全局搜索start(const)，我们可以跟着注释方法进去一探究竟：
uintptr_t start(const dyld3::MachOLoaded* appsMachHeader, int argc, const char* argv[],
const dyld3::MachOLoaded* dyldsMachHeader, uintptr_t* startGlue)
{
// Emit kdebug tracepoint to indicate dyld bootstrap has started
dyld3::kdebug_trace_dyld_marker(DBG_DYLD_TIMING_BOOTSTRAP_START, 0, 0, 0, 0);
// if kernel had to slide dyld, we need to fix up load sensitive locations
// we have to do this before using any global variables
// dyld 重定位
rebaseDyld(dyldsMachHeader);
// kernel sets up env pointer to be just past end of agv array
const char** envp = &argv[argc+1];
// kernel sets up apple pointer to be just past end of envp array
const char** apple = envp;
while(*apple != NULL) { ++apple; }
++apple;
// set up random value for stack canary
// 栈溢出保护
__guard_setup(apple);
#if DYLD_INITIALIZER_SUPPORT // 0
// run all C++ initializers inside dyld
// 初始化dyld
runDyldInitializers(argc, argv, envp, apple);
#endif
// now that we are done bootstrapping dyld, call dyld's main
uintptr_t appsSlide = appsMachHeader->getSlide();
return dyld::_main((macho_header*)appsMachHeader, appsSlide, argc, argv, envp, apple, startGlue);
}
start方法主要做了很多dyld的初始化工作：
-
rebaseDylddyld重定位 -
__guard_setup栈溢出保护
在结束dyld初始化工作后，函数调用 dyld::_main() 函数，再将返回值传递给__dyld_start去调用真正的main函数。
uintptr_t
_main(const macho_header* mainExecutableMH, uintptr_t mainExecutableSlide,
int argc, const char* argv[], const char* envp[], const char* apple[],
uintptr_t* startGlue)
{
if (dyld3::kdebug_trace_dyld_enabled(DBG_DYLD_TIMING_LAUNCH_EXECUTABLE)) {
launchTraceID = dyld3::kdebug_trace_dyld_duration_start(DBG_DYLD_TIMING_LAUNCH_EXECUTABLE, (uint64_t)mainExecutableMH, 0, 0);
}
//Check and see if there are any kernel flags
dyld3::BootArgs::setFlags(hexToUInt64(_simple_getenv(apple, "dyld_flags"), nullptr));
// Grab the cdHash of the main executable from the environment
// 第一步：设置运行环境
uint8_t mainExecutableCDHashBuffer[20];
const uint8_t* mainExecutableCDHash = nullptr;
if ( hexToBytes(_simple_getenv(apple, "executable_cdhash"), 40, mainExecutableCDHashBuffer) )
// 获取主程序的hash
mainExecutableCDHash = mainExecutableCDHashBuffer;
#if !TARGET_OS_SIMULATOR
// Trace dyld's load
notifyKernelAboutImage((macho_header*)&__dso_handle, _simple_getenv(apple, "dyld_file"));
// Trace the main executable's load
notifyKernelAboutImage(mainExecutableMH, _simple_getenv(apple, "executable_file"));
#endif
uintptr_t result = 0;
// 获取主程序的macho_header结构
sMainExecutableMachHeader = mainExecutableMH;
// 获取主程序的slide值
sMainExecutableSlide = mainExecutableSlide;
// Set the platform ID in the all image infos so debuggers can tell the process type
// FIXME: This can all be removed once we make the kernel handle it in rdar://43369446
if (gProcessInfo->version >= 16) {
__block bool platformFound = false;
((dyld3::MachOFile*)mainExecutableMH)->forEachSupportedPlatform(^(dyld3::Platform platform, uint32_t minOS, uint32_t sdk) {
if (platformFound) {
halt("MH_EXECUTE binaries may only specify one platform");
}
gProcessInfo->platform = (uint32_t)platform;
platformFound = true;
});
if (gProcessInfo->platform == (uint32_t)dyld3::Platform::unknown) {
// There were no platforms found in the binary. This may occur on macOS for alternate toolchains and old binaries.
// It should never occur on any of our embedded platforms.
#if __MAC_OS_X_VERSION_MIN_REQUIRED
gProcessInfo->platform = (uint32_t)dyld3::Platform::macOS;
#else
halt("MH_EXECUTE binaries must specify a minimum supported OS version");
#endif
}
}
#if __MAC_OS_X_VERSION_MIN_REQUIRED
// Check to see if we need to override the platform
const char* forcedPlatform = _simple_getenv(envp, "DYLD_FORCE_PLATFORM");
if (forcedPlatform) {
if (strncmp(forcedPlatform, "6", 1) != 0) {
halt("DYLD_FORCE_PLATFORM is only supported for platform 6");
}
const dyld3::MachOFile* mf = (dyld3::MachOFile*)sMainExecutableMachHeader;
if (mf->allowsAlternatePlatform()) {
gProcessInfo->platform = PLATFORM_IOSMAC;
}
}
// if this is host dyld, check to see if iOS simulator is being run
const char* rootPath = _simple_getenv(envp, "DYLD_ROOT_PATH");
if ( (rootPath != NULL) ) {
// look to see if simulator has its own dyld
char simDyldPath[PATH_MAX];
strlcpy(simDyldPath, rootPath, PATH_MAX);
strlcat(simDyldPath, "/usr/lib/dyld_sim", PATH_MAX);
int fd = my_open(simDyldPath, O_RDONLY, 0);
if ( fd != -1 ) {
const char* errMessage = useSimulatorDyld(fd, mainExecutableMH, simDyldPath, argc, argv, envp, apple, startGlue, &result);
if ( errMessage != NULL )
halt(errMessage);
return result;
}
}
else {
((dyld3::MachOFile*)mainExecutableMH)->forEachSupportedPlatform(^(dyld3::Platform platform, uint32_t minOS, uint32_t sdk) {
if ( dyld3::MachOFile::isSimulatorPlatform(platform) )
halt("attempt to run simulator program outside simulator (DYLD_ROOT_PATH not set)");
});
}
#endif
CRSetCrashLogMessage("dyld: launch started");
// 设置上下文信息
setContext(mainExecutableMH, argc, argv, envp, apple);
// Pickup the pointer to the exec path.
