使用pthreads時objc_release中的SIGSEGV

我在執行緒中運行一些 Metal 代碼，但遇到了一些我不完全理解的問題。USE_THREAD 0使用和運行以下代碼USE_AUTORELEASEPOOL 0作業正常，但將其中之一設定為 1 會導致 SIGSEGV in objc_release：

* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x20)
  * frame #0: 0x00007fff2020d4af libobjc.A.dylib`objc_release   31
    frame #1: 0x00007fff2022b20f libobjc.A.dylib`AutoreleasePoolPage::releaseUntil(objc_object**)   167
    frame #2: 0x00007fff2020de30 libobjc.A.dylib`objc_autoreleasePoolPop   161
    frame #3: 0x0000000100003d60 a.out`render(void*)   896
    frame #4: 0x0000000100003dd8 a.out`main   24
    frame #5: 0x00007fff20388f3d libdyld.dylib`start   1
    frame #6: 0x00007fff20388f3d libdyld.dylib`start   1

使用 autoreleasepool 我可以理解，因為物件已經被釋放（因為release在它們上手動呼叫），但是為什么當代碼在執行緒內運行時會出現同樣的問題？這與pthreads具體有關嗎？我缺少的地方是否有“隱藏的”自動釋放池？

我知道使用自動釋放池而不是手動釋放會達到相同的結果，但我想了解這里發生了什么。

// clang   main.mm -lobjc -framework Metal
#define USE_THREAD 0
#define USE_AUTORELEASEPOOL 1

#import <Metal/Metal.h>

void * render(void *) {
    #if USE_AUTORELEASEPOOL
    @autoreleasepool {
    #else
    {
    #endif
        NSArray<id<MTLDevice>> * devices = MTLCopyAllDevices();
        id<MTLDevice> device = devices[0];

        id<MTLCommandQueue> command_queue = [device newCommandQueue];

        MTLTextureDescriptor * texture_descriptor = [MTLTextureDescriptor texture2DDescriptorWithPixelFormat:MTLPixelFormatBGRA8Unorm width:640 height:480 mipmapped:NO];
        texture_descriptor.usage = MTLTextureUsageRenderTarget;

        id<MTLTexture> texture = [device newTextureWithDescriptor:texture_descriptor];

        [texture_descriptor release];
        texture_descriptor = NULL;

        id<MTLCommandBuffer> command_buffer = [command_queue commandBuffer];

        MTLRenderPassDescriptor * render_pass_descriptor = [MTLRenderPassDescriptor renderPassDescriptor];
        render_pass_descriptor.colorAttachments[0].texture = texture;

        id<MTLRenderCommandEncoder> render_command_encoder = [command_buffer renderCommandEncoderWithDescriptor:render_pass_descriptor];

        [render_pass_descriptor release];
        render_pass_descriptor = NULL;

        [render_command_encoder endEncoding];

        [render_command_encoder release];
        render_command_encoder = nil;

        [command_buffer commit];
        [command_buffer waitUntilCompleted];

        [command_buffer release];
        command_buffer = nil;

        [texture release];
        texture = nil;

        [command_queue release];
        command_queue = nil;
    }

    return 0;
}

#include <pthread.h>

int main() {
    #if USE_THREAD
        pthread_t thread;
        pthread_create(&thread, NULL, render, NULL);

        pthread_join(thread, NULL);
    #else
        render(NULL);
    #endif

    return 0;
}

uj5u.com熱心網友回復：

我不確定 Apple 是否更新過關于此行為的檔案，但大約從 macOS 10.7 開始，所有 POSIX 執行緒都自動填充了自動釋放池塊。dealloc您可以從自動釋放的物件中清楚地看到它：

#import <Foundation/Foundation.h>
#import <pthread.h>

@interface TDWObject : NSObject

@end

@implementation TDWObject

- (void)dealloc {
    [super dealloc];
    NSLog(@"%@", [NSThread callStackSymbols]);
}

@end


void *run(void *objPtr) {
    TDWObject *obj = (__bridge TDWObject *)objPtr;
    [obj autorelease];
    return NULL;
}


int main() {
    pthread_t thread;
    TDWObject *obj = [TDWObject new];
    pthread_create(&thread, NULL, run, (__bridge void *)obj);
    pthread_join(thread, NULL);
    return 0;
}

此代碼（使用 MRC 編譯時）將列印以下堆疊跟蹤：

-[TDWObject dealloc]   83
_ZN19AutoreleasePoolPage12releaseUntilEPP11objc_object   168
objc_autoreleasePoolPop   227
_ZN20objc_tls_direct_baseIP19AutoreleasePoolPageL7tls_key3ENS0_14HotPageDeallocEE5dtor_EPv   140
_pthread_tsd_cleanup   607
_pthread_exit   70
_pthread_start   136
thread_start   15

objc_autoreleasePoolPop函式正是如今在自動釋放池塊末尾呼叫的函式。

標籤：目标-c苹果系统线程金属目标-C

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