关于ios:探究-iOS-内存问题

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本文从 Tagged Pointer、objc 源码、dealloc 原理、AutoreleasePool 原理、野指针探索等技术点开展聊了聊 iOS 内存相干问题。

定时器内存透露

NSTimer、CADisplayLink 的 根底 API [NSTimer scheduledTimersWithTimeInterval:1 repeat:YES block:nil] 和以后的 VC 都会相互持有，造成环，会存在内存透露问题。

定时器内存透露起因，解决方案以及高精度定时器，具体能够看这篇 NSTimer 中的内存泄露。

iOS 内存布局

栈、堆、BSS、数据段、代码段

栈（stack）：又称作堆栈，用来存储程序的局部变量（但不包含 static 申明的变量，static 润饰的数据寄存于数据段中）。除此之外，在函数被调用时，栈用来传递参数和返回值。栈内存地址越来越少

func a {
    变量 1 地址最大
    变量 2 地址第二大
    // ...
    变量 n  地址最小
}

堆（heap）：用于存储程序运行中被动态分配的内存段，它的大小并不固定，可动静的扩张和缩减。操作函数(malloc/free)。调配的内存空间地址越来越大。

BSS 段（bss segment）：通常用来存储程序中未被初始化的全局变量和动态变量的一块内存区域。BSS 是英文 Block Started by Symbol 的简称。BSS 段输出动态内存调配

数据段（data segment）：通常用来存储程序中已被初始化的全局变量和动态变量和字符串的一块内存区域。数据段蕴含 3 局部：

  • 字符串常量。比方 NSString *str = @"杭城小刘";
  • 已初始化数据：曾经初始化的全局变量、动态变量等
  • 未初始化数据：未初始化的全局变量、动态变量等

代码段（code segment）：编译之后的代码。通常是指用来存储程序可执行代码的一块内存区域。这部分区域的大小在程序运行前就曾经确定，并且内存区域通常属于只读，某些架构也容许代码段为可写，即容许批改程序。

上 Demo 验证

int a = 10;
static int b;
int main () {
    NSString *name = @"杭城小刘";
    int age = 27;
    int height = 177;
    NSObject *obj = [[NSObject alloc] init];
    NSLog(@"\na: %p\nb: %p\n name: %p\nage: %p\n height: %p\nobj:%p", &a, &b, &name, &age, &height, obj);
}
a: 0x107b09b80
b: 0x107b09c48
name: 0x7ff7b83fdbc0
age: 0x7ff7b83fdbbc
height: 0x7ff7b83fdbb8
obj:0x6000012780e0

咱们依照内存地址由低到高排个序(如下)，发现和咱们总结的法则统一。

// 字符串常量
name:   0x7ff7b83fdbc0
// 已初始化的全局变量、动态变量
a:      0x107b09b80
// 未初始化的全局变量、动态变量
b:      0x107b09c48
// 堆
obj:    0x6000012780e0
// 栈
height: 0x7ff7b83fdbb8
age:    0x7ff7b83fdbbc
NSObject *obj = [[NSObject alloc] init];
NSLog(@"%p %p %@", obj, &obj, obj);

别离打印 obj 指针指向的堆上的内存地址、obj 指针在栈上的地址、obj 内容

Tagged Pointer

先来一个 Demo 开启本局部内容（画外音：代码很短，但让我产生了一个大大的问号）

- (bool)isTaggedPointer:(const void *)ptr
{return ((uintptr_t)ptr & (1UL<<63)) == (1UL<<63);
}

NSNumber *number = [NSNumber numberWithInt:10]; // 0xb0000000000000a2 b:12  1100
NSLog(@"%p %d %@", number, [self isTaggedPointer:(__bridge const void *)number], number.class);

NSString *name1 = [NSString stringWithFormat:@"ss"]; // 0xa000000000073732 a:11 1011
NSLog(@"%p %d %@", name1, [self isTaggedPointer:(__bridge const void *)name1], name1.class);

前提阐明：

创立一个 NSNumer 类型的变量 number，NSString 类型的 name1，代码打印地址、类型。产生一个问题：为什么 NSNumber 是 TaggedPointer，然而 class 却显示 __NSCFNumber？

static inline bool _objc_isTaggedPointer(const void * _Nullable ptr) 
{return ((uintptr_t)ptr & _OBJC_TAG_MASK) == _OBJC_TAG_MASK;
}
  • 通过 objc4 源码钻研写了个判断对象是否是 Tagged Pointer 类型的办法。通过零碎源码参考写了判断办法 isTaggedPointer。调用办法失去 number 对象是 Tagged Pointer 类型
  • 依据 iOS 平台个性，依据内存地址高位剖析的确是 TaggedPointer 类型 ​
  • 同样的 NSString 指针指向的字符串内容比拟少，占用内存没必要开翻新的内存时，name1 就是 NSTaggedPointerString，打印出 class 也是 NSTaggedPointerString。调用 isTaggedPointer 失去也是 Tageed Pointer 类型

带着问题开始吧

什么是 Tagged Pointer

iOS 从 64bit 开始引入了 Tagged Pointer 技术，用于优化 NSNumber、NSDate、NSString 等小对象的存储。

在此之前，创建对象须要动静分配内存、保护援用计数等，对象指针存储的是堆中对象的地址值创立一个对象的流程。先在堆上申请一块内存，而后再在栈上减少一个指针类型，指针指向堆上这块内存。如果是 NSNumber *value = [NSNumber numberWithInt:2] value 是指针长度为 8 字节，堆上内存 16 字节。加起来 24 字节就存一个 int 2。

此外还须要保护援用技术，沿用一个真正对象那一套，太大材小用了。

Tagged Pointer 格局，对象指针外面存储的数据变成了：Tag + Data，将数据间接存储在了指针中。当指针不够存储数据时，才会应用动静分配内存的形式来存储数据

objc_msgSend 能辨认 Tagged Pointer，比方 NSNumber 的 intValue 办法，间接从指针提取数据，节俭了调用开销。

经典问题

Demo1

- (void)test {dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    for (NSInteger i = 0; i<1000; i++) {
        dispatch_async(queue, ^{self.name = [NSString stringWithFormat:@"和好多好多好多好多事看看上课上课上课"];
        });
    }
}

运行该代码会 Crash，报错信息如下

阐明：一开始的报错信息只说坏内存拜访，然而并没有显示具体的办法调用堆。想晓得具体 Crash 起因还是须要看看堆栈比拟不便。输出 bt 查看最初是因为 objc_release 办法造成 crash。

小窍门：利用 LLDB 模式下输出 bt，能够查看堆栈。也就是 backtrace 的缩写。

不认真想可能发现不了问题，看到 objc_release 就会想到是在多线程状况下 NSString 的 setter 办法内，ARC 代码通过编译器最初会依照 MRC 去运行。所以 Setter 相似上面代码。

-(void)setName:(NSString *)name 
{if (_name!=name) {[name retain];
        [_name release];
        _name = name;
    }
}

Demo

- (void)test {dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    for (NSInteger i = 0; i<1000; i++) {
        dispatch_async(queue, ^{self.name = [NSString stringWithFormat:@"ss"];
            if (i == 100) {NSLog(@"%p %@", self.name, self.name.class);
            }
        });
    }
} 
// 0xa000000000073732 NSTaggedPointerString

同样的代码字符串变短竟然不 crash 了？因为命中 Tagged Pointer 逻辑了，查看类型是 NSTaggedPointerString

本问题实质是

  • ARC 代码在编译后真正运行阶段是走 MRC 的，strong、copy 外部都会 release 旧的，copy/retain 新的
  • 多线程状况下拜访 setter 须要加锁
  • 字符串在 NSTaggedPointerString 状况下不存在像 OC 对象的 setter 办法内的 release、copy 操作