// 获取主程序路径
sExecPath = _simple_getenv(apple, "executable_path");
// Remove interim apple[0] transition code from dyld
if (!sExecPath) sExecPath = apple[0];
#if __IPHONE_OS_VERSION_MIN_REQUIRED && !TARGET_OS_SIMULATOR
// kernel is not passing a real path for main executable
if ( strncmp(sExecPath, "/var/containers/Bundle/Application/", 35) == 0 ) {
if ( char* newPath = (char*)malloc(strlen(sExecPath)+10) ) {
strcpy(newPath, "/private");
strcat(newPath, sExecPath);
sExecPath = newPath;
}
}
#endif
if ( sExecPath[0] != '/' ) {
// have relative path, use cwd to make absolute
char cwdbuff[MAXPATHLEN];
if ( getcwd(cwdbuff, MAXPATHLEN) != NULL ) {
// maybe use static buffer to avoid calling malloc so early...
char* s = new char[strlen(cwdbuff) + strlen(sExecPath) + 2];
strcpy(s, cwdbuff);
strcat(s, "/");
strcat(s, sExecPath);
sExecPath = s;
}
}
// Remember short name of process for later logging
// 进程的名字
sExecShortName = ::strrchr(sExecPath, '/');
if ( sExecShortName != NULL )
++sExecShortName;
else
sExecShortName = sExecPath;
// 进程的头环境配置
configureProcessRestrictions(mainExecutableMH, envp);
// Check if we should force dyld3. Note we have to do this outside of the regular env parsing due to AMFI
if ( dyld3::internalInstall() ) {
if (const char* useClosures = _simple_getenv(envp, "DYLD_USE_CLOSURES")) {
if ( strcmp(useClosures, "0") == 0 ) {
sClosureMode = ClosureMode::Off;
} else if ( strcmp(useClosures, "1") == 0 ) {
#if __MAC_OS_X_VERSION_MIN_REQUIRED
#if __i386__
// don't support dyld3 for 32-bit macOS
#else
// Also don't support dyld3 for iOSMac right now
if ( gProcessInfo->platform != PLATFORM_IOSMAC ) {
sClosureMode = ClosureMode::On;
}
#endif // __i386__
#else
sClosureMode = ClosureMode::On;
#endif // __MAC_OS_X_VERSION_MIN_REQUIRED
} else {
dyld::warn("unknown option to DYLD_USE_CLOSURES. Valid options are: 0 and 1\n");
}
}
}
#if __MAC_OS_X_VERSION_MIN_REQUIRED
if ( !gLinkContext.allowEnvVarsPrint && !gLinkContext.allowEnvVarsPath && !gLinkContext.allowEnvVarsSharedCache ) {
pruneEnvironmentVariables(envp, &apple);
// set again because envp and apple may have changed or moved
setContext(mainExecutableMH, argc, argv, envp, apple);
}
else
#endif
{
// 检测环境变量
checkEnvironmentVariables(envp);
defaultUninitializedFallbackPaths(envp);
}
#if __MAC_OS_X_VERSION_MIN_REQUIRED
if ( gProcessInfo->platform == PLATFORM_IOSMAC ) {
gLinkContext.rootPaths = parseColonList("/System/iOSSupport", NULL);
gLinkContext.iOSonMac = true;
if ( sEnv.DYLD_FALLBACK_LIBRARY_PATH == sLibraryFallbackPaths )
sEnv.DYLD_FALLBACK_LIBRARY_PATH = sRestrictedLibraryFallbackPaths;
if ( sEnv.DYLD_FALLBACK_FRAMEWORK_PATH == sFrameworkFallbackPaths )
sEnv.DYLD_FALLBACK_FRAMEWORK_PATH = sRestrictedFrameworkFallbackPaths;
}
else if ( ((dyld3::MachOFile*)mainExecutableMH)->supportsPlatform(dyld3::Platform::driverKit) ) {
gLinkContext.driverKit = true;
gLinkContext.sharedRegionMode = ImageLoader::kDontUseSharedRegion;
}
#endif
if ( sEnv.DYLD_PRINT_OPTS )
printOptions(argv);
if ( sEnv.DYLD_PRINT_ENV )
printEnvironmentVariables(envp);
// Parse this envirionment variable outside of the regular logic as we want to accept
// this on binaries without an entitelment
#if !TARGET_OS_SIMULATOR
if ( _simple_getenv(envp, "DYLD_JUST_BUILD_CLOSURE") != nullptr ) {
#if TARGET_OS_IPHONE
const char* tempDir = getTempDir(envp);
if ( (tempDir != nullptr) && (geteuid() != 0) ) {
// Use realpath to prevent something like TMPRIR=/tmp/../usr/bin
char realPath[PATH_MAX];
if ( realpath(tempDir, realPath) != NULL )
tempDir = realPath;
if (strncmp(tempDir, "/private/var/mobile/Containers/", strlen("/private/var/mobile/Containers/")) == 0) {
sJustBuildClosure = true;
}
}
#endif
// If we didn't like the format of TMPDIR, just exit. We don't want to launch the app as that would bring up the UI
if (!sJustBuildClosure) {
_exit(EXIT_SUCCESS);
}
}
#endif
if ( sJustBuildClosure )
sClosureMode = ClosureMode::On;
// 获取主机信息 可理解为 程序结构
getHostInfo(mainExecutableMH, mainExecutableSlide);
//--- 第一步结束
// load shared cache
// 第二步：加载共享缓存
// 检查缓存共享区域是否开启，iOS必须开启
checkSharedRegionDisable((dyld3::MachOLoaded*)mainExecutableMH, mainExecutableSlide);
if ( gLinkContext.sharedRegionMode != ImageLoader::kDontUseSharedRegion ) {
#if TARGET_OS_SIMULATOR
if ( sSharedCacheOverrideDir)
mapSharedCache();
#else
// 共享缓存加载
mapSharedCache();
#endif
}
// If we haven't got a closure mode yet, then check the environment and cache type
if ( sClosureMode == ClosureMode::Unset ) {
// First test to see if we forced in dyld2 via a kernel boot-arg
if ( dyld3::BootArgs::forceDyld2() ) {
sClosureMode = ClosureMode::Off;
} else if ( inDenyList(sExecPath) ) {
sClosureMode = ClosureMode::Off;
} else if ( sEnv.hasOverride ) {
sClosureMode = ClosureMode::Off;
} else if ( dyld3::BootArgs::forceDyld3() ) {
sClosureMode = ClosureMode::On;
} else {
sClosureMode = getPlatformDefaultClosureMode();
}
}
#if !TARGET_OS_SIMULATOR
if ( sClosureMode == ClosureMode::Off ) {
if ( gLinkContext.verboseWarnings )
dyld::log("dyld: not using closure because of DYLD_USE_CLOSURES or -force_dyld2=1 override\n");
} else {
const dyld3::closure::LaunchClosure* mainClosure = nullptr;
dyld3::closure::LoadedFileInfo mainFileInfo;
mainFileInfo.fileContent = mainExecutableMH;
mainFileInfo.path = sExecPath;
// FIXME: If we are saving this closure, this slice offset/length is probably wrong in the case of FAT files.
