[翻譯]does NSThread create autoreleasepool automaticly now?

前言

很不錯的一個stackoverflow的問題，來自于does NSThread create autoreleasepool automaticly now?。

之前寫過一篇探索子線程autorelease對象的釋放時機，但感覺沒有很好的深入找到問題根源，而這個答案就鎖定了。

這裡翻譯一下，當作學習。

問題

I have test code like this

- (void)viewDidLoad
{
    [super viewDidLoad];
    NSThread *thread = [[NSThread alloc] initWithTarget:self selector:@selector(test) object:nil];
    [thread start];
}

-(void)test
{
    MyClass *my = [[[MyClass alloc] init] autorelease];
    NSLog(@"%@",[my description]);
}

I did not create any autoreleasepool for my own thread,but when the thread exit, object “my” just dealloc.why?

even though I change my test code as below

- (void)viewDidLoad
{
    [super viewDidLoad];

    NSThread *thread = [[NSThread alloc] initWithTarget:self selector:@selector(test) object:nil];
    [thread start];
} 

-(void)test
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    MyClass *my = [[[MyClass alloc] init] autorelease];
    NSLog(@"%@",[my description]);
}

I create my own autoreleasepool but not drain it when the thread exit. object “my” can still dealloc anyway. why!!!!!!!??????

btw, I use Xcode5 and not using ARC

anyone can help me to figure out what happening now?

回答原文

It’s not documented, but the answer appears to be Yes, on OS X 10.9+ and iOS 7+.

The Objective-C runtime is open-source so you can read the source to see what’s going on. The latest version of the runtime (646, which shipped with OS X 10.10 and iOS 8) does indeed add a pool if you perform an autorelease

without a pool on the current thread. In NSObject.mm:

static __attribute__((noinline))
id *autoreleaseNoPage(id obj)
{
    // No pool in place.
    assert(!hotPage());

    if (obj != POOL_SENTINEL  &&  DebugMissingPools) {
        // We are pushing an object with no pool in place, 
        // and no-pool debugging was requested by environment.
        _objc_inform("MISSING POOLS: Object %p of class %s "
                     "autoreleased with no pool in place - "
                     "just leaking - break on "
                     "objc_autoreleaseNoPool() to debug", 
                     (void*)obj, object_getClassName(obj));
        objc_autoreleaseNoPool(obj);
        return nil;
    }

    // Install the first page.
    AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
    setHotPage(page);

    // Push an autorelease pool boundary if it wasn't already requested.
    if (obj != POOL_SENTINEL) {
        page->add(POOL_SENTINEL);
    }

    // Push the requested object.
    return page->add(obj);
}

This function is called when you push the first pool (in which case the thing pushed is POOL_SENTINEL), or you autorelease with no pool. When the first pool is pushed, it sets up the autorelease stack. But as you see from the code, as long as the DebugMissingPools environmental variable is not set (it’s not set by default), when autorelease is done with no pool, it also sets up the autorelease stack, and then pushes a pool (pushes a POOL_SENTINEL).

Similarly, (it’s a little hard to follow without looking at the other code, but this is the relevant part) when the thread is destroyed (and the Thread-Local Storage is destroyed), it releases everything in the autorelease stack (that’s what the pop(0); does) so it doesn’t rely on the user to pop the last pool:

static void tls_dealloc(void *p) 
{
    // reinstate TLS value while we work
    setHotPage((AutoreleasePoolPage *)p);
    pop();
    setHotPage(nil);
}

The previous version of the runtime (551.1, which came with OS X 10.9 and iOS 7), also did this, as you can see from its NSObject.mm:

static __attribute__((noinline))
id *autoreleaseSlow(id obj)
{
    AutoreleasePoolPage *page;
    page = hotPage();

    // The code below assumes some cases are handled by autoreleaseFast()
    assert(!page || page->full());

    if (!page) {
        // No pool. Silently push one.
        assert(obj != POOL_SENTINEL);

        if (DebugMissingPools) {
            _objc_inform("MISSING POOLS: Object %p of class %s "
                         "autoreleased with no pool in place - "
                         "just leaking - break on "
                         "objc_autoreleaseNoPool() to debug", 
                         (void*)obj, object_getClassName(obj));
            objc_autoreleaseNoPool(obj);
            return nil;
        }

        push();
        page = hotPage();
    }

    do {
        if (page->child) page = page->child;
        else page = new AutoreleasePoolPage(page);
    } while (page->full());

    setHotPage(page);
    return page->add(obj);
}

But the version before that (532.2, which came with OS X 10.8 and iOS 6), does not:

static __attribute__((noinline))
id *autoreleaseSlow(id obj)
{
    AutoreleasePoolPage *page;
    page = hotPage();

    // The code below assumes some cases are handled by autoreleaseFast()
    assert(!page || page->full());

    if (!page) {
        assert(obj != POOL_SENTINEL);
        _objc_inform("Object %p of class %s autoreleased "
                     "with no pool in place - just leaking - "
                     "break on objc_autoreleaseNoPool() to debug", 
                     obj, object_getClassName(obj));
        objc_autoreleaseNoPool(obj);
        return NULL;
    }

    do {
        if (page->child) page = page->child;
        else page = new AutoreleasePoolPage(page);
    } while (page->full());

    setHotPage(page);
    return page->add(obj);
}

Note that the above works for any pthreads, not just NSThreads.

So basically, if you are running on OS X 10.9+ or iOS 7+, autoreleasing on a thread without a pool should not lead to a leak. This is not documented and is an internal implementation detail, so be careful relying on this as Apple could change it in a future OS. However, I don’t see any reason why they would remove this feature as it is simple and only has benefits and no downsides, unless they completely re-write the way autorelease pools work or something.