如何判断一个指针是否为 Tagged Pointer

查看 objc4 源码

#if TARGET_OS_OSX && __x86_64__
    // 64-bit Mac - tag bit is LSB
#   define OBJC_MSB_TAGGED_POINTERS 0
#else
    // Everything else - tag bit is MSB
#   define OBJC_MSB_TAGGED_POINTERS 1
#endif

#if OBJC_MSB_TAGGED_POINTERS
#   define _OBJC_TAG_MASK (1UL<<63)
#   define _OBJC_TAG_INDEX_SHIFT 60
#   define _OBJC_TAG_SLOT_SHIFT 60
#   define _OBJC_TAG_PAYLOAD_LSHIFT 4
#   define _OBJC_TAG_PAYLOAD_RSHIFT 4
#   define _OBJC_TAG_EXT_MASK (0xfUL<<60)
#   define _OBJC_TAG_EXT_INDEX_SHIFT 52
#   define _OBJC_TAG_EXT_SLOT_SHIFT 52
#   define _OBJC_TAG_EXT_PAYLOAD_LSHIFT 12
#   define _OBJC_TAG_EXT_PAYLOAD_RSHIFT 12
#else
#   define _OBJC_TAG_MASK 1UL
#   define _OBJC_TAG_INDEX_SHIFT 1
#   define _OBJC_TAG_SLOT_SHIFT 0
#   define _OBJC_TAG_PAYLOAD_LSHIFT 0
#   define _OBJC_TAG_PAYLOAD_RSHIFT 4
#   define _OBJC_TAG_EXT_MASK 0xfUL
#   define _O  BJC_TAG_EXT_INDEX_SHIFT 4
#   define _OBJC_TAG_EXT_SLOT_SHIFT 4
#   define _OBJC_TAG_EXT_PAYLOAD_LSHIFT 0
#   define _OBJC_TAG_EXT_PAYLOAD_RSHIFT 12
#endif

static inline bool _objc_isTaggedPointer(const void * _Nullable ptr) 
{return ((uintptr_t)ptr & _OBJC_TAG_MASK) == _OBJC_TAG_MASK;
}

能够看到源码通过 _objc_isTaggedPointer 办法判断是否是 Tagged Pointer 类型。传入对象地址，外部通过 _OBJC_TAG_MASK 按位与运算。

其中 _OBJC_TAG_MASK 是一个宏，宏定义内部有个 if 判读，判断是 Mac OS 并且是 x86_64 架构则为 0，否则为 1。也就是 Mac OS 并且是 x86_64 架构状况下则与 1UL 按位与，否则与 1UL<<63 按位与。

  • iOS 平台 | Mac 非 x86 平台：最高无效位是 1（第 64bit）1UL<<63
  • Mac 且 x86 平台：最低无效位是 1 1UL

比方 iOS 平台下

0xb0000000000000a2 b:12  1100

  1100
& 1000
-------
  1000

tips：某些对象尽管是 TaggedPointer 类型，然而打印 class 发现不是，猜想可能是零碎用类簇暗藏了某些实现细节。比方上面

针对 NSNumber 的 TaggedPoniter 的 case，查看 class 打印出 __NSCFNumber。但依据源码和内存高地址位剖析的确是 TaggedPoniter。

疑难是：为什么 NSNumber 的 TaggedPpinter case 下打印 class 是 __NSCFNumber。如果是类簇暗藏细节实现，为什么同样 KVO 也扭转了 isa，然而命名是一个新的名字，而不是类簇的实现？

和敌人探讨后有 2 种观点（观点不是独立的，而是并且同时成立的。对错难以断定，仅供参考）：

  • 类簇，为了暗藏细节实现
  • KVO 和以后 case 不统一。类簇是零碎类的设计，KVO 是针对开发者写的对象所以没有类簇，只能动静生成类，扭转原类的 isa，命名为 NSKVONotifying_*** 这样的规定。

类簇

类簇（Class Cluster）是形象工厂模式在 OC 数组中的实现，NSArray、NSNumber、NSString、NSDictionary 都有体现。借口简略性和拓展性的衡量体现。零碎暗藏了较多实现细节，只暴露出简略接口。

- (void)classCus
{id obj1 = [NSArray alloc]; // __NSPlaceholderArray
    id obj2 = [NSMutableArray alloc]; // __NSPlaceholderArray
    id obj3 = [obj1 init]; // __NSArray0
    id obj4 = [obj2 init]; // __NSArrayM
    NSLog(@"%@ %@ %@ %@", obj1, obj2, obj3, obj4);
}

调用 alloc 之后产生的是 __NSPlaceholderArray 不合乎预期。持续调用 init 发现满足冀望了。所以猜想 __NSPlaceholderArray  是一个两头对象，后续的 init 办法就是给两头对象发消息，再由它做工厂，生成真的对象，这里的 __NSArray0、__NSArrayM 对应 NSArray、NSMutableArray

Foundation 用了动态实例地址形式来实现，伪代码如下：

static __NSPlacehodlerArray *GetPlaceholderForNSArray() {
    static __NSPlacehodlerArray *instanceForNSArray;
    if (!instanceForNSArray) {instanceForNSArray = [[__NSPlacehodlerArray alloc] init];
    }
    return instanceForNSArray;
}

static __NSPlacehodlerArray *GetPlaceholderForNSMutableArray() {
    static __NSPlacehodlerArray *instanceForNSMutableArray;
    if (!instanceForNSMutableArray) {instanceForNSMutableArray = [[__NSPlacehodlerArray alloc] init];
    }
    return instanceForNSMutableArray;
}
// NSArray 实现
+ (id)alloc {if (self == [NSArray class]) {return GetPlaceholderForNSArray()
    }
}
// NSMutableArray 实现
+ (id)alloc {if (self == [NSMutableArray class]) {return GetPlaceholderForNSMutableArray()
    }
}
// __NSPlacehodlerArray 实现
- (id)init {if (self == GetPlaceholderForNSArray()) {self = [[__NSArrayI alloc] init];
    }
    else if (self == GetPlaceholderForNSMutableArray()) {self = [[__NSArrayM alloc] init];
    }
    return self;
}

另外 iOS Foundation 对动态不可变空对象（以后 case 为数组）做了优化

NSArray *a1 = [[NSArray alloc] init];
NSArray *a2 = [[NSArray alloc] init];
NSArray *a3 = [[NSArray alloc] init];
(lldb) p a1
(__NSArray0 *) $0 = 0x0000000109f50a10 @"0 elements"
(lldb) p a2
(__NSArray0 *) $1 = 0x0000000109f50a10 @"0 elements"
(lldb) p a3
(__NSArray0 *) $2 = 0x0000000109f50a10 @"0 elements"
(lldb)

若干个不可变的空数组间没有任何特异性，返回一个动态对象。

OC 对象内存治理

iOS 中应用援用计数来治理 OC 对象的内存。一个新创建的 OC 对象援用计数默认是 1，当援用计数减为 0，OC 对象就会销毁，开释其占用的内存空间

调用 retain/copy 会让 OC 对象的援用计数 +1，调用 release 会让 OC 对象的援用计数 -1。

内存治理的经验总结

  • 当调用 alloc、new、copy、mutableCopy 办法返回了一个对象，在不须要这个对象时，要调用 release 或者 autorelease 来开释它
  • 想领有某个对象，就让它的援用计数 +1；不想再领有某个对象，就让它的援用计数 -1
  • 能够通过以下公有函数来查看主动开释池的状况extern void _objc_autoreleasePoolPrint(void);

僵尸对象：反复开释内存造成的。一个典型场景是屡次 setter。setter 外部实现不合理，比方上面 setter。

Person *p = [[Person aloc] init]; // 1
Cat *cat = [[Cat alloc] init]; // 1  
[p setCat:cat];  // 2
[cat release]; // 1
[p setCat:cat]; // 0
[p setCat:cat]; // badAccess
- (void)setCat:(Cat *)cat
{[_cat release];
    _cat = [cat retain];
}

改良

- (void)setCat:(Cat *)cat
{if (_cat != cat) {[_cat release];
        _cat = [cat retain];
    }
}