mainFileInfo.sliceOffset = 0;
mainFileInfo.sliceLen = -1;
struct stat mainExeStatBuf;
if ( ::stat(sExecPath, &mainExeStatBuf) == 0 ) {
mainFileInfo.inode = mainExeStatBuf.st_ino;
mainFileInfo.mtime = mainExeStatBuf.st_mtime;
}
// check for closure in cache first
if ( sSharedCacheLoadInfo.loadAddress != nullptr ) {
mainClosure = sSharedCacheLoadInfo.loadAddress->findClosure(sExecPath);
if ( gLinkContext.verboseWarnings && (mainClosure != nullptr) )
dyld::log("dyld: found closure %p (size=%lu) in dyld shared cache\n", mainClosure, mainClosure->size());
}
// We only want to try build a closure at runtime if its an iOS third party binary, or a macOS binary from the shared cache
bool allowClosureRebuilds = false;
if ( sClosureMode == ClosureMode::On ) {
allowClosureRebuilds = true;
} else if ( (sClosureMode == ClosureMode::PreBuiltOnly) && (mainClosure != nullptr) ) {
allowClosureRebuilds = true;
}
if ( (mainClosure != nullptr) && !closureValid(mainClosure, mainFileInfo, mainExecutableCDHash, true, envp) )
mainClosure = nullptr;
// If we didn't find a valid cache closure then try build a new one
if ( (mainClosure == nullptr) && allowClosureRebuilds ) {
// if forcing closures, and no closure in cache, or it is invalid, check for cached closure
if ( !sForceInvalidSharedCacheClosureFormat )
mainClosure = findCachedLaunchClosure(mainExecutableCDHash, mainFileInfo, envp);
if ( mainClosure == nullptr ) {
// if no cached closure found, build new one
mainClosure = buildLaunchClosure(mainExecutableCDHash, mainFileInfo, envp);
}
}
// exit dyld after closure is built, without running program
if ( sJustBuildClosure )
_exit(EXIT_SUCCESS);
// try using launch closure
if ( mainClosure != nullptr ) {
CRSetCrashLogMessage("dyld3: launch started");
bool launched = launchWithClosure(mainClosure, sSharedCacheLoadInfo.loadAddress, (dyld3::MachOLoaded*)mainExecutableMH,
mainExecutableSlide, argc, argv, envp, apple, &result, startGlue);
if ( !launched && allowClosureRebuilds ) {
// closure is out of date, build new one
mainClosure = buildLaunchClosure(mainExecutableCDHash, mainFileInfo, envp);
if ( mainClosure != nullptr ) {
launched = launchWithClosure(mainClosure, sSharedCacheLoadInfo.loadAddress, (dyld3::MachOLoaded*)mainExecutableMH,
mainExecutableSlide, argc, argv, envp, apple, &result, startGlue);
}
}
if ( launched ) {
gLinkContext.startedInitializingMainExecutable = true;
#if __has_feature(ptrauth_calls)
// start() calls the result pointer as a function pointer so we need to sign it.
result = (uintptr_t)__builtin_ptrauth_sign_unauthenticated((void*)result, 0, 0);
#endif
if (sSkipMain)
result = (uintptr_t)&fake_main;
return result;
}
else {
if ( gLinkContext.verboseWarnings ) {
dyld::log("dyld: unable to use closure %p\n", mainClosure);
}
}
}
}
#endif // TARGET_OS_SIMULATOR
// could not use closure info, launch old way
// install gdb notifier
stateToHandlers(dyld_image_state_dependents_mapped, sBatchHandlers)->push_back(notifyGDB);
stateToHandlers(dyld_image_state_mapped, sSingleHandlers)->push_back(updateAllImages);
// make initial allocations large enough that it is unlikely to need to be re-alloced
sImageRoots.reserve(16);
sAddImageCallbacks.reserve(4);
sRemoveImageCallbacks.reserve(4);
sAddLoadImageCallbacks.reserve(4);
sImageFilesNeedingTermination.reserve(16);
sImageFilesNeedingDOFUnregistration.reserve(8);
#if !TARGET_OS_SIMULATOR
#ifdef WAIT_FOR_SYSTEM_ORDER_HANDSHAKE
// Add gating mechanism to dyld support system order file generation process
WAIT_FOR_SYSTEM_ORDER_HANDSHAKE(dyld::gProcessInfo->systemOrderFlag);
#endif
#endif
try {
// add dyld itself to UUID list
addDyldImageToUUIDList();
#if SUPPORT_ACCELERATE_TABLES
#if __arm64e__
// Disable accelerator tables when we have threaded rebase/bind, which is arm64e executables only for now.
