为什么此简单的NSWindow创建代码在ARC下关闭时触发自动释放池崩溃? [英] Why does this simple NSWindow creation code trigger an autorelease pool crash on shutdown under ARC?
问题描述
我在关机时遇到自动释放池崩溃的问题，我将其简化为下面的小测试用例，它只是创建一个窗口，然后将其关闭.如果取消了-fobjc-arc标志，崩溃将消失.在OS X 10.8.2，Clang 4.1(421.11.66)上运行.我希望对ARC有更深入了解的人能启发我这里发生的事情-运行带有僵尸对象的程序显示，正是NSWindow对象被释放了太多次，或者保留得不够多，但是我以为ARC应该负责所有这一切?
I am having an issue with an autorelease pool crash on shutdown which I've reduced to the small test case below that simply creates a window and then closes it. The crash disappears if the -fobjc-arc flag is taken away. Running on OS X 10.8.2, Clang 4.1 (421.11.66). I am hoping that someone with a more in depth understanding of ARC can enlighten me as to what is going on here - running with zombie objects on shows that it is the NSWindow object that is getting released too many times, or not retained enough, but I thought ARC was meant to take care of all this?
堆栈跟踪为:
0 libobjc.A.dylib 0x00007fff8fad4f5e objc_release + 14
1 libobjc.A.dylib 0x00007fff8fad4230 (anonymous namespace)::AutoreleasePoolPage::pop(void*) + 464
2 com.apple.CoreFoundation 0x00007fff99d22342 _CFAutoreleasePoolPop + 34
3 com.apple.Foundation 0x00007fff936e84fa -[NSAutoreleasePool drain] + 154
4 com.apple.Foundation 0x00007fff936effa0 _NSAppleEventManagerGenericHandler + 125
5 com.apple.AE 0x00007fff93a5ab48 aeDispatchAppleEvent(AEDesc const*, AEDesc*, unsigned int, unsigned char*) + 307
6 com.apple.AE 0x00007fff93a5a9a9 dispatchEventAndSendReply(AEDesc const*, AEDesc*) + 37
7 com.apple.AE 0x00007fff93a5a869 aeProcessAppleEvent + 318
8 com.apple.HIToolbox 0x00007fff8d0c18e9 AEProcessAppleEvent + 100
9 com.apple.AppKit 0x00007fff8e95c916 _DPSNextEvent + 1456
10 com.apple.AppKit 0x00007fff8e95bed2 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 128
11 com.apple.AppKit 0x00007fff8e953283 -[NSApplication run] + 517
12 Test 0x00000001070e1d68 main + 152 (Test.mm:31)
13 libdyld.dylib 0x00007fff8e10c7e1 start + 1
测试用例的代码是:
// Tested with `clang++ -fobjc-arc -g Test.mm -framework Cocoa -o Test && ./Test`
#import <Cocoa/Cocoa.h>
@interface MyApplication : NSApplication
@end
@implementation MyApplication
- (void) applicationDidFinishLaunching: (NSNotification *) note
{
NSWindow * window = [[NSWindow alloc] initWithContentRect: NSMakeRect(100, 100, 100, 100)
styleMask: NSTitledWindowMask backing: NSBackingStoreBuffered defer: YES];
[window close];
[super stop: self];
}
@end
int main()
{
@autoreleasepool
{
const ProcessSerialNumber psn = { 0, kCurrentProcess };
TransformProcessType(&psn, kProcessTransformToForegroundApplication);
SetFrontProcess(&psn);
[MyApplication sharedApplication];
[NSApp setDelegate: NSApp];
[NSApp run];
}
return 0;
}
推荐答案
使用Instruments的Zombies配置文件显示，通过调用close:将NSWindow对象放入自动释放池中.一旦applicationDidFinishLaunching:完成并销毁了NSWindow实例，ARC就会正确地以零的引用计数结束.但是，自动释放池仍然知道现在已终止的NSWindow实例，然后尝试在关闭时释放它，从而导致崩溃.
Using Instruments' Zombies profile showed that the NSWindow object gets put into the autorelease pool by the call to close:. ARC then correctly ends up with a reference count of zero once applicationDidFinishLaunching: completes and destroys the NSWindow instance. However, the autorelease pool still knows about the now-defunct NSWindow instance and then tries to release it on shutdown, causing the crash.
在ARC下管理自动释放对象似乎是个坏主意，除非自动释放池将对其对象的弱引用归零，这似乎在这里没有做.
Autoreleasing objects being managed under ARC seems like a bad idea unless the autorelease pool holds zeroing weak references to its objects, which it doesn't seem to be doing here.
通过添加[window setReleasedWhenClosed: NO];告诉窗口在关闭时不要自动释放，可以防止此问题.
The problem can be prevented by telling the window not to autorelease on close by adding [window setReleasedWhenClosed: NO];.
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