晚期在 MRC 时代，在 .h 文件中 @property 只会属性的 getter、setter 申明，@synthesize 会主动生成成员变量和属性的 setter、getter 的实现。随着编译器提高，当初 @property 会做齐全部的事件。

晚期 VC 中应用属性

@property (nonatomic, strong) NSMutableDictionary *dict;

NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
self.dict = dict;
[dict release];

通过 Foundation 框架中类办法创立进去的对象，会主动调用 autorelease 办法。

简写为 self.dict = [NSMutableDictionary dictionary];

上述能够查看 GUNStep 源码  NSDictionary.m

#define    AUTORELEASE(object)    [(id)(object) autorelease]
+ (id) dictionary {return AUTORELEASE([[self allocWithZone: NSDefaultMallocZone()] init]);
}

QA：ARC 做了什么

ARC 其实是 LLVM + Runtime 独特作用的后果。LLVM 编译器主动插入 retain、release 内存治理代码。Runtime 运行时帮咱们解决相似 __weak 程序运行过程中弱援用革除掉。

copy/mutableCopy

OC 有 2 个拷贝办法

  • copy 不可变拷贝，产生新不可变对象
  • mutableCopy 可变拷贝，产生新可变对象

上个 Demo1

NSArray *array1 = [[NSArray alloc] initWithObjects:@"1", @"2", nil];
NSLog(@"array1 --- %zd", array1.retainCount);
NSArray *array2 = [array1 copy];
NSLog(@"array1 --- %zd", array1.retainCount);
NSLog(@"array2 --- %zd", array2.retainCount);
NSMutableArray *array3 = [array1 mutableCopy];
NSLog(@"array1 --- %zd", array1.retainCount);
NSLog(@"array2 --- %zd", array2.retainCount);
NSLog(@"array3 --- %zd" array3.retainCount);

[array3 release];
NSLog(@"array3 --- %zd", array3.retainCount);
[array2 release];
NSLog(@"array2 --- %zd", array2.retainCount);
NSLog(@"array1 --- %zd", array1.retainCount);
[array1 release];
NSLog(@"array1 --- %zd", array1.retainCount);
2022-04-12 20:50:43.639296+0800 Main[4408:60897] array1 --- 1
2022-04-12 20:50:43.639715+0800 Main[4408:60897] array1 --- 2
2022-04-12 20:50:43.639772+0800 Main[4408:60897] array2 --- 2
2022-04-12 20:50:43.639846+0800 Main[4408:60897] array1 --- 2
2022-04-12 20:50:43.639899+0800 Main[4408:60897] array2 --- 2
2022-04-12 20:50:43.639957+0800 Main[4408:60897] array3 --- 1
2022-04-12 20:50:43.640013+0800 Main[4408:60897] array3 --- 0
2022-04-12 20:50:43.640059+0800 Main[4408:60897] array2 --- 1
2022-04-12 20:50:43.640105+0800 Main[4408:60897] array1 --- 1
2022-04-12 20:50:43.640159+0800 Main[4408:60897] array1 --- 0

疑难 1: 为什么在 array2 创立之后 array2、array1 的援用技术都是 2.

因为 array1 指针指向堆上一块内存（NSArray 类型），创立好后 array1 援用计数为 1。在创立 array2 的时候发现是对 array1 的浅拷贝，零碎为了内存的节俭优化，array2 的指针也指向堆上的这一块内存，copy 自身会对 array1 援用技术 +1，变为 2。所以这时候 array2 指针指向的内存，援用计数也是 2.

基于此，咱们略微批改下，看看 Demo2

NSArray *array1 = [[NSArray alloc] initWithObjects:@"1", @"2", nil];
NSLog(@"array1 --- %zd", array1.retainCount);
NSArray *array2 = [array1 mutableCopy];
NSLog(@"array1 --- %zd", array1.retainCount);
NSLog(@"array2 --- %zd", array2.retainCount);

2022-04-12 20:55:36.539060+0800 Main[4576:65031] array1 --- 1
2022-04-12 20:55:36.539514+0800 Main[4576:65031] array1 --- 1
2022-04-12 20:55:36.539631+0800 Main[4576:65031] array2 --- 1

因为 array1 指针指向堆上一块内存（NSArray 类型），创立好后 array1 援用计数为 1。在创立 array2 的时候发现是对 array1 的深拷贝，要产生不可变对象，所以堆上申请内存空间，array2 指针指向这块内存，援用技术为 1。

此外 mutableCopy 是 Foundation 针对汇合类提供的。如果自定义对象须要反对 copy 办法，需遵循对应的NSCopyint 协定，实现协定办法 -(id)copyWithZone:(NSZone *)zone

总结：

NSString NSMutableString NSArray NSMutableArray NSDictionary NSMutableDictionary
copy NSString 浅拷贝 NSString 深拷贝 NSArray 浅拷贝 NSArray 深拷贝 NSDictionary 浅拷贝 NSDictionary 深拷贝
mutableCopy NSMutableString 深拷贝 NSMutableString 深拷贝 NSMutableArray 深拷贝 NSMutableArray 深拷贝 NSMutableDictionary 深拷贝 NSMutableDictionary 深拷贝

援用计数

union isa_t {
    Class cls;
    uintptr_t bits;
    struct {
        uintptr_t nonpointer        : 1;
        uintptr_t has_assoc         : 1;
        uintptr_t has_cxx_dtor      : 1;
        uintptr_t shiftcls          : 33; // MACH_VM_MAX_ADDRESS 0x1000000000
        uintptr_t magic             : 6;
        uintptr_t weakly_referenced : 1;
        uintptr_t deallocating      : 1;
        uintptr_t has_sidetable_rc  : 1;
        uintptr_t extra_rc          : 19;
    };
}

iOS 从 64 位开始开始，对 isa 进行了优化，信息寄存于 union 构造中

  • extra_rc 存储援用计数信息 -1，能够看到是 19 位。存储援用计数器 -1
  • has_sidetable_rc 援用计数是否过大无奈存储在 isa。当过大无奈存储与 isa 中时，has_sidetable_rc 这位会变为 1，援用计数存储在 SideTable 的类的属性中

也就是说，iOS 从 64 位开始，援用计数寄存于 isa 构造体的一个 union 中，字段为 extra_rc，值为对象援用计数值 -1。当援用计数过大无奈寄存的时候 union 中 has_sidetable_rc 为 1，则援用计数寄存于 SideTable 构造体中。

SideTable 构造如下

struct SideTable {
    spinlock_t slock;
    RefcountMap refcnts;
    weak_table_t weak_table;
};

其中 refcnts 是一个寄存着对象援用计数的散列表

查看 objc4 对于援用计数的实现

uintptr_t _objc_rootRetainCount(id obj) {assert(obj);
    return obj->rootRetainCount();}

inline uintptr_t objc_object::rootRetainCount() {if (isTaggedPointer()) return (uintptr_t)this;
    sidetable_lock();
    isa_t bits = LoadExclusive(&isa.bits);
    ClearExclusive(&isa.bits);
    if (bits.nonpointer) { // 优化过的 isa
        uintptr_t rc = 1 + bits.extra_rc; 
        if (bits.has_sidetable_rc) { // 援用计数不是存储在 isa 中，而是 SideTable 
            rc += sidetable_getExtraRC_nolock();}
        sidetable_unlock();
        return rc;
    }
    sidetable_unlock();
    return sidetable_retainCount();}

size_t  objc_object::sidetable_getExtraRC_nolock() {assert(isa.nonpointer);
    SideTable& table = SideTables()[this];
    RefcountMap::iterator it = table.refcnts.find(this); // key 拿值
    if (it == table.refcnts.end()) return 0;
    else return it->second >> SIDE_TABLE_RC_SHIFT;
}