if (sMainExecutableMachHeader->cpusubtype == CPU_SUBTYPE_ARM64E)
sDisableAcceleratorTables = true;
#endif
bool mainExcutableAlreadyRebased = false;
if ( (sSharedCacheLoadInfo.loadAddress != nullptr) && !dylibsCanOverrideCache() && !sDisableAcceleratorTables && (sSharedCacheLoadInfo.loadAddress->header.accelerateInfoAddr != 0) ) {
struct stat statBuf;
if ( ::stat(IPHONE_DYLD_SHARED_CACHE_DIR "no-dyld2-accelerator-tables", &statBuf) != 0 )
sAllCacheImagesProxy = ImageLoaderMegaDylib::makeImageLoaderMegaDylib(&sSharedCacheLoadInfo.loadAddress->header, sSharedCacheLoadInfo.slide, mainExecutableMH, gLinkContext);
}
reloadAllImages:
#endif
#if __MAC_OS_X_VERSION_MIN_REQUIRED
gLinkContext.strictMachORequired = false;
// be less strict about old macOS mach-o binaries
((dyld3::MachOFile*)mainExecutableMH)->forEachSupportedPlatform(^(dyld3::Platform platform, uint32_t minOS, uint32_t sdk) {
if ( (platform == dyld3::Platform::macOS) && (sdk >= DYLD_PACKED_VERSION(10,15,0)) ) {
gLinkContext.strictMachORequired = true;
}
});
if ( gLinkContext.iOSonMac )
gLinkContext.strictMachORequired = true;
#else
// simulators, iOS, tvOS, watchOS, are always strict
gLinkContext.strictMachORequired = true;
#endif
CRSetCrashLogMessage(sLoadingCrashMessage);
// instantiate ImageLoader for main executable
// 第三步：实例化主程序
sMainExecutable = instantiateFromLoadedImage(mainExecutableMH, mainExecutableSlide, sExecPath);
gLinkContext.mainExecutable = sMainExecutable;
gLinkContext.mainExecutableCodeSigned = hasCodeSignatureLoadCommand(mainExecutableMH);
#if TARGET_OS_SIMULATOR
// check main executable is not too new for this OS
{
if ( ! isSimulatorBinary((uint8_t*)mainExecutableMH, sExecPath) ) {
throwf("program was built for a platform that is not supported by this runtime");
}
uint32_t mainMinOS = sMainExecutable->minOSVersion();
// dyld is always built for the current OS, so we can get the current OS version
// from the load command in dyld itself.
uint32_t dyldMinOS = ImageLoaderMachO::minOSVersion((const mach_header*)&__dso_handle);
if ( mainMinOS > dyldMinOS ) {
#if TARGET_OS_WATCH
throwf("app was built for watchOS %d.%d which is newer than this simulator %d.%d",
mainMinOS >> 16, ((mainMinOS >> 8) & 0xFF),
dyldMinOS >> 16, ((dyldMinOS >> 8) & 0xFF));
#elif TARGET_OS_TV
throwf("app was built for tvOS %d.%d which is newer than this simulator %d.%d",
mainMinOS >> 16, ((mainMinOS >> 8) & 0xFF),
dyldMinOS >> 16, ((dyldMinOS >> 8) & 0xFF));
#else
throwf("app was built for iOS %d.%d which is newer than this simulator %d.%d",
mainMinOS >> 16, ((mainMinOS >> 8) & 0xFF),
dyldMinOS >> 16, ((dyldMinOS >> 8) & 0xFF));
#endif
}
}
#endif
#if SUPPORT_ACCELERATE_TABLES
sAllImages.reserve((sAllCacheImagesProxy != NULL) ? 16 : INITIAL_IMAGE_COUNT);
#else
sAllImages.reserve(INITIAL_IMAGE_COUNT);
#endif
// Now that shared cache is loaded, setup an versioned dylib overrides
#if SUPPORT_VERSIONED_PATHS
checkVersionedPaths();
#endif
// dyld_all_image_infos image list does not contain dyld
// add it as dyldPath field in dyld_all_image_infos
// for simulator, dyld_sim is in image list, need host dyld added
#if TARGET_OS_SIMULATOR
// get path of host dyld from table of syscall vectors in host dyld
void* addressInDyld = gSyscallHelpers;
#else
// get path of dyld itself
void* addressInDyld = (void*)&__dso_handle;
#endif
char dyldPathBuffer[MAXPATHLEN+1];
int len = proc_regionfilename(getpid(), (uint64_t)(long)addressInDyld, dyldPathBuffer, MAXPATHLEN);
if ( len > 0 ) {
dyldPathBuffer[len] = '\0'; // proc_regionfilename() does not zero terminate returned string
if ( strcmp(dyldPathBuffer, gProcessInfo->dyldPath) != 0 )
gProcessInfo->dyldPath = strdup(dyldPathBuffer);
}
// load any inserted libraries
// 第四步：加载插入的动态库
if ( sEnv.DYLD_INSERT_LIBRARIES != NULL ) {
for (const char* const* lib = sEnv.DYLD_INSERT_LIBRARIES; *lib != NULL; ++lib)
loadInsertedDylib(*lib);
}
// record count of inserted libraries so that a flat search will look at
// inserted libraries, then main, then others.
