Accessing unallocated memory C++

Question

I am having this piece of code:

try
{
    int* myTestArray = new int[2];

    myTestArray[4] = 54;

    cout << \"Should throw ex \"  << myTestAr         


        

Answer 1

Knowing what's going on here for sure is very hard to do. But I can give you a rough idea.

Most operating systems have a minimum size for memory allocations. In Unix it is the native page size. On x86 and amd64 systems this is 4 kB. In Windows it is 64 kB (I think).

The memory allocator used by malloc and new gets memory from the operating system in chunks of this size. It sets up data structures (often a linked list, sometimes a bitmap, or a tree) and hands out small pieces of the requested sizes.

One other confusing thing is that before your program even starts running main() it has run quite a bit of other code and allocated memory. For std::cout and other static and global objects, and for shared library linking.

But assume that when you call new your program first gets a chunk of 4 kB and gives you a pointer to 8 bytes of it (two integers). Your program has the entire 4 kB allocated and you can write there without crashing. However, what happens if you call new again? It is very likely that the memory allocator wrote some important tracking information somewhere into that 4 kB. The next bytes might be the size of the following block. Writing 54 into it might make it think it has more or less memory than it does. Or those bytes might be a pointer to the next block of free memory, and your 54 will cause the next memory allocation to crash the program.