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?

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:表格来自我的一些笔记 - 它们可能不是 100% 正确(或连贯).

/*
 * 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 */