// 记录插入的动态库的数量
sInsertedDylibCount = sAllImages.size()-1;
// link main executable
gLinkContext.linkingMainExecutable = true;
#if SUPPORT_ACCELERATE_TABLES
if ( mainExcutableAlreadyRebased ) {
// previous link() on main executable has already adjusted its internal pointers for ASLR
// work around that by rebasing by inverse amount
sMainExecutable->rebase(gLinkContext, -mainExecutableSlide);
}
#endif
// 第五步：链接主程序 Executable：可执行的意思
link(sMainExecutable, sEnv.DYLD_BIND_AT_LAUNCH, true, ImageLoader::RPathChain(NULL, NULL), -1);
sMainExecutable->setNeverUnloadRecursive();
if ( sMainExecutable->forceFlat() ) {
gLinkContext.bindFlat = true;
gLinkContext.prebindUsage = ImageLoader::kUseNoPrebinding;
}
// 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
// 第六步：链接插入的动态库
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 ) {
// only INSERTED libraries can interpose
// register interposing info after all inserted libraries are bound so chaining works
for(unsigned int i=0; i < sInsertedDylibCount; ++i) {
ImageLoader* image = sAllImages[i+1];
image->registerInterposing(gLinkContext);
}
}
}
if ( gLinkContext.allowInterposing ) {
// dyld should support interposition even without DYLD_INSERT_LIBRARIES
for (long i=sInsertedDylibCount+1; i < sAllImages.size(); ++i) {
ImageLoader* image = sAllImages[i];
if ( image->inSharedCache() )
continue;
image->registerInterposing(gLinkContext);
}
}
#if g
if ( (sAllCacheImagesProxy != NULL) && ImageLoader::haveInterposingTuples() ) {
// Accelerator tables cannot be used with implicit interposing, so relaunch with accelerator tables disabled
ImageLoader::clearInterposingTuples();
// unmap all loaded dylibs (but not main executable)
for (long i=1; i < sAllImages.size(); ++i) {
ImageLoader* image = sAllImages[i];
if ( image == sMainExecutable )
continue;
if ( image == sAllCacheImagesProxy )
continue;
image->setCanUnload();
ImageLoader::deleteImage(image);
}
// note: we don't need to worry about inserted images because if DYLD_INSERT_LIBRARIES was set we would not be using the accelerator table
sAllImages.clear();
sImageRoots.clear();
sImageFilesNeedingTermination.clear();
sImageFilesNeedingDOFUnregistration.clear();
sAddImageCallbacks.clear();
sRemoveImageCallbacks.clear();
sAddLoadImageCallbacks.clear();
sAddBulkLoadImageCallbacks.clear();
sDisableAcceleratorTables = true;
sAllCacheImagesProxy = NULL;
sMappedRangesStart = NULL;
mainExcutableAlreadyRebased = true;
gLinkContext.linkingMainExecutable = false;
resetAllImages();
goto reloadAllImages;
}
#endif
// apply interposing to initial set of images
for(int i=0; i < sImageRoots.size(); ++i) {
sImageRoots[i]->applyInterposing(gLinkContext);
}
ImageLoader::applyInterposingToDyldCache(gLinkContext);
// Bind and notify for the main executable now that interposing has been registered
uint64_t bindMainExecutableStartTime = mach_absolute_time();
sMainExecutable->recursiveBindWithAccounting(gLinkContext, sEnv.DYLD_BIND_AT_LAUNCH, true);
uint64_t bindMainExecutableEndTime = mach_absolute_time();
ImageLoaderMachO::fgTotalBindTime += bindMainExecutableEndTime - bindMainExecutableStartTime;
gLinkContext.notifyBatch(dyld_image_state_bound, false);
// Bind and notify for the inserted images now interposing has been registered
if ( sInsertedDylibCount > 0 ) {
for(unsigned int i=0; i < sInsertedDylibCount; ++i) {
ImageLoader* image = sAllImages[i+1];
image->recursiveBind(gLinkContext, sEnv.DYLD_BIND_AT_LAUNCH, true);
}
}
// do weak binding only after all inserted images linked
// 第七步：弱引用绑定主程序
sMainExecutable->weakBind(gLinkContext);
gLinkContext.linkingMainExecutable = false;
sMainExecutable->recursiveMakeDataReadOnly(gLinkContext);
CRSetCrashLogMessage("dyld: launch, running initializers");
#if SUPPORT_OLD_CRT_INITIALIZATION
// Old way is to run initializers via a callback from crt1.o
if ( ! gRunInitializersOldWay )
initializeMainExecutable();
#else
// run all initializers
// 第八步：执行初始化方法
initializeMainExecutable();
#endif
// notify any montoring proccesses that this process is about to enter main()
// 第九步：查找程序入口函数main并返回
notifyMonitoringDyldMain();
if (dyld3::kdebug_trace_dyld_enabled(DBG_DYLD_TIMING_LAUNCH_EXECUTABLE)) {
dyld3::kdebug_trace_dyld_duration_end(launchTraceID, DBG_DYLD_TIMING_LAUNCH_EXECUTABLE, 0, 0, 2);
}
ARIADNEDBG_CODE(220, 1);
#if __MAC_OS_X_VERSION_MIN_REQUIRED
if ( gLinkContext.driverKit ) {
result = (uintptr_t)sEntryOveride;
if ( result == 0 )
halt("no entry point registered");
*startGlue = (uintptr_t)gLibSystemHelpers->startGlueToCallExit;
}
else
#endif
{
// find entry point for main executable
result = (uintptr_t)sMainExecutable->getEntryFromLC_MAIN();
if ( result != 0 ) {
// main executable uses LC_MAIN, we need to use helper in libdyld to call into main()
if ( (gLibSystemHelpers != NULL) && (gLibSystemHelpers->version >= 9) )
*startGlue = (uintptr_t)gLibSystemHelpers->startGlueToCallExit;
else
halt("libdyld.dylib support not present for LC_MAIN");
}
else {
// main executable uses LC_UNIXTHREAD, dyld needs to let "start" in program set up for main()
result = (uintptr_t)sMainExecutable->getEntryFromLC_UNIXTHREAD();
*startGlue = 0;
}
}
#if __has_feature(ptrauth_calls)
// start() calls the result pointer as a function pointer so we need to sign it.