__unsafe_unretained 不平安如何体现？上 Demo

__weak Person *p2;
__unsafe_unretained Person *p3;
{Person *p = [[Person alloc] init];
    p2 = p;
}
NSLog(@"%@", p2);
2022-04-12 21:39:30.308917+0800 Main[5307:98296] -[Person dealloc]
2022-04-12 21:39:30.309413+0800 Main[5307:98296] (null)

能够看到出了代码块，之后 p2 尽管指向 p，然而 p 没有强指针指向，所以回收了，此时打印 p2，是 null。

__unsafe_unretained Person *p3;
{Person *p = [[Person alloc] init];
    p3 = p;
}
NSLog(@"%@", p3);
2022-04-12 21:40:47.558581+0800 Main[5342:99598] -[Person dealloc]
2022-04-12 21:40:47.559330+0800 Main[5342:99598] <Person: 0x101206130>

当用 __unsafe_unretained 润饰后，尽管开释了，然而内存还没回收，这时候去应用很容易出错。

dealloc 是如何工作的？

在 MRC 时代，写完代码都须要显示在 dealloc 办法中做一些内存回收之类的工作。对象析构时将外部对象先 release 掉，非 OC 对象(比方定时器、c 对象、CF 对象等) 也须要回收内存，最初调用 [super dealloc] 持续将父类对象做析构。

- (void)dealloc {CFRelease(XX);
    self.timer = nil;
    [super dealloc];
}

但在 ARC 时代，dealloc 中个别只须要写一些非 OC 对象的内存开释工作，比方 CFRelease()

带来 2 个问题：

  • 类中的实例变量在哪开释？
  • 以后类中没有显示调用 [super dealloc]，父类的析构如何触发？

LLVM 文档对 dealloc 的形容

LLVM ARC 文档对 dealloc 形容 如下

A class may provide a method definition for an instance method named dealloc. This method will be called after the final release of the object but before it is deallocated or any of its instance variables are destroyed. The superclass’s implementation of dealloc will be called automatically when the method returns.

The instance variables for an ARC-compiled class will be destroyed at some point after control enters the dealloc method for the root class of the class. The ordering of the destruction of instance variables is unspecified, both within a single class and between subclasses and superclasses.

依据形容能够看到 dealloc 办法在最初一次 release 办法调用后触发，但实例变量（ivars）还未开释，父类的 dealloc 办法将会在子类 dealloc 办法返回后主动调用。

ARC 模式下，对象的实例变量会在根类 [NSObject dealloc] 中开释，然而开释的程序是不肯定的。

也就是说会主动调用 [super dealloc]，那到底如何实现的，探索下。

查看 objc4 源码

- (void)dealloc {_objc_rootDealloc(self);
}

void _objc_rootDealloc(id obj) {assert(obj);
    obj->rootDealloc();}

inline void objc_object::rootDealloc() {if (isTaggedPointer()) return;  // fixme necessary?
    // fastpath 判断以后对象是否满足条件。if (fastpath(isa.nonpointer  &&      // nonpointer
                 !isa.weakly_referenced  &&   // 是否有弱援用
                 !isa.has_assoc  &&   // 关联对象
                 !isa.has_cxx_dtor  &&   // c++ 析构函数
                 !isa.has_sidetable_rc)) // 是否有 SideTable
    {assert(!sidetable_present());
        free(this);
    }  else {object_dispose((id)this);
    }
}

id object_dispose(id obj){if (!obj) return nil;
    objc_destructInstance(obj);    
    free(obj);
    return nil;
}

void *objc_destructInstance(id obj) {if (obj) {
        // Read all of the flags at once for performance.
        bool cxx = obj->hasCxxDtor();
        bool assoc = obj->hasAssociatedObjects();

        // This order is important.
        if (cxx) object_cxxDestruct(obj); // 革除成员变量
        if (assoc) _object_remove_assocations(obj);
        obj->clearDeallocating(); // 将指向以后对象的弱指针置为 nil}
    return obj;
}

inline void objc_object::clearDeallocating() {if (slowpath(!isa.nonpointer)) {
        // Slow path for raw pointer isa.
        sidetable_clearDeallocating();}
    else if (slowpath(isa.weakly_referenced  ||  isa.has_sidetable_rc)) {
        // Slow path for non-pointer isa with weak refs and/or side table data.
        clearDeallocating_slow();}
    assert(!sidetable_present());
}
void objc_object::sidetable_clearDeallocating(){SideTable& table = SideTables()[this];
    // clear any weak table items
    // clear extra retain count and deallocating bit
    // (fixme warn or abort if extra retain count == 0 ?)
    table.lock();
    RefcountMap::iterator it = table.refcnts.find(this);
    if (it != table.refcnts.end()) {if (it->second & SIDE_TABLE_WEAKLY_REFERENCED) {weak_clear_no_lock(&table.weak_table, (id)this);
        }
        table.refcnts.erase(it);
    }
    table.unlock();}

void 
weak_clear_no_lock(weak_table_t *weak_table, id referent_id) 
{objc_object *referent = (objc_object *)referent_id;

    weak_entry_t *entry = weak_entry_for_referent(weak_table, referent);
    if (entry == nil) {
        /// XXX shouldn't happen, but does with mismatched CF/objc
        //printf("XXX no entry for clear deallocating %p\n", referent);
        return;
    }

    // zero out references
    weak_referrer_t *referrers;
    size_t count;

    if (entry->out_of_line()) {
        referrers = entry->referrers;
        count = TABLE_SIZE(entry);
    } 
    else {
        referrers = entry->inline_referrers;
        count = WEAK_INLINE_COUNT;
    }

    for (size_t i = 0; i < count; ++i) {objc_object **referrer = referrers[i];
        if (referrer) {if (*referrer == referent) {*referrer = nil;}
            else if (*referrer) {
                _objc_inform("__weak variable at %p holds %p instead of %p."
                             "This is probably incorrect use of"
                             "objc_storeWeak() and objc_loadWeak()."
                             "Break on objc_weak_error to debug.\n", 
                             referrer, (void*)*referrer, (void*)referent);
                objc_weak_error();}
        }
    }

    weak_entry_remove(weak_table, entry);
}

能够分明看到在 objc_destructInstance 办法中调用了 3 个外围办法

  • object_cxxDestruct(obj)：革除成员变量
  • object_remove_assocations(obj)：去除该对象相干的关联属性(Category 增加的)
  • obj->clearDeallocating()：清空援用技术表和弱援用表，将 weak 援用设置为 nil

持续看看 object_cxxDestruct 办法外部细节。

神秘的 cxx_destruct

object_cxxDestruct 办法最终会调用到 object_cxxDestructFromClass

void object_cxxDestruct(id obj) {if (_objc_isTaggedPointerOrNil(obj)) return;
    object_cxxDestructFromClass(obj, obj->ISA());
}

static void object_cxxDestructFromClass(id obj, Class cls) {void (*dtor)(id);
    // Call cls's dtor first, then superclasses's dtors.
    for (; cls; cls = cls->getSuperclass()) {if (!cls->hasCxxDtor()) return; 
        dtor = (void(*)(id))
            lookupMethodInClassAndLoadCache(cls, SEL_cxx_destruct);
        // 调用
        if (dtor != (void(*)(id))_objc_msgForward_impcache) {if (PrintCxxCtors) {
                _objc_inform("CXX: calling C++ destructors for class %s", 
                             cls->nameForLogging());
            }
            (*dtor)(obj);
        }
    }
}

做的事件就是遍历，一直寻找父类中 SEL_cxx_destruct这个 selector，找到函数实现并调用。

void sel_init(size_t selrefCount){
#if SUPPORT_PREOPT
    if (PrintPreopt) {_objc_inform("PREOPTIMIZATION: using dyld selector opt");
    }
#endif
  namedSelectors.init((unsigned)selrefCount);
    // Register selectors used by libobjc
    mutex_locker_t lock(selLock)
    SEL_cxx_construct = sel_registerNameNoLock(".cxx_construct", NO);
    SEL_cxx_destruct = sel_registerNameNoLock(".cxx_destruct", NO);
}

持续翻阅源码发现 SEL_cxx_destruct 其实就是 .cxx_destruct。在《Effective Objective-C 2.0》中阐明：

When the compiler saw that an object contained C++ objects, it would generate a method called .cxx_destruct. ARC piggybacks on this method and emits the required cleanup code within it.