答案翻譯

這沒有被文檔化，但這個答案顯然是：是的。在OS X 10.9+和iOS 7+上。

Objective-c的runtime對你來說是開源的，是以你可以讀源代碼看如何運作。最新版本的runtime(646，用在OS X 10.10和IOS 8)确實添加了一個pool如果你仔沒有pool的情況下運作autorelease。

static __attribute__((noinline))
id *autoreleaseNoPage(id obj)
{
    // No pool in place.
    assert(!hotPage());

    if (obj != POOL_SENTINEL  &&  DebugMissingPools) {
        // We are pushing an object with no pool in place, 
        // and no-pool debugging was requested by environment.
        _objc_inform("MISSING POOLS: Object %p of class %s "
                     "autoreleased with no pool in place - "
                     "just leaking - break on "
                     "objc_autoreleaseNoPool() to debug", 
                     (void*)obj, object_getClassName(obj));
        objc_autoreleaseNoPool(obj);
        return nil;
    }

    // Install the first page.
    AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
    setHotPage(page);

    // Push an autorelease pool boundary if it wasn't already requested.
    if (obj != POOL_SENTINEL) {
        page->add(POOL_SENTINEL);
    }

    // Push the requested object.
    return page->add(obj);
}

這個函數的調用時在你第一次壓入pool（事實上push的是

POOL_SENTIAL

），或者你沒有pool下autorelease。當第一個pool被壓入，它建立autorelease的棧。但當你從code來看，因為

DebugMissingPools

的環境變量沒有設定（不會被設為預設）。當autolrease沒有pool下結束了，它同樣會建立autorelease棧，然後壓入一個pool(壓入一個

POOL_SENTINEL

)

同樣的，（如果沒有看其他代碼，有點難了解，但這個是相關部分）。當thread被銷毀（同樣線程本地的存儲也被銷毀），它釋放了每一個在autorelease棧中的對象（使用

pop(0);

）是以它不建立在使用者去彈出最後一個pool。

static void tls_dealloc(void *p) 
{
    // reinstate TLS value while we work
    setHotPage((AutoreleasePoolPage *)p);
    pop();
    setHotPage(nil);
}

這個目前版本的runtime(551.1，來自于OS X 10.9與iOS 7)也同樣做了這個，是以你可以看

NSObject.mm

static __attribute__((noinline))
id *autoreleaseSlow(id obj)
{
    AutoreleasePoolPage *page;
    page = hotPage();

    // The code below assumes some cases are handled by autoreleaseFast()
    assert(!page || page->full());

    if (!page) {
        // No pool. Silently push one.
        assert(obj != POOL_SENTINEL);

        if (DebugMissingPools) {
            _objc_inform("MISSING POOLS: Object %p of class %s "
                         "autoreleased with no pool in place - "
                         "just leaking - break on "
                         "objc_autoreleaseNoPool() to debug", 
                         (void*)obj, object_getClassName(obj));
            objc_autoreleaseNoPool(obj);
            return nil;
        }

        push();
        page = hotPage();
    }

    do {
        if (page->child) page = page->child;
        else page = new AutoreleasePoolPage(page);
    } while (page->full());

    setHotPage(page);
    return page->add(obj);
}

但之前的版本(532.2，來自于OS X 10.8與 iOS 6)沒有做。

static __attribute__((noinline))
id *autoreleaseSlow(id obj)
{
    AutoreleasePoolPage *page;
    page = hotPage();

    // The code below assumes some cases are handled by autoreleaseFast()
    assert(!page || page->full());

    if (!page) {
        assert(obj != POOL_SENTINEL);
        _objc_inform("Object %p of class %s autoreleased "
                     "with no pool in place - just leaking - "
                     "break on objc_autoreleaseNoPool() to debug", 
                     obj, object_getClassName(obj));
        objc_autoreleaseNoPool(obj);
        return NULL;
    }

    do {
        if (page->child) page = page->child;
        else page = new AutoreleasePoolPage(page);
    } while (page->full());

    setHotPage(page);
    return page->add(obj);
}

注意上面的代碼為任何

pthread

執行，不僅僅是

NSThread

。

基本上，如果你運作在OS X 10.9+或者iOS 7+，不用pool去autorelease在一個線程上不回引起記憶體洩漏。這個沒有被文檔記錄并且是一個内部的實作細節，是以小心依賴這個因為Apple可能會在之後的系統修改它。然而，我看不到任何他們會移除這個功能的理由，因為這個簡單，并且隻有益處而沒有壞處，除非他們重寫autorelease pool的工作或者其他内容。

來源

does NSThread create autoreleasepool automaticly now?

本文簡書位址

[翻譯]does NSThread create autoreleasepool automaticly now?

前言

問題

回答原文

答案翻譯

來源

繼續閱讀

吃透MVC，馴服爛代碼吃透MVC，馴服爛代碼

PAT (Advanced Level) Practise 1083 List Grades (25)

PAT (Advanced Level) Practise 1135 Is It A Red-Black Tree (30)

UVA 348 Optimal Array Multiplication Sequence

PAT (Advanced Level) Practise 1066 Root of AVL Tree (25)

CSU 1806 Toll

CodeChef PALPROB Palindromeness

ZOJ 1032 Area 2

UVA 590 Always on the run

FZU 1978 Repair the brackets

UVA 10344- 23 out of 5

POJ 2115 C Looooops

ZOJ 3700 Ever Dream

UVA 620 Cellular Structure

swift資料合集

蘋果稽核 Guideline 2.1 - Information Needed