All calls to heap functions such as malloc, free, calloc, realloc, new, and delete resolve to Debug versions of those functions that operate in the debug heap. When you free a memory block, the debug heap automatically checks the integrity of the buffers on either side of your allocated area and issues an error report if overwriting has occurred.

Debug Heap Features Accessed From Within Code

_CrtCheckMemory

Many of the debug heap's features must be accessed from within your code. You can use a call to _CrtCheckMemory, for example, to check the heap's integrity at any point. This function inspects every memory block in the heap, verifies that the memory block header information is valid, and confirms that the buffers were not modified.

_CrtSetDbgFlag

You can control how the debug heap keeps track of allocations using an internal flag, _crtDbgFlag, which can be read and set using _CrtSetDbgFlag. By changing this flag, you can instruct the debug heap to check for memory leaks when the program exits and report any leaks that are detected. Similarly, you can specify that freed memory blocks not be removed from the linked list, to simulate low-memory situations. When the heap is checked, these freed blocks are inspected in their entirety to ensure that they have not been disturbed.

The _crtDbgFlag flag contains these bit fields:

Bit field	Description
_CRTDBG_ALLOC_MEM_DF	ON (default): Turns on debug allocation. OFF: Allocations remain chained together but their block type is _IGNORE_BLOCK.
_CRTDBG_DELAY_FREE_MEM_DF	ON: Prevents memory from actually being freed, as for simulating low-memory conditions. OFF (default): Keeps freed blocks in the debug heap's linked list but marks them as _FREE_BLOCK and fills them with a special byte value.
_CRTDBG_CHECK_ALWAYS_DF	ON: Causes _CrtCheckMemory to be called at every allocation and deallocation. This slows execution, but catches errors quickly. OFF (default): Does not call _CrtCheckMemory.
_CRTDBG_CHECK_CRT_DF	ON: Causes blocks marked as type _CRT_BLOCK to be included in leak-detection and state-difference operations. When this bit is off, the memory used internally by the run-time library is ignored during such operations. OFF (default): Does not include _CRT_BLOCK blocks in leak-detection and state-difference operations.
_CRTDBG_LEAK_CHECK_DF	ON: Causes leak checking to be performed at program exit via a call to _CrtDumpMemoryLeaks. An error report is generated if the application has failed to free all the memory that it allocated. OFF (default): Does not perform leak checking at program exit.

To change one or more _crtDbgFlag bit fields and create a new state for the flag.

1. Call _CrtSetDbgFlag with the newFlag parameter set to _CRTDBG_REPORT_FLAG (to obtain the current _crtDbgFlag state) and store the returned value in a temporary variable.
2. Turn on any bits by OR-ing (bitwise | symbol) the temporary variable with the corresponding bitmasks (represented in the application code by manifest constants).
3. Turn off the other bits by AND-ing (bitwise & symbol) the variable with a NOT (bitwise ~ symbol) of the appropriate bitmasks.
4. Call _CrtSetDbgFlag with the newFlag parameter set to the value stored in the temporary variable to create the new state for _crtDbgFlag.

For example, the following lines of code turn on automatic leak detection and turn off checking for blocks of type _CRT_BLOCK:

// Get current flag
int tmpFlag = _CrtSetDbgFlag( _CRTDBG_REPORT_FLAG );

// Turn on leak-checking bit
tmpFlag |= _CRTDBG_LEAK_CHECK_DF;

// Turn off CRT block checking bit
tmpFlag &= ~_CRTDBG_CHECK_CRT_DF;

// Set flag to the new value
_CrtSetDbgFlag( tmpFlag );