也就是说，当编译器看到 C++ 对象的时候，它将会生成 .cxx_destruct 析构办法，然而 ARC 借用这个办法，并在其中插入了代码以实现主动内存开释的性能。

探索啥时候生成 .cxx_destruct 办法

@interface Person : NSObject
@property (nonatomic, strong) NSString *name;
@end
// 
- (void)viewDidLoad {[super viewDidLoad];
    {NSLog(@"comes");
        Person *p = [[Person alloc] init];
        p.name = @"杭城小刘";
        NSLog(@"gone");
    }
}

在 gone 处加断点，利用 runtime 查看类中的办法信息

发现存在 .cxx_destruct 办法。

咱们一开要钻研的是 ivars 啥时候开释，所以控制变量，将属性改为成员对象

@interface Person : NSObject
{
    @public
    NSString *name;
}
@end

{NSLog(@"comes");
    Person *p = [[Person alloc] init];
    p->name = @"杭城小刘";
    NSLog(@"gone");
}

也有 .cxx_destruct 办法

将成员变量换为根本数据类型

@interface Person : NSObject
{
    @public
    int age;
}
@end

Tips：@property 会主动生成成员变量，另外类前面加 {} 在外部也能够加成员变量，如果成员变量是对象类型，比方 NSString，则叫实例变量。

得出结论：

  • 只有 ARC 模式下才有 .cxx_destruct 办法
  • 类领有实例变量的时候({} 或者 @property) 才有 .cxx_destruct，父类成员对象的实例变量不会让子类领有该办法

应用 watchpoint 察看内存开释机会

在 gone 的中央加断点，输出 watchpoint set variable p->_name，则会将 _name 实例变量退出 watchpoint，当变量被批改时会触发断点，能够看出从某个值变为 0x0，也就是 nil。此时边上调用堆栈显示在 objc_storestrong 办法中，被设置为 nil.

深刻 .cxx_destruct

简略梳理下，在 ARC 模式下，类领有实例变量的时候会在 .cxx_destruct 办法内调用 objc_storeStrong 去开释的内存。

咱们也晓得 .cxx_destruct 是编译器生成的代码。去查问材料 .cxx_destruct site:clang.llvm.org

在 clang 的 doxygen 文档中 CodeGenModule 模块源码发现了相干逻辑。在 5907 行代码

void CodeGenModule::EmitObjCIvarInitializations(ObjCImplementationDecl *D) {
  // We might need a .cxx_destruct even if we don't have any ivar initializers.
  if (needsDestructMethod(D)) {IdentifierInfo *II = &getContext().Idents.get(".cxx_destruct");
    Selector cxxSelector = getContext().Selectors.getSelector(0, &II);
    ObjCMethodDecl *DTORMethod = ObjCMethodDecl::Create(getContext(), D->getLocation(), D->getLocation(), cxxSelector,
        getContext().VoidTy, nullptr, D,
        /*isInstance=*/true, /*isVariadic=*/false,
        /*isPropertyAccessor=*/true, /*isSynthesizedAccessorStub=*/false,
        /*isImplicitlyDeclared=*/true,
        /*isDefined=*/false, ObjCMethodDecl::Required);
    D->addInstanceMethod(DTORMethod);
    CodeGenFunction(*this).GenerateObjCCtorDtorMethod(D, DTORMethod, false);
    D->setHasDestructors(true);
  }

  // If the implementation doesn't have any ivar initializers, we don't need
  // a .cxx_construct.
  if (D->getNumIvarInitializers() == 0 ||
      AllTrivialInitializers(*this, D))
    return;

  IdentifierInfo *II = &getContext().Idents.get(".cxx_construct");
  Selector cxxSelector = getContext().Selectors.getSelector(0, &II);
  // The constructor returns 'self'.
  ObjCMethodDecl *CTORMethod = ObjCMethodDecl::Create(getContext(), D->getLocation(), D->getLocation(), cxxSelector,
      getContext().getObjCIdType(), nullptr, D, /*isInstance=*/true,
      /*isVariadic=*/false,
      /*isPropertyAccessor=*/true, /*isSynthesizedAccessorStub=*/false,
      /*isImplicitlyDeclared=*/true,
      /*isDefined=*/false, ObjCMethodDecl::Required);
  D->addInstanceMethod(CTORMethod);
  CodeGenFunction(*this).GenerateObjCCtorDtorMethod(D, CTORMethod, true);
  D->setHasNonZeroConstructors(true);
}

源码大略做的事件就是：获取 .cxx_destructor 的 selector，创立 Method，而后将新创建的 Method 插入到 class 办法列表中。调用 GenerateObjCCtorDtorMethod 办法，才创立这个办法的实现。查看 GenerateObjCCtorDtorMethod 的实现。在 https://clang.llvm.org/doxyge… 的 1626 行处。

static void emitCXXDestructMethod(CodeGenFunction &CGF,
                                   ObjCImplementationDecl *impl) {CodeGenFunction::RunCleanupsScope scope(CGF);

   llvm::Value *self = CGF.LoadObjCSelf();

   const ObjCInterfaceDecl *iface = impl->getClassInterface();
   for (const ObjCIvarDecl *ivar = iface->all_declared_ivar_begin();
        ivar; ivar = ivar->getNextIvar()) {QualType type = ivar->getType();

     // Check whether the ivar is a destructible type.
     QualType::DestructionKind dtorKind = type.isDestructedType();
     if (!dtorKind) continue;

     CodeGenFunction::Destroyer *destroyer = nullptr;

     // Use a call to objc_storeStrong to destroy strong ivars, for the
     // general benefit of the tools.
     if (dtorKind == QualType::DK_objc_strong_lifetime) {
       destroyer = destroyARCStrongWithStore;

     // Otherwise use the default for the destruction kind.
     } else {destroyer = CGF.getDestroyer(dtorKind);
     }

     CleanupKind cleanupKind = CGF.getCleanupKind(dtorKind);

     CGF.EHStack.pushCleanup<DestroyIvar>(cleanupKind, self, ivar, destroyer,
                                          cleanupKind & EHCleanup);
   }

   assert(scope.requiresCleanups() && "nothing to do in .cxx_destruct?");
 }

能够看到：遍历了以后对象的所有实例变量，调用 objc_storeStrong，从 clang 文档上能够看出

id objc_storeStrong(id *object, id value) {value = [value retain];
  id oldValue = *object;
  *object = value;
  [oldValue release];
  return value;
}

在 .cxx_destruct 办法外部会对所有的实例变量调用 objc_storeStrong(&ivar, null)，实例变量就会 release。

主动调用 [super dealloc] 的原理

同理，CodeGen 也会做主动调用 [super dealloc] 的事件。https://clang.llvm.org/doxyge…，第 751 行 StartObjCMethod 办法。

  751 void CodeGenFunction::StartObjCMethod(const ObjCMethodDecl *OMD,
  752                                       const ObjCContainerDecl *CD) {
  // ...  
  789   // In ARC, certain methods get an extra cleanup.
  790   if (CGM.getLangOpts().ObjCAutoRefCount &&
  791       OMD->isInstanceMethod() &&
  792       OMD->getSelector().isUnarySelector()) {
  793     const IdentifierInfo *ident =
  794       OMD->getSelector().getIdentifierInfoForSlot(0);
  795     if (ident->isStr("dealloc"))
  796       EHStack.pushCleanup<FinishARCDealloc>(getARCCleanupKind());
  797   }
  798 }

