编译器何时以及为何将内存初始化为 malloc/free/new/delete 上的 0xCD、0xDD 等? [英] When and why will a compiler initialise memory to 0xCD, 0xDD, etc. on malloc/free/new/delete?

问题描述

我知道编译器有时会使用某些模式初始化内存，例如 0xCD 和 0xDD.我想知道的是何时以及为什么会发生这种情况.

I know that the compiler will sometimes initialize memory with certain patterns such as 0xCD and 0xDD. What I want to know is when and why this happens.

什么时候

这是特定于使用的编译器吗?

Is this specific to the compiler used?

malloc/new 和 free/delete 在这方面的工作方式是否相同?

Do malloc/new and free/delete work in the same way with regard to this?

是否特定于平台?

是否会出现在其他操作系统上，例如 Linux 或 VxWorks?

Will it occur on other operating systems, such as Linux or VxWorks?

为什么

我的理解是这只发生在Win32调试配置中，用于检测内存溢出并帮助编译器捕获异常.

My understanding is this only occurs in Win32 debug configuration, and it is used to detect memory overruns and to help the compiler catch exceptions.

你能举出一些实际例子来说明这种初始化是如何有用的吗?

Can you give any practical examples as to how this initialization is useful?

我记得读过一些(可能在 Code Complete 2 中)说在分配内存时将内存初始化为已知模式是好的，并且某些模式会在 Win32 中触发中断，从而导致异常在调试器中显示.

I remember reading something (maybe in Code Complete 2) saying that it is good to initialize memory to a known pattern when allocating it, and certain patterns will trigger interrupts in Win32 which will result in exceptions showing in the debugger.

这有多便携?

推荐答案

在为调试模式编译时，Microsoft 编译器对各种未拥有/未初始化内存使用的内容的快速摘要(支持可能因编译器版本而异):

A quick summary of what Microsoft's compilers use for various bits of unowned/uninitialized memory when compiled for debug mode (support may vary by compiler version):

Value     Name           Description 
------   --------        -------------------------
0xCD     Clean Memory    Allocated memory via malloc or new but never 
                         written by the application. 

0xDD     Dead Memory     Memory that has been released with delete or free. 
                         It is used to detect writing through dangling pointers. 

0xED or  Aligned Fence   'No man's land' for aligned allocations. Using a 
0xBD                     different value here than 0xFD allows the runtime
                         to detect not only writing outside the allocation,
                         but to also identify mixing alignment-specific
                         allocation/deallocation routines with the regular
                         ones.

0xFD     Fence Memory    Also known as "no mans land." This is used to wrap 
                         the allocated memory (surrounding it with a fence) 
                         and is used to detect indexing arrays out of 
                         bounds or other accesses (especially writes) past
                         the end (or start) of an allocated block.

0xFD or  Buffer slack    Used to fill slack space in some memory buffers 
0xFE                     (unused parts of `std::string` or the user buffer 
                         passed to `fread()`). 0xFD is used in VS 2005 (maybe 
                         some prior versions, too), 0xFE is used in VS 2008 
                         and later.

0xCC                     When the code is compiled with the /GZ option,
                         uninitialized variables are automatically assigned 
                         to this value (at byte level). 


// the following magic values are done by the OS, not the C runtime:

0xAB  (Allocated Block?) Memory allocated by LocalAlloc(). 

0xBAADF00D Bad Food      Memory allocated by LocalAlloc() with LMEM_FIXED,but 
                         not yet written to. 

0xFEEEFEEE               OS fill heap memory, which was marked for usage, 
                         but wasn't allocated by HeapAlloc() or LocalAlloc(). 
                         Or that memory just has been freed by HeapFree(). 

免责声明:表格来自我身边的一些笔记 - 它们可能不是 100% 正确(或连贯).

Disclaimer: the table is from some notes I have lying around - they may not be 100% correct (or coherent).

