c++ - 编译器何时以及为什么会在 malloc/free/new/delete 上将内存初始化为 0xCD、0xDD 等？

Question

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。

当

这是特定于使用的编译器吗？

malloc/new 和 free/delete 在这方面的工作方式相同吗？

它是特定于平台的吗？

会不会出现在其他操作系统上，例如 Linux 或 VxWorks？

为什么

我的理解是这只发生在Win32调试配置中，它是用来检测内存溢出和帮助编译器捕捉异常的。

你能举出任何实际的例子来说明这个初始化是如何有用的吗？

我记得读过一些东西(可能在 Code Complete 2 中)说在分配内存时最好将内存初始化为已知模式，并且某些模式会在 Win32 中触发中断，这将导致异常显示在调试器中。

这有多便携？

Answer 1

Microsoft 编译器在为 Debug模式编译时用于各种未拥有/未初始化内存的快速摘要(支持可能因编译器版本而异):

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 self 的一些笔记 - 它们可能不是 100% 正确(或连贯)。

其中许多值在 vc/crt/src/dbgheap.c 中定义:

/*
 * 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 */

还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“松弛”空间或传递给 fread() 的缓冲区。这些情况使用一个名为 _SECURECRT_FILL_BUFFER_PATTERN 的值(在 crtdefs.h 中定义)。我不确定它是何时引入的，但它至少在 VS 2005 (VC++8) 的调试运行时中。

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，填充操作可能会超出缓冲区的末尾，例如，如果调用者传入的缓冲区大小对于 fread() 来说太大。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 1，则填充值将与用于初始化金丝雀的无人地带值相同.无人区没有变化意味着越界不会被注意到。

所以在 VS 2008 中更改了填充值，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。

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 32 位 Windows 配置中，用户模式地址不会高于 0x7fffffff。