result = (uintptr_t)__builtin_ptrauth_sign_unauthenticated((void*)result, 0, 0);
#endif
}
catch(const char* message) {
syncAllImages();
halt(message);
}
catch(...) {
dyld::log("dyld: launch failed\n");
}
CRSetCrashLogMessage("dyld2 mode");
#if !TARGET_OS_SIMULATOR
if (sLogClosureFailure) {
// We failed to launch in dyld3, but dyld2 can handle it. synthesize a crash report for analytics
dyld3::syntheticBacktrace("Could not generate launchClosure, falling back to dyld2", true);
}
#endif
if (sSkipMain) {
notifyMonitoringDyldMain();
if (dyld3::kdebug_trace_dyld_enabled(DBG_DYLD_TIMING_LAUNCH_EXECUTABLE)) {
dyld3::kdebug_trace_dyld_duration_end(launchTraceID, DBG_DYLD_TIMING_LAUNCH_EXECUTABLE, 0, 0, 2);
}
ARIADNEDBG_CODE(220, 1);
result = (uintptr_t)&fake_main;
*startGlue = (uintptr_t)gLibSystemHelpers->startGlueToCallExit;
}
return result;
}
这代码也太长了。。。代码规范呢？哪去了？
dyld流程可总结为九个步骤：
- 第一步：
设置运行环境 - 第二步：
加载共享缓存 - 第三步：
实例化主程序 - 第四步：
加载插入的动态库 - 第五步：
链接主程序 - 第六步：
链接插入的动态库 - 第七步：
执行弱引用绑定 - 第八步：
执行初始化方法 - 第九步：
查找程序入口main然后返回
第一步：设置运行环境
这一步主要是设置程序的运行环境，运行条件等准备工作，包括环境，平台，版本，路径，主机信息，设置程序上下文信息等
// 获取主程序的hash
mainExecutableCDHash = mainExecutableCDHashBuffer;
// 获取主程序的macho_header结构
sMainExecutableMachHeader = mainExecutableMH;
// 获取主程序的slide值
sMainExecutableSlide = mainExecutableSlide;
// 设置上下文信息
setContext(mainExecutableMH, argc, argv, envp, apple);
// 获取主程序路径
sExecPath = _simple_getenv(apple, "executable_path");
// 进程的头环境配置
configureProcessRestrictions(mainExecutableMH, envp);
// 检测环境变量
checkEnvironmentVariables(envp);
defaultUninitializedFallbackPaths(envp);
// 获取主机信息 可理解为 程序结构
getHostInfo(mainExecutableMH, mainExecutableSlide);
第二步：加载共享缓存
首先检查dyld共享缓存区是否禁用，iOS必须开启，在checkSharedRegionDisable里面iOS环境下注释：
// iOS cannot run without shared region
// 检查缓存共享区域是否开启
checkSharedRegionDisable((dyld3::MachOLoaded*)mainExecutableMH, mainExecutableSlide);
// 共享缓存加载
mapSharedCache();
// mapSharedCache 函数
static void mapSharedCache()
{
dyld3::SharedCacheOptions opts;
opts.cacheDirOverride = sSharedCacheOverrideDir;
opts.forcePrivate = (gLinkContext.sharedRegionMode == ImageLoader::kUsePrivateSharedRegion);
#if __x86_64__ && !TARGET_OS_SIMULATOR
opts.useHaswell = sHaswell;
#else
opts.useHaswell = false;
#endif
opts.verbose = gLinkContext.verboseMapping;
//--- 加载dyld缓存主函数
loadDyldCache(opts, &sSharedCacheLoadInfo);
// update global state
if ( sSharedCacheLoadInfo.loadAddress != nullptr ) {
gLinkContext.dyldCache = sSharedCacheLoadInfo.loadAddress;
dyld::gProcessInfo->processDetachedFromSharedRegion = opts.forcePrivate;
dyld::gProcessInfo->sharedCacheSlide = sSharedCacheLoadInfo.slide;
dyld::gProcessInfo->sharedCacheBaseAddress = (unsigned long)sSharedCacheLoadInfo.loadAddress;
sSharedCacheLoadInfo.loadAddress->getUUID(dyld::gProcessInfo->sharedCacheUUID);
dyld3::kdebug_trace_dyld_image(DBG_DYLD_UUID_SHARED_CACHE_A, sSharedCacheLoadInfo.path, (const uuid_t *)&dyld::gProcessInfo->sharedCacheUUID[0], {0,0}, {{ 0, 0 }}, (const mach_header *)sSharedCacheLoadInfo.loadAddress);
}
}
// loadDyldCache函数
bool loadDyldCache(const SharedCacheOptions& options, SharedCacheLoadInfo* results)
{
results->loadAddress = 0;
results->slide = 0;
results->errorMessage = nullptr;
#if TARGET_OS_SIMULATOR
// simulator only supports mmap()ing cache privately into process
return mapCachePrivate(options, results);
#else
if ( options.forcePrivate ) {
// mmap cache into this process only
return mapCachePrivate(options, results);
}
else {
// fast path: when cache is already mapped into shared region
bool hasError = false;
if ( reuseExistingCache(options, results) ) {
hasError = (results->errorMessage != nullptr);
} else {
// slow path: this is first process to load cache
hasError = mapCacheSystemWide(options, results);
}
return hasError;
}
#endif
}
mapSharedCache函数主要调用loadDyldCache 函数，loadDyldCache函数主要有三种方式加载共享缓存：
-
mapCachePrivate()仅加载到当前进程 - 共享缓存已经加载过，不做任何处理
-
mapCacheSystemWide()未加载过，首次加载
第三步：实例化主程序
这一步主要是将主程序的Mach-O加载进内存，并实例化一个ImageLoader。instantiateFromLoadedImage() 首先调用isCompatibleMachO() 函数检测当前进程的magic、cputype、cpusubtype 等相关属性，判断Mach-O文件的兼容性，如果兼容性满足，就调用ImageLoaderMachO::instantiateMainExecutable()实例化主程序的ImageLoader：
// 第三步：实例化主程序
sMainExecutable = instantiateFromLoadedImage(mainExecutableMH, mainExecutableSlide, sExecPath);
gLinkContext.mainExecutable = sMainExecutable;
gLinkContext.mainExecutableCodeSigned = hasCodeSignatureLoadCommand(mainExecutableMH);
// The kernel maps in main executable before dyld gets control. We need to
// make an ImageLoader* for the already mapped in main executable.
static ImageLoaderMachO* instantiateFromLoadedImage(const macho_header* mh, uintptr_t slide, const char* path)
{
// try mach-o loader
if ( isCompatibleMachO((const uint8_t*)mh, path) ) {
ImageLoader* image = ImageLoaderMachO::instantiateMainExecutable(mh, slide, path, gLinkContext);
addImage(image);
return (ImageLoaderMachO*)image;
}
throw "main executable not a known format";
}
// create image for main executable
ImageLoader* ImageLoaderMachO::instantiateMainExecutable(const macho_header* mh, uintptr_t slide, const char* path, const LinkContext& context)
{
//dyld::log("ImageLoader=%ld, ImageLoaderMachO=%ld, ImageLoaderMachOClassic=%ld, ImageLoaderMachOCompressed=%ld\n",
// sizeof(ImageLoader), sizeof(ImageLoaderMachO), sizeof(ImageLoaderMachOClassic), sizeof(ImageLoaderMachOCompressed));
bool compressed;
unsigned int segCount;
unsigned int libCount;
const linkedit_data_command* codeSigCmd;
const encryption_info_command* encryptCmd;
sniffLoadCommands(mh, path, false, &compressed, &segCount, &libCount, context, &codeSigCmd, &encryptCmd);
// instantiate concrete class based on content of load commands
if ( compressed )
return ImageLoaderMachOCompressed::instantiateMainExecutable(mh, slide, path, segCount, libCount, context);
else
#if SUPPORT_CLASSIC_MACHO
return ImageLoaderMachOClassic::instantiateMainExecutable(mh, slide, path, segCount, libCount, context);
#else
throw "missing LC_DYLD_INFO load command";
#endif
}
instantiateMainExecutable() 函数中通过sniffLoadCommands() 判断这个mach-o文件是普通的还是压缩的LINKEDIT，以及它有多少段。
最后根据compressed 是否压缩来实例化最后返回的ImageLoader。
第四步：加载插入的动态库
// 第四步：加载插入的动态库
if ( sEnv.DYLD_INSERT_LIBRARIES != NULL ) {
for (const char* const* lib = sEnv.DYLD_INSERT_LIBRARIES; *lib != NULL; ++lib)
loadInsertedDylib(*lib);
}
loadInsertedDylib() 函数中设置了一个LoadContext，并为它配置一些参数后，调用load() 方法：
ImageLoader* load(const char* path, const LoadContext& context, unsigned& cacheIndex)
{
......
