Storage Allocator Issues

The debug version of the MFC runtime allocates storage differently than the release version. In particular, the debug version allocates some space at the beginning and end of each block of storage, so its allocation patterns are somewhat different. The changes in storage allocation can cause problems to appear that would not appear in the debug version--but almost always these are genuine problems, as in bugs in your program, which somehow managed to not be detected in the debug version. These are usually rare.

Why are they rare? Because the debug version of the MFC allocator initializes all storage to really bogus values, so an attempt to use a chunk of storage that you have failed to allocate will give you an immediate access fault in the debug version. Furthermore, when a block of storage is freed, it is initialized to another pattern, so that if you have retained any pointers to the storage and try to use the block after it is freed you will also see some immediately bogus behavior.

The debug allocator also checks the storage at the start and end of the block it allocated to see if it has been damaged in any way. The typical problem is that you have allocated a block of n values as an array and then accessed elements 0 through n, instead of 0 through n-1, thus overwriting the area at the end of the array. This condition will cause an assertion failure most of the time. But not all of the time. And this leads to a potential for failure.

Storage is allocated in quantized chunks, where the quantum is unspecified but is something like 16, or 32 bytes. Thus, if you allocated a DWORD array of six elements (size = 6 * sizeof(DWORD) bytes = 24 bytes) then the allocator will actually deliver 32 bytes (one 32-byte quantum or two 16-byte quanta). So if you write element [6] (the seventh element) you overwrite some of the "dead space" and the error is not detected. But in the release version, the quantum might be 8 bytes, and three 8-byte quanta would be allocated, and writing the [6] element of the array would overwrite a part of the storage allocator data structure that belongs to the next chunk. After that it is all downhill. There error might not even show up until the program exits! You can construct similar "boundary condition" situations for any size quantum. Because the quantum size is the same for both versions of the allocator, but the debug version of the allocator adds hidden space for its own purposes, you will get different storage allocation patterns in debug and release mode.