能够看到在调用到 dealloc 办法时，插入了代码，实现如下

struct FinishARCDealloc : EHScopeStack::Cleanup {void Emit(CodeGenFunction &CGF, Flags flags) override {const ObjCMethodDecl *method = cast<ObjCMethodDecl>(CGF.CurCodeDecl);

     const ObjCImplDecl *impl = cast<ObjCImplDecl>(method->getDeclContext());
     const ObjCInterfaceDecl *iface = impl->getClassInterface();
     if (!iface->getSuperClass()) return;

     bool isCategory = isa<ObjCCategoryImplDecl>(impl);

     // Call [super dealloc] if we have a superclass.
     llvm::Value *self = CGF.LoadObjCSelf();

     CallArgList args;
     CGF.CGM.getObjCRuntime().GenerateMessageSendSuper(CGF, ReturnValueSlot(),
                                                       CGF.getContext().VoidTy,
                                                       method->getSelector(),
                                                       iface,
                                                       isCategory,
                                                       self,
                                                       /*is class msg*/ false,
                                                       args,
                                                       method);
   }
};

代码大略就是向父类转发 dealloc 的调用实现，外部主动调用 [super dealloc] 办法。

总结下：

  • ARC 模式下，实例变量由编译器插入 .cxx_destruct 办法主动开释
  • ARC 模式下 [super dealloc] 由 llvm 编译器主动插入(CodeGen)

AutoreleasePool 底层原理摸索

单 AutoreleasePool 的 case

int main(int argc, const char * argv[]) {
    @autoreleasepool {Person *p = [[[Person alloc] init] autorelease];
    }
    return 0;
}

clang 转为 c++ xcrun -sdk iphonesimulator clang -rewrite-objc main.m

int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ {  __AtAutoreleasePool __autoreleasepool; 
        Person *p = ((Person *(*)(id, SEL))(void *)objc_msgSend)((id)((Person *(*)(id, SEL))(void *)objc_msgSend)((id)((Person *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("alloc")), sel_registerName("init")), sel_registerName("autorelease"));
    }
    return 0;
}

上面的代码其实就是 objc_msgSend，无效代码是 __AtAutoreleasePool __autoreleasepool;

持续查找

struct __AtAutoreleasePool {__AtAutoreleasePool() {atautoreleasepoolobj = objc_autoreleasePoolPush();
    }
    ~__AtAutoreleasePool() {objc_autoreleasePoolPop(atautoreleasepoolobj);
    }
    void * atautoreleasepoolobj;
};

OC 对象实质就是构造体

  • __AtAutoreleasePool 构造体中 __AtAutoreleasePool 是构造方法，在创立构造体的时候调用
  • ~__AtAutoreleasePool 是析构函数，在构造体销毁的时候调用

main 内的代码作用域，来到代表销毁。所以下面代码等价于

atautoreleasepoolobj = objc_autoreleasePoolPush();
Person *p = [[[Person alloc] init] autorelease];
objc_autoreleasePoolPop(atautoreleasepoolobj);

利用要害函数持续查看 objc4 源码

void *objc_autoreleasePoolPush(void) {return AutoreleasePoolPage::push();
}
void objc_autoreleasePoolPop(void *ctxt) {AutoreleasePoolPage::pop(ctxt);
}

主动开释池的次要实现依附 2 个对象：__AtAutoreleasePool、AutoreleasePoolPage

objc_autoreleasePoolPush、objc_autoreleasePoolPop 底层都是调用了 AutoreleasePoolPage 对象来治理的。

查看源码

class AutoreleasePoolPage {
    magic_t const magic;
    id *next;
    pthread_t const thread;
    AutoreleasePoolPage * const parent;
    AutoreleasePoolPage *child;
    uint32_t const depth;
    uint32_t hiwat;
}
  • 每个 AutoreleasePoolPage 对象占用 4096 字节内存，除了用来寄存它外部的成员变量，剩下的空间用来寄存 autorelease 对象的地址
  • 所有的 AutoreleasePoolPage 对象通过 双向链表 的模式连贯在一起。child 指向下一个对象，parent 指向上一个对象
id * begin() {return (id *) ((uint8_t *)this+sizeof(*this));
}

id * end() {return (id *) ((uint8_t *)this+SIZE);
}

其中 begin 办法返回 autoreleasePoolPage 对象中开始存储 autorelease 对象的开始地址

end 办法返回 autoreleasePoolPage 对象中完结存储 autorelease 对象的开始地址

调用 AutoreleasePoolPage::push 办法会将一个 POOL_BOUNDARY 入栈，并且返回其寄存的内存地址

调用 AutoreleasePoolPage::pop 办法时传入一个 POOL_BOUNDARY 的内存地址，零碎会从最初一个入栈的对象开始发送 release 消 息，直到遇到这个 POOL_BOUNDARY

id *next 指向了下一个能寄存 autorelease 对象地址的区域

static inline void *push() {
    id *dest;
    if (DebugPoolAllocation) {
        // Each autorelease pool starts on a new pool page.
        dest = autoreleaseNewPage(POOL_BOUNDARY);
    } else {dest = autoreleaseFast(POOL_BOUNDARY);
    }
    assert(dest == EMPTY_POOL_PLACEHOLDER || *dest == POOL_BOUNDARY);
    return dest;
}

多 AutoreleasePool 的 case

来个骚一些的例子

int main(int argc, const char * argv[]) {
    @autoreleasepool {Person *p1 = [[[Person alloc] init] autorelease];
        Person *p2 = [[[Person alloc] init] autorelease];
        @autoreleasepool {Person *p3 = [[[Person alloc] init] autorelease];
            @autoreleasepool {Person *p4 = [[[Person alloc] init] autorelease];
            }
        }
    }
    return 0;
}

main 办法外部 3 个 autoreleasepool 底层怎么样工作的？

3 个 @auto releasepool，零碎遇到第一个的时候底层就是初始化一个构造体 __AtAutoreleasePool，构造体构造方法外部调用 AutoreleasePoolPage::push 办法，零碎给 AutoreleasePoolPage 真正保留 autorelease 对象的中央存储进一个 POOL_BOUNDARY 对象，而后贮存 P1、P2 对象地址，遇到第二个则持续初始化构造体，调用 push 办法，存储一个 POOL_BOUNDARY 对象，持续保留 P3，遇到第三个则持续初始化构造体，调用 push 办法，存储一个 POOL_BOUNDARY 对象，持续保留 P4。

当完结第三个大括号的时候，第三个构造体对象，调用析构函数，外部调用 AutoreleasePoolPage::pop 办法，会从最初一个入栈的对象开始发送 release 音讯，直到遇到 POOL_BOUNDARY 对象。

紧接着第二个大括号完结，第二个构造体对象析构函数执行，外部调用 AutoreleasePoolPage::pop 办法，会从最初一个入栈的对象开始发送 release 音讯，直到遇到 POOL_BOUNDARY 对象。

所以，嵌套的 AutoreleasePool 就非常简单了，pop 的时候总会开释到上次 push 的地位为止，多层的 pool 就是多个哨兵对象而已，就像剥洋葱一样，每次一层，互不影响

第一个同理。

小窍门，对于上述原理的剖析能够用源码中看到的 AutoreleasePoolPage 对象的 printAll 办法。

static void printAll() {_objc_inform("##############");
    _objc_inform("AUTORELEASE POOLS for thread %p", pthread_self());

    AutoreleasePoolPage *page;
    ptrdiff_t objects = 0;
    for (page = coldPage(); page; page = page->child) {objects += page->next - page->begin();
    }
    _objc_inform("%llu releases pending.", (unsigned long long)objects);

    if (haveEmptyPoolPlaceholder()) {_objc_inform("[%p]  ................  PAGE (placeholder)", 
                        EMPTY_POOL_PLACEHOLDER);
        _objc_inform("[%p]  ################  POOL (placeholder)", 
                        EMPTY_POOL_PLACEHOLDER);
    }
    else {for (page = coldPage(); page; page = page->child) {page->print();
        }
    }
    _objc_inform("##############");
}


void _objc_autoreleasePoolPrint(void) {AutoreleasePoolPage::printAll();
}

查了下 printAll 函数的应用方，就只有 _objc_autoreleasePoolPrint 函数。且能够看到在 objc4 objc-internal.h 头文件中有将该函数 export 进来，也就是能够在内部链接该符号。

