为什么AddRef返回零 [英] Why AddRef returns zero

问题描述

我正在调试C ++/COM应用程序，查看我们如何添加和释放COM对象.当AddRef返回0时，我遇到了奇怪的情况.这是我如何获取返回值的方法:

I'm debugging C++/COM application, looking at how we AddRef and Release COM objects. I came around weird case when AddRef returns 0. Here is how I get the return value:

ULONG TraceAddRef(LPUNKNOWN pUnk, const std::string &a_msg) {
    ULONG count = pUnk->AddRef(); // count == 0 at some point after execution
    ATLTRACE("%s *** AddRef:  pUnk = 0x%p, referenceCount = %lu\n", a_msg.c_str(), pUnk, count);
    return count;
}

pUnk实际上是Web控件的IWebBrowser2 COM接口:

pUnk is actually IWebBrowser2 COM interface to a web control:

pUnk    0x20d763ac  IUnknown *
__vfptr 0x5d85b0d8  const CFrameWebOC::`vftable'{for `IWebBrowser2'}

我已经查看了该行的反汇编(调试构建模式):

I've looked into Disassembly (Debug build mode) for that line:

    ULONG count = pUnk->AddRef();
6515A52C  mov         eax,dword ptr [pUnk]  
6515A52F  mov         ecx,dword ptr [eax]  
6515A531  mov         esi,esp  
6515A533  mov         edx,dword ptr [pUnk]  
6515A536  push        edx  
6515A537  mov         eax,dword ptr [ecx+4]  
6515A53A  call        eax  
6515A53C  cmp         esi,esp  
6515A53E  call        _RTC_CheckEsp (65323F90h)  
6515A543  mov         dword ptr [count],eax

那时6515A543在行6515A543上为0.

在调试器中，当我进入6515A53A行时，它将显示以下代码:

In the debugger when I go into the line 6515A53A, there it will show the following code:

    CFrameWebOC::AddRef:
5D707B6D  mov         edi,edi  
5D707B6F  push        ebp  
5D707B70  mov         ebp,esp  
5D707B72  push        edi  
5D707B73  mov         edi,dword ptr [ebp+8]  
5D707B76  inc         dword ptr [edi-18h]  
5D707B79  cmp         dword ptr [edi-18h],2  
5D707B7D  je          CFrameWebOC::AddRef+26h (5D707B93h)  
5D707B7F  test        dword ptr [edi-4],0FFFFFFFCh  
5D707B86  jne         5DF2DD04  
5D707B8C  xor         eax,eax  
5D707B8E  pop         edi  
5D707B8F  pop         ebp  
5D707B90  ret         4  
5D707B93  push        esi  
5D707B94  lea         esi,[edi-8]  
5D707B97  call        CTrackerHelper::SetAsRoot (5D85AD2Fh)  
5D707B9C  pop         esi  
5D707B9D  jmp         CFrameWebOC::AddRef+12h (5D707B7Fh)  
5D707B9F  nop  

在5D707B76行上，值dword ptr [edi-18h]似乎是一个参考计数器，其值是正确的，在inc命令之前和之后都不为零.

On the line 5D707B76 the value dword ptr [edi-18h] seems to be a reference counter, the value of which is correct, non-zero before and after the inc command.

我知道AddRef的返回值仅用于调试目的. AddRef返回0似乎是一个错误.这个错误会影响我正在使用的COM对象的行为，尤其是生命周期吗?

I know that the return value of AddRef is for debugging purposes only. AddRef returning 0 seems like a bug. Can this bug affect the behavior of COM objects I'm using, lifetime in particular?

如果有帮助，我可以使用MSVS 2010在VirtualBox内的Win7 64bit上运行.DLLAddRef位于:mshtml.dll

If that helps I'm on Win7 64bit inside of VirtualBox, using MSVS 2010. The DLL AddRef is in: mshtml.dll

mshtml.dll  C:\Windows\SysWOW64\mshtml.dll  Symbols loaded (source information stripped).   C:\SYMBOLS\PUBLIC\mshtml.pdb\049E32F8F9F84F8EB494D8324AC1C3112\mshtml.pdb   104 10.00.9200.16521 (win8_gdr_soc_ie.130216-2100)  10/24/2013 8:37 PM  5D380000-5E137000   [0x21DFC] MyApplication.exe: Native

推荐答案

这似乎很奇怪，但是艾伦(Alan)的猜测听起来是正确的.从AddRef返回零不会影响COM内部的任何内容，因为正如您提到的，该值仅用于调试.

It does seem weird, but Alan's guess sound about right. Returning zero from AddRef should not affect anything inside COM because as you mention the value is used for debugging only.

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