// try all path permutations and check against existing loaded images
ImageLoader* image = loadPhase0(path, orgPath, context, cacheIndex, NULL);
if ( image != NULL ) {
CRSetCrashLogMessage2(NULL);
return image;
}
// try all path permutations and try open() until first success
std::vector exceptions;
image = loadPhase0(path, orgPath, context, cacheIndex, &exceptions);
#if !TARGET_OS_SIMULATOR
// support symlinks on disk to a path in dyld shared cache
if ( image == NULL)
image = loadPhase2cache(path, orgPath, context, cacheIndex, &exceptions);
#endif
CRSetCrashLogMessage2(NULL);
if ( image != NULL ) {
// leak in dyld during dlopen when using DYLD_ variables
for (std::vector::iterator it = exceptions.begin(); it != exceptions.end(); ++it) {
free((void*)(*it));
}
// if loaded image is not from cache, but original path is in cache
// set gSharedCacheOverridden flag to disable some ObjC optimizations
if ( !gSharedCacheOverridden && !image->inSharedCache() && image->isDylib() && dyld3::MachOFile::isSharedCacheEligiblePath(path) && inSharedCache(path) ) {
gSharedCacheOverridden = true;
}
return image;
}
......
}
第五步：链接主程序
// 第五步：链接主程序 Executable：可执行的意思
link(sMainExecutable, sEnv.DYLD_BIND_AT_LAUNCH,
true, ImageLoader::RPathChain(NULL, NULL), -1);
调用link()函数将实例化后的主程序进行动态修正，让二进制变为可正常执行的状态。link()函数内部调用了ImgaeLoader::link() 函数，主要做了下面几件事：
- this->
recursiveLoadLibraries()递归加载依赖库进内存 - this->
recursiveUpdateDepth()递归更新依赖库的路径 - this->
recursiveRebaseWithAccounting()递归重定位主程序和依赖库 - this->
recursiveBindWithAccounting()递归将主程序和依赖库执行符号表绑定（链接动态库使用） - this->
weakBind()如果不是正在链接主程序二进制，那就主程序弱符号绑定（链接动态库使用） - this->
recursiveApplyInterposing()递归申请可插入依赖库权限 - this->
recursiveMakeDataReadOnly()递归设置所有信息只读（链接动态库使用） - this->
recursiveGetDOFSections()注册DOF节
代码如下所示：
void ImageLoader::link(const LinkContext& context, bool forceLazysBound, bool preflightOnly, bool neverUnload, const RPathChain& loaderRPaths, const char* imagePath)
{
// clear error strings
(*context.setErrorStrings)(0, NULL, NULL, NULL);
uint64_t t0 = mach_absolute_time();
// 递归加载依赖库进内存
this->recursiveLoadLibraries(context, preflightOnly, loaderRPaths, imagePath);
context.notifyBatch(dyld_image_state_dependents_mapped, preflightOnly);
// we only do the loading step for preflights
if ( preflightOnly )
return;
uint64_t t1 = mach_absolute_time();
// 递归更新依赖库的路径
context.clearAllDepths();
this->recursiveUpdateDepth(context.imageCount());
__block uint64_t t2, t3, t4, t5;
{
dyld3::ScopedTimer(DBG_DYLD_TIMING_APPLY_FIXUPS, 0, 0, 0);
t2 = mach_absolute_time();
// 递归重定位主程序和依赖库
this->recursiveRebaseWithAccounting(context);
context.notifyBatch(dyld_image_state_rebased, false);
t3 = mach_absolute_time();
if ( !context.linkingMainExecutable )
// 递归将主程序和依赖库执行符号表绑定
this->recursiveBindWithAccounting(context, forceLazysBound, neverUnload);
t4 = mach_absolute_time();
if ( !context.linkingMainExecutable )
// 如果不是正在链接主程序二进制，那就主程序弱符号绑定
this->weakBind(context);
t5 = mach_absolute_time();
}
// interpose any dynamically loaded images
if ( !context.linkingMainExecutable && (fgInterposingTuples.size() != 0) ) {
dyld3::ScopedTimer timer(DBG_DYLD_TIMING_APPLY_INTERPOSING, 0, 0, 0);
// 递归申请可插入依赖库权限
this->recursiveApplyInterposing(context);
}
// now that all fixups are done, make __DATA_CONST segments read-only
if ( !context.linkingMainExecutable )
// 递归设置所有信息只读
this->recursiveMakeDataReadOnly(context);
if ( !context.linkingMainExecutable )
context.notifyBatch(dyld_image_state_bound, false);
uint64_t t6 = mach_absolute_time();
if ( context.registerDOFs != NULL ) {
std::vector dofs;
// 注册DOF节
this->recursiveGetDOFSections(context, dofs);
context.registerDOFs(dofs);
}
uint64_t t7 = mach_absolute_time();
// clear error strings
(*context.setErrorStrings)(0, NULL, NULL, NULL);
fgTotalLoadLibrariesTime += t1 - t0;
fgTotalRebaseTime += t3 - t2;
fgTotalBindTime += t4 - t3;
fgTotalWeakBindTime += t5 - t4;
fgTotalDOF += t7 - t6;
// done with initial dylib loads
fgNextPIEDylibAddress = 0;
}
第六步：链接插入的动态库
这一步跟链接主程序一样，将sAllImages中的ImageLoader遍历出来，然后调用link()进行链接，需要注意的是，sAllImages中保存的第一个是主程序的镜像，所以要获取所有的动态库的ImageLoader，就必须从i + 1 开始遍历：
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();
}
第七步：执行主程序弱符号绑定
weakBind()首先通过getCoalescedImages()合并所有动态库的弱符号到一个列表里，然后调用initializeCoalIterator()对需要绑定的弱符号进行排序，接着调用incrementCoalIterator()读取dyld_info_command结构的weak_bind_off和weak_bind_size字段，确定弱符号的数据偏移与大小，最终进行弱符号绑定
