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// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file or at
#import "GPBRootObject_PackagePrivate.h"
#import <objc/runtime.h>
#import <os/lock.h>
#import <CoreFoundation/CoreFoundation.h>
#import "GPBDescriptor.h"
#import "GPBExtensionRegistry.h"
#import "GPBUtilities_PackagePrivate.h"
@interface GPBExtensionDescriptor (GPBRootObject)
// Get singletonName as a c string.
- (const char *)singletonNameC;
// We need some object to conform to the MessageSignatureProtocol to make sure
// the selectors in it are recorded in our Objective C runtime information.
// GPBMessage is arguably the more "obvious" choice, but given that all messages
// inherit from GPBMessage, conflicts seem likely, so we are using GPBRootObject
// instead.
@interface GPBRootObject () <GPBMessageSignatureProtocol>
@implementation GPBRootObject
// Taken from
// Public Domain
static uint32_t jenkins_one_at_a_time_hash(const char *key) {
uint32_t hash = 0;
for (uint32_t i = 0; key[i] != '\0'; ++i) {
hash += key[i];
hash += (hash << 10);
hash ^= (hash >> 6);
hash += (hash << 3);
hash ^= (hash >> 11);
hash += (hash << 15);
return hash;
// Key methods for our custom CFDictionary.
// Note that the dictionary lasts for the lifetime of our app, so no need
// to worry about deallocation. All of the items are added to it at
// startup, and so the keys don't need to be retained/released.
// Keys are NULL terminated char *.
static const void *GPBRootExtensionKeyRetain(__unused CFAllocatorRef allocator, const void *value) {
return value;
static void GPBRootExtensionKeyRelease(__unused CFAllocatorRef allocator,
__unused const void *value) {}
static CFStringRef GPBRootExtensionCopyKeyDescription(const void *value) {
const char *key = (const char *)value;
return CFStringCreateWithCString(kCFAllocatorDefault, key, kCFStringEncodingUTF8);
static Boolean GPBRootExtensionKeyEqual(const void *value1, const void *value2) {
const char *key1 = (const char *)value1;
const char *key2 = (const char *)value2;
return strcmp(key1, key2) == 0;
static CFHashCode GPBRootExtensionKeyHash(const void *value) {
const char *key = (const char *)value;
return jenkins_one_at_a_time_hash(key);
// Long ago, this was an OSSpinLock, but then it came to light that there were issues for that on
// iOS:
// It was changed to a dispatch_semaphore_t, but that has potential for priority inversion issues.
// The minOS versions are now high enough that os_unfair_lock can be used, and should provide
// all the support we need. For more information in the concurrency/locking space see:
static os_unfair_lock gExtensionSingletonDictionaryLock = OS_UNFAIR_LOCK_INIT;
static CFMutableDictionaryRef gExtensionSingletonDictionary = NULL;
static GPBExtensionRegistry *gDefaultExtensionRegistry = NULL;
+ (void)initialize {
// Ensure the global is started up.
if (!gExtensionSingletonDictionary) {
CFDictionaryKeyCallBacks keyCallBacks = {
// See description above for reason for using custom dictionary.
gExtensionSingletonDictionary = CFDictionaryCreateMutable(kCFAllocatorDefault, 0, &keyCallBacks,
gDefaultExtensionRegistry = [[GPBExtensionRegistry alloc] init];
if ([self superclass] == [GPBRootObject class]) {
// This is here to start up all the per file "Root" subclasses.
// This must be done in initialize to enforce thread safety of start up of
// the protocol buffer library.
[self extensionRegistry];
+ (GPBExtensionRegistry *)extensionRegistry {
// Is overridden in all the subclasses that provide extensions to provide the
// per class one.
return gDefaultExtensionRegistry;
+ (void)globallyRegisterExtension:(GPBExtensionDescriptor *)field {
const char *key = [field singletonNameC];
CFDictionarySetValue(gExtensionSingletonDictionary, key, field);
static id ExtensionForName(id self, SEL _cmd) {
// Really fast way of doing "classname_selName".
// This came up as a hotspot (creation of NSString *) when accessing a
// lot of extensions.
const char *selName = sel_getName(_cmd);
if (selName[0] == '_') {
return nil; // Apple internal selector.
size_t selNameLen = 0;
while (1) {
char c = selName[selNameLen];
if (c == '\0') { // String end.
if (c == ':') {
return nil; // Selector took an arg, not one of the runtime methods.
const char *className = class_getName(self);
size_t classNameLen = strlen(className);
char key[classNameLen + selNameLen + 2];
memcpy(key, className, classNameLen);
key[classNameLen] = '_';
memcpy(&key[classNameLen + 1], selName, selNameLen);
key[classNameLen + 1 + selNameLen] = '\0';
// NOTE: Even though this method is called from another C function,
// gExtensionSingletonDictionaryLock and gExtensionSingletonDictionary
// will always be initialized. This is because this call flow is just to
// lookup the Extension, meaning the code is calling an Extension class
// message on a Message or Root class. This guarantees that the class was
// initialized and Message classes ensure their Root was also initialized.
NSAssert(gExtensionSingletonDictionary, @"Startup order broken!");
id extension = (id)CFDictionaryGetValue(gExtensionSingletonDictionary, key);
// We can't remove the key from the dictionary here (as an optimization),
// two threads could have gone into +resolveClassMethod: for the same method,
// and ended up here; there's no way to ensure both return YES without letting
// both try to wire in the method.
return extension;
BOOL GPBResolveExtensionClassMethod(Class self, SEL sel) {
// Another option would be to register the extensions with the class at
// globallyRegisterExtension:
// Timing the two solutions, this solution turned out to be much faster
// and reduced startup time, and runtime memory.
// The advantage to globallyRegisterExtension is that it would reduce the
// size of the protos somewhat because the singletonNameC wouldn't need
// to include the class name. For a class with a lot of extensions it
// can add up. You could also significantly reduce the code complexity of this
// file.
id extension = ExtensionForName(self, sel);
if (extension != nil) {
const char *encoding = GPBMessageEncodingForSelector(@selector(getClassValue), NO);
Class metaClass = objc_getMetaClass(class_getName(self));
IMP imp = imp_implementationWithBlock(^(__unused id obj) {
return extension;
BOOL methodAdded = class_addMethod(metaClass, sel, imp, encoding);
// class_addMethod() is documented as also failing if the method was already
// added; so we check if the method is already there and return success so
// the method dispatch will still happen. Why would it already be added?
// Two threads could cause the same method to be bound at the same time,
// but only one will actually bind it; the other still needs to return true
// so things will dispatch.
if (!methodAdded) {
methodAdded = GPBClassHasSel(metaClass, sel);
return methodAdded;
return NO;
+ (BOOL)resolveClassMethod:(SEL)sel {
if (GPBResolveExtensionClassMethod(self, sel)) {
return YES;
return [super resolveClassMethod:sel];