What are Guard bytes ?
A. peterchen quoted :- malloc_dbg calls malloc, but requests a few more bytes. These bytes are filled with a predefined value. free_dbg then checks if these extra bytes still hold this value. If they did, you probably wrote beyond the allocated memory (the VC Runtime prints a diagnostic message to the debug output). Typical implementations add 4 bytes at the end. In this case, using free instead of _free_dbg is often not a problem, but depending on the implementation, it might leak the guard bytes. But if an implementation adds guard bytes *before* the memory provided to you (to detect underflows, which are less common), using free instead of -free_dbg is likely to corrupt the heap.