代码略，太多了。。。
第八步：执行初始化方法
这一步就开始进行初始化工作了，initializeMainExecutable() 函数中调用 runInitializers() 函数，接着依次调用 processInitializers() 函数进行一些初始化准备工作，接着recursiveInitialization() 函数调用进行初始化工作，接着全局搜索recursiveInitialization(，找到ImageLoader.cpp中的此方法定义，看重点，函数里面我们看到noffitySingle()单个通知注入：
void ImageLoader::recursiveInitialization(const LinkContext& context, mach_port_t this_thread, const char* pathToInitialize,
InitializerTimingList& timingInfo, UninitedUpwards& uninitUps)
{
recursive_lock lock_info(this_thread);
recursiveSpinLock(lock_info);
if ( fState < dyld_image_state_dependents_initialized-1 ) {
uint8_t oldState = fState;
// break cycles
fState = dyld_image_state_dependents_initialized-1;
try {
// initialize lower level libraries first
// 先初始化底层依赖库
for(unsigned int i=0; i < libraryCount(); ++i) {
ImageLoader* dependentImage = libImage(i);
if ( dependentImage != NULL ) {
// don't try to initialize stuff "above" me yet
if ( libIsUpward(i) ) {
uninitUps.imagesAndPaths[uninitUps.count] = { dependentImage, libPath(i) };
uninitUps.count++;
}
else if ( dependentImage->fDepth >= fDepth ) {
dependentImage->recursiveInitialization(context, this_thread, libPath(i), timingInfo, uninitUps);
}
}
}
// record termination order
// 记录终端序号
if ( this->needsTermination() )
context.terminationRecorder(this);
// let objc know we are about to initialize this image
// 单个通知注入 通知告知大家我要开始初始化啦，你们赶紧去做初始化的工作
uint64_t t1 = mach_absolute_time();
fState = dyld_image_state_dependents_initialized;
oldState = fState;
context.notifySingle(dyld_image_state_dependents_initialized, this, &timingInfo);
// initialize this image
bool hasInitializers = this->doInitialization(context);
// let anyone know we finished initializing this image
fState = dyld_image_state_initialized;
oldState = fState;
context.notifySingle(dyld_image_state_initialized, this, NULL);
if ( hasInitializers ) {
uint64_t t2 = mach_absolute_time();
timingInfo.addTime(this->getShortName(), t2-t1);
}
}
catch (const char* msg) {
// this image is not initialized
fState = oldState;
recursiveSpinUnLock();
throw;
}
}
recursiveSpinUnLock();
}
我们跟踪notifySingle方法进来，我们发现下面一段代码：
if ( (state == dyld_image_state_dependents_initialized) && (sNotifyObjCInit != NULL) && image->notifyObjC() ) {
uint64_t t0 = mach_absolute_time();
dyld3::ScopedTimer timer(DBG_DYLD_TIMING_OBJC_INIT, (uint64_t)image->machHeader(), 0, 0);
(*sNotifyObjCInit)(image->getRealPath(), image->machHeader());
uint64_t t1 = mach_absolute_time();
uint64_t t2 = mach_absolute_time();
uint64_t timeInObjC = t1-t0;
uint64_t emptyTime = (t2-t1)*100;
if ( (timeInObjC > emptyTime) && (timingInfo != NULL) ) {
timingInfo->addTime(image->getShortName(), timeInObjC);
}
}
sNotifyObjCInit关键词，搜索查找此关键词,发现一个赋值的地方：
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;
.....
}
发现在registerObjCNotifiers 方法中对sNotifyObjCInit 进行了赋值操作，registerObjCNotifiers 可以理解为注册objc通知，继续查找调用registerObjCNotifiers()的地方：
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);
}
那么问题来了，继续查找下去，dyld代码全局搜索找不到_dyld_objc_notify_register() 函数调用的方法了。。。那咋办？
我们不妨利用工程及断点来查找，_dyld_objc_notify_register() 下断点继续观察一下，在xcode 新建 Symbolic Breakpoint.. 符号断点:
_dyld_objc_notify_register调用探索
可以看到，在
_dyld_objc_notify_register方法调用前，_objc_init方法调用了，我们不妨试着看看是不是在_objc_init里面有调用_dyld_objc_notify_register方法：
_objc_init 在objc源码里面，所以我们在先前获取的objc4 - 781找到_objc_init源码如下：
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();
cache_init();
_imp_implementationWithBlock_init();
_dyld_objc_notify_register(&map_images, load_images, unmap_image);
#if __OBJC2__
didCallDyldNotifyRegister = true;
#endif
}
的确，在_objc_init 方法中调用了_dyld_objc_notify_register() 函数。可以看出此处注册的就是load_images()，回调函数里调用了call_load_methods()来执行所有的+load方法:
void
load_images(const char *path __unused, const struct mach_header *mh)
{
if (!didInitialAttachCategories && didCallDyldNotifyRegister) {
didInitialAttachCategories = true;
// 加载所有的分类信息
loadAllCategories();
}
// Return without taking locks if there are no +load methods here.
if (!hasLoadMethods((const headerType *)mh)) return;
recursive_mutex_locker_t lock(loadMethodLock);
// Discover load methods
{
mutex_locker_t lock2(runtimeLock);
prepare_load_methods((const headerType *)mh);
}
// Call +load methods (without runtimeLock - re-entrant)
// 调用所有的load方法
call_load_methods();
}
现在回到文章刚开始的例子处，我们在ViewController.m 文件中的+load() 方法中打下断点来查看调用堆栈信息：
调用堆栈信息展示
从堆栈信息可以看出，从下往上，因为我用的是模拟器，所以会有start_sim 方法的调用。