OBJC_EXPORT void _objc_autoreleasePoolPrint(void) OBJC_AVAILABLE(10.7, 5.0, 9.0, 1.0, 2.0);

所以咱们在测试 Demo 中将 _objc_autoreleasePoolPrint 函数申明下。在打印下

extern void _objc_autoreleasePoolPrint(void);
int main(int argc, const char * argv[]) {
    @autoreleasepool {Person *p1 = [[[Person alloc] init] autorelease];
        Person *p2 = [[[Person alloc] init] autorelease];
        @autoreleasepool {Person *p3 = [[[Person alloc] init] autorelease];
            @autoreleasepool {Person *p4 = [[[Person alloc] init] autorelease];
                _objc_autoreleasePoolPrint();}
        }
    }
    return 0;
}
objc[23132]: ##############
objc[23132]: AUTORELEASE POOLS for thread 0x100094600
objc[23132]: 7 releases pending.
objc[23132]: [0x10080a000]  ................  PAGE  (hot) (cold)
objc[23132]: [0x10080a038]  ################  POOL 0x10080a038
objc[23132]: [0x10080a040]       0x10075f060  Person
objc[23132]: [0x10080a048]       0x10075f0c0  Person
objc[23132]: [0x10080a050]  ################  POOL 0x10080a050
objc[23132]: [0x10080a058]       0x10075f0e0  Person
objc[23132]: [0x10080a060]  ################  POOL 0x10080a060
objc[23132]: [0x10080a068]       0x10075f100  Person
objc[23132]: ##############

能够看到打印后果和下面的剖析是统一的（和下面的图片比照看看）

再来个 Demo，验证下 AutoreleasePoolPage 一页满状况

extern void _objc_autoreleasePoolPrint(void);
int main(int argc, const char * argv[]) {
    @autoreleasepool {Person *p1 = [[[Person alloc] init] autorelease];
        Person *p2 = [[[Person alloc] init] autorelease];
        @autoreleasepool {for (NSInteger index = 0; index<600; index++) {Person *p3 = [[[Person alloc] init] autorelease];
            }
            @autoreleasepool {Person *p4 = [[[Person alloc] init] autorelease];
                _objc_autoreleasePoolPrint();}
        }
    }
    return 0;
}

objc[23504]: ##############
objc[23504]: AUTORELEASE POOLS for thread 0x100094600
objc[23504]: 606 releases pending.
objc[23504]: [0x10080d000]  ................  PAGE (full)  (cold)
objc[23504]: [0x10080d038]  ################  POOL 0x10080d038
objc[23504]: [0x10080d040]       0x1007092f0  Person
objc[23504]: [0x10080d048]       0x100709350  Person
objc[23504]: [0x10080d050]  ################  POOL 0x10080d050
objc[23504]: [0x10080d058]       0x100753250  Person
objc[23504]: [0x10080d060]       0x100753270  Person
objc[23504]: [0x10080d068]       0x100753290  Person
objc[23504]: [0x10080d070]       0x1007532b0  Person
objc[23504]: [0x10080d078]       0x1007532d0  Person
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objc[23504]: [0x1008172a8]       0x100755b90  Person
objc[23504]: [0x1008172b0]       0x100755bb0  Person
objc[23504]: [0x1008172b8]       0x100755bd0  Person
objc[23504]: [0x1008172c0]       0x100755bf0  Person
objc[23504]: [0x1008172c8]       0x100755c10  Person
objc[23504]: [0x1008172d0]       0x100755c30  Person
objc[23504]: [0x1008172d8]       0x100755c50  Person
objc[23504]: [0x1008172e0]       0x100755c70  Person
objc[23504]: [0x1008172e8]       0x100755c90  Person
objc[23504]: [0x1008172f0]       0x100755cb0  Person
objc[23504]: [0x1008172f8]       0x100755cd0  Person
objc[23504]: [0x100817300]       0x100755cf0  Person
objc[23504]: [0x100817308]       0x100755d10  Person
objc[23504]: [0x100817310]       0x100755d30  Person
objc[23504]: [0x100817318]       0x100755d50  Person
objc[23504]: [0x100817320]       0x100755d70  Person
objc[23504]: [0x100817328]       0x100755d90  Person
objc[23504]: [0x100817330]       0x100755db0  Person
objc[23504]: [0x100817338]       0x100755dd0  Person
objc[23504]: [0x100817340]       0x100755df0  Person
objc[23504]: [0x100817348]       0x100755e10  Person
objc[23504]: [0x100817350]  ################  POOL 0x100817350
objc[23504]: [0x100817358]       0x100755e30  Person
objc[23504]: ##############

能够看到当 600*8=4800 字节，所以一页必定存不下，能够看到

................ PAGE (full) (cold) page 左边有个 cold、hot。cold 代表不是当前页，hot 代表当前页。

持续看看对象调用 autorelease 办法做了什么事件？

- (id)autorelease {return ((id)self)->rootAutorelease();}

inline id objc_object::rootAutorelease() {if (isTaggedPointer()) return (id)this;
    if (prepareOptimizedReturn(ReturnAtPlus1)) return (id)this;
    return rootAutorelease2();}
_attribute__((noinline,used)) id objc_object::rootAutorelease2() {assert(!isTaggedPointer());
    return AutoreleasePoolPage::autorelease((id)this);
}

static inline id autorelease(id obj) {assert(obj);
    assert(!obj->isTaggedPointer());
    id *dest __unused = autoreleaseFast(obj);
    assert(!dest  ||  dest == EMPTY_POOL_PLACEHOLDER  ||  *dest == obj);
    return obj;
}

static inline id *autoreleaseFast(id obj) {AutoreleasePoolPage *page = hotPage();
    if (page && !page->full()) {return page->add(obj);
    } else if (page) {return autoreleaseFullPage(obj, page);
    } else {return autoreleaseNoPage(obj);
    }
}

查看 NSObject autorelease 办法调用链路能够看到最初还是调用 AutoreleasePoolPage 的 add 办法（会判断有没有页、有没有满）

容器类会主动增加 AutoreleasePool

零碎容器类，在应用 block 枚举器的时候，外部会主动创立 AutoreleasePool

[array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
    @autoreleasepool {<#statements#>}
}];

所以，咱们老老实实写的 for、while 循环中须要手加部分 AutoreleasePool。举荐应用零碎提供的容器类的 block 枚举器。

autorelease 对象什么时候调用 release 办法

每当进行一次 objc_autoreleasePoolPush 调用时，runtime 向以后的 AutoreleasePoolPage 中 add 进一个 哨兵对象，值为 0（也就是个 nil），那么这一个 page 就变成了上面的样子：

objc_autoreleasePoolPush的返回值正是这个哨兵对象的地址，被 objc_autoreleasePoolPop(哨兵对象) 作为入参，于是：

  1. 依据传入的哨兵对象地址找到哨兵对象所处的 page
  2. 在以后 page 中，将晚于哨兵对象插入的所有 autorelease 对象都发送一次 - release 音讯，并向回挪动 next 指针到正确地位
  3. 补充 2：从最新退出的对象始终向前清理，能够向前逾越若干个 page，直到哨兵所在的 page

其次，AutoreleasePool 和 RunLoop 的也有关系

iOS 在主线程的 Runloop 中注册了 2 个 Observer

  • 第 1 个 Observer 监听了 kCFRunLoopEntry 事件，会调用objc_autoreleasePoolPush()
  • 第 2 个 Observer 监听了 kCFRunLoopBeforeWaiting 事件，会调用 objc_autoreleasePoolPop()、objc_autoreleasePoolPush()。还监听了kCFRunLoopBeforeExit 事件，会调用 objc_autoreleasePoolPop()