其中许多值都在 vc/crt/src/dbgheap.c 中定义:

Many of these values are defined in vc/crt/src/dbgheap.c:

/*
 * The following values are non-zero, constant, odd, large, and atypical
 *      Non-zero values help find bugs assuming zero filled data.
 *      Constant values are good, so that memory filling is deterministic
 *          (to help make bugs reproducible).  Of course, it is bad if
 *          the constant filling of weird values masks a bug.
 *      Mathematically odd numbers are good for finding bugs assuming a cleared
 *          lower bit.
 *      Large numbers (byte values at least) are less typical and are good
 *          at finding bad addresses.
 *      Atypical values (i.e. not too often) are good since they typically
 *          cause early detection in code.
 *      For the case of no man's land and free blocks, if you store to any
 *          of these locations, the memory integrity checker will detect it.
 *
 *      _bAlignLandFill has been changed from 0xBD to 0xED, to ensure that
 *      4 bytes of that (0xEDEDEDED) would give an inaccessible address under 3gb.
 */

static unsigned char _bNoMansLandFill = 0xFD;   /* fill no-man's land with this */
static unsigned char _bAlignLandFill  = 0xED;   /* fill no-man's land for aligned routines */
static unsigned char _bDeadLandFill   = 0xDD;   /* fill free objects with this */
static unsigned char _bCleanLandFill  = 0xCD;   /* fill new objects with this */

也有几次调试运行时会用已知值填充缓冲区(或缓冲区的一部分)，例如，std::string 分配中的slack"空间或传递给 fread() 的缓冲区.这些情况使用给定名称 _SECURECRT_FILL_BUFFER_PATTERN(在 crtdefs.h 中定义)的值.我不确定它是什么时候引入的，但至少在 VS 2005 (VC++8) 之前它就在调试运行时中.

There are also a few times where the debug runtime will fill buffers (or parts of buffers) with a known value, for example, the 'slack' space in std::string's allocation or the buffer passed to fread(). Those cases use a value given the name _SECURECRT_FILL_BUFFER_PATTERN (defined in crtdefs.h). I'm not sure exactly when it was introduced, but it was in the debug runtime by at least VS 2005 (VC++8).

最初，用于填充这些缓冲区的值是 0xFD - 与用于无人区的值相同.但是，在 VS 2008 (VC++9) 中，该值已更改为 0xFE.我认为这是因为在某些情况下填充操作可能会超过缓冲区的末尾，例如，如果调用方传入的缓冲区大小对于 fread() 来说太大了.在这种情况下，值 0xFD 可能不会触发检测此溢出，因为如果缓冲区大小仅大一，填充值将与用于初始化金丝雀的无人区值相同.无人区没有变化意味着不会注意到超支.

Initially, the value used to fill these buffers was 0xFD - the same value used for no man's land. However, in VS 2008 (VC++9) the value was changed to 0xFE. I assume that's because there could be situations where the fill operation would run past the end of the buffer, for example, if the caller passed in a buffer size that was too large to fread(). In that case, the value 0xFD might not trigger detecting this overrun since if the buffer size were too large by just one, the fill value would be the same as the no man's land value used to initialize that canary. No change in no man's land means the overrun wouldn't be noticed.

所以在VS 2008中改变了填充值，这样这种情况就会改变无人区金丝雀，导致运行时检测到问题.

So the fill value was changed in VS 2008 so that such a case would change the no man's land canary, resulting in the detection of the problem by the runtime.

正如其他人所指出的，这些值的关键属性之一是，如果取消引用具有这些值之一的指针变量，则会导致访问冲突，因为在标准的 32 位 Windows 配置上，用户模式地址不会高于 0x7fffffff.

As others have noted, one of the key properties of these values is that if a pointer variable with one of these values is de-referenced, it will result in an access violation, since on a standard 32-bit Windows configuration, user mode addresses will not go higher than 0x7fffffff.

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