联合 RunLoop 运行图

  • 01 告诉 Observer 进入 Loop 会调用 objc_autoreleasePoolPush
  • 做一堆其余事件
  • 07 在将要休眠的时候先调用 objc_autoreleasePoolPop，再调用 objc_autoreleasePoolPush
  • 期待唤醒做一堆其余事件，回到第二步
  • 07 又开始休眠，先调用 objc_autoreleasePoolPop，再调用 objc_autoreleasePoolPush
  • 11 没工作将要休眠，调用 objc_autoreleasePoolPop

能够看到 objc_autoreleasePoolPush、objc_autoreleasePoolPop 成对调用，贯通 RunLoop

内存问题典型 case

OC 中有没有不对内存进行强持有的汇合类型？

NSHashMap、NSMapTable 都能够形容 key、value 的内存润饰。

数组有 NSPointerArray 外部持有的是对象的指针，并非间接保留对象。不过 oc 转指针须要加 (__bridge void*) 进行润饰。NSPointerArray 的构造方法中能够通过 NSPointerFunctionsOptions 来申明内存的管制。

- (void)viewDidLoad {[super viewDidLoad];
    Person *p1 = [[Person alloc] init];
    Person *p2 = [[Person alloc] init];
    Person *p3 = [[Person alloc] init];
    NSPointerArray *arrays = [[NSPointerArray alloc] initWithOptions:NSPointerFunctionsWeakMemory];
//    NSMutableArray *array = [NSMutableArray array];
//    [array addObject:p1];
//    [array addObject:p2];
//    [array addObject:p3];
    [arrays addPointer:(__bridge void *)p1];
    [arrays addPointer:(__bridge void *)p2];
    [arrays addPointer:(__bridge void *)p3];
    p1 = nil;
    p2 = nil;
    // 断点设置到 NSLog，能够看到 Person 马上开释了
    NSLog(@"%@", arrays);
}
2022-05-24 21:57:27.071793+0800 TTTTW[63427:2087468] -[Person dealloc]
2022-05-24 21:57:27.071916+0800 TTTTW[63427:2087468] -[Person dealloc]
(lldb)

NSError 内存透露的 case

共事问了一个问题，上面的代码存在什么问题？

据说是 Zoom 这个公司的面试题，看了下其实就是考查 NSError 有没有踩过坑。怎么了解呢

- (BOOL)isZoomUserWithUserID:(NSInteger)userID error:(NSError **)error
{
    @autoreleasepool {NSString *errorMessage = [[NSString alloc] initWithFormat:@"the user is not zoom user"];
        if (userID == 100) {*error = [NSError errorWithDomain:@"com.test" code:userID userInfo:@{NSLocalizedDescriptionKey: errorMessage}];
            return NO;
        }
    }
    return YES;
}

- (void)viewDidLoad {[super viewDidLoad];
    [self test];
}

- (void)test {for (NSInteger index = 0; index <= 100; index++) {
        NSString *str;
        str = [NSString stringWithFormat:@"welcome to zoom:%ld", index];
        str = [str stringByAppendingString:@"user"];
        NSError *error = NULL;
        if ([self isZoomUserWithUserID:index error:&error]) {NSLog(@"%@", str);
        } else {NSLog(@"%@", error);
        }
    }
}

这段代码运行会 crash，信息如下

起因是 NSError 构造方法外部会加 autorelease。源码如下

#define    AUTORELEASE(object)    [(id)(object) autorelease]
+ (id) errorWithDomain: (NSErrorDomain)aDomain
          code: (NSInteger)aCode
          userInfo: (NSDictionary*)aDictionary
{NSError    *e = [self allocWithZone: NSDefaultMallocZone()];

  e = [e initWithDomain: aDomain code: aCode userInfo: aDictionary];
  return AUTORELEASE(e);
}

MRC 下的 [(id)(object) autorelease] 等价于 ARC 下的 id __autoreleasing obj

所以这个问题的实质就是 autoreleasepool 和 __autoreleasing 的问题

__autoreleasing is used to denote arguments that are passed by reference (id *) and are autoreleased on return.

用 __autoreleasing 润饰的变量会被增加到以后的 autoreleasepool 中。

办法的 Out Parameters 参数会主动增加 __autoreleasing 属性。当办法参数外面有 Out Parameters 参数时，就是有指针的指针类型时，编译器会主动为参数加上__autoreleasing 属性。改如下

- (BOOL)isZoomUserWithUserID:(NSInteger)userID error:(NSError **)error
{
    NSError *temp;
    @autoreleasepool {NSString *errorMessage = [[NSString alloc] initWithFormat:@"the user is not zoom user"];
        if (userID == 100) {temp = [NSError errorWithDomain:@"com.test" code:userID userInfo:@{NSLocalizedDescriptionKey: errorMessage}];
        }
    }
    *error = temp;
    return YES;
}

- (void)viewDidLoad {[super viewDidLoad];
    [self test];
}

- (void)test {for (NSInteger index = 0; index <= 100; index++) {
        NSString *str;
        str = [NSString stringWithFormat:@"welcome to zoom:%ld", index];
        str = [str stringByAppendingString:@"user"];
        NSError * __autoreleasing error = NULL;
        if ([self isZoomUserWithUserID:index error:&error]) {NSLog(@"%@", str);
        } else {NSLog(@"%@", error);
        }
    }
}

我写了个僵尸对象检测工具，成果如下

能够定位僵尸对象，并且打印出具体堆栈，并模拟系统行为调用 abort。对监控原理和工具实现感兴趣的能够查看这里带你打造一套 APM 监控零碎 - 内存监控之野指针 / 内存透露监控

Demo 👇这里

内存是间断的吗？

利用启动后，Mach-O 文件是分段载入内存的。咱们应用的内存都是虚拟内存，通过内存映射表来做。

每个过程在创立加载时，会被调配一个大小大略为 1～2 倍实在地内存的间断虚拟地址空间，让以后软件认为本人领有一块很大内存空间。实际上是把磁盘的一小部分作为假想内存来应用。

CPU 不间接和物理内存打交道，而是通过 MMU（Memory Manage Unit，内存治理单元），MMU 是一种硬件电路，速度很快，次要工作是内存治理，地址转换是性能之一。

每个过程都会有本人的页表 Page Table，页表存储了过程中虚拟地址到物理地址的映射关系，所以就相当于地图。MMU 收到 CPU 的虚拟地址之后就开始查问页表，确定是否存在映射以及读写权限是否失常。

iOS 程序在进行加载时，会依据一 page 大小 16kb 将程序宰割为多页，启动时局部的页加载进实在内存，局部页还在磁盘中，两头的调度记录在一张内存映射表（Page Table），这个表用来调度磁盘和内存两者之间的数据交换。

如上图，App 运行时执行某个工作时，会先拜访虚构页表，如果页表的标记为 1，则阐明该页面数据曾经存在于内存中，能够间接拜访。如果页表为 0，则阐明数据未在物理内存中，这时候零碎会阻塞过程，叫做缺页中断（page fault），过程会从用户态切换到内核态，并将缺页中断交给内核的 page Fault Handler 解决。等将对应的 page 从磁盘加载到内存之后再进行拜访，这个过程叫做 page in。

因为磁盘访问速度较慢，所以 page in 比拟耗时，而且 iOS 不仅仅是将数据加载到内存中，还要对这页做 Code Sign 签名认证，所以 iOS 耗时更长

Tips：Code Sign 加密哈希并不少针对于整个文件，而是针对于每一个 Page 的，保障了在 dyld 进行加载的时候，能够对每一个 page 进行独立验证。

等到程序运行时用到了才去内存中寻找虚拟地址对应的页帧，找不到才进行调配，这就是内存的惰性（延时）分配机制。

检测

依据 Instrucments 提供的工具的工作原理，写一个野指针探针工具去发现并定位问题。具体见野指针监控工具

正文完
ios
发表至： ios
2022-12-29
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