What happens if my Visual C++ custom memory allocator returns a block not properly aligned?

Question

In my Visual C++ program I use a custom operator new that uses malloc() to allocate memory. My custom operator new stores extra data in the first 4 bytes of memory and returns an offset pointer as the beginning of the block (the program is 32 bit):

 void* operator new( size_t size )
 {
     size += sizeof( int );//assume it doesn't overflow
     int* result = static_cast<int*>( malloc( size ) );
     if( result == 0 ) {
         throw std::bad_alloc;
     }
     *result = ... //write extra data
     return result + 1;
 }

Now if caller code wants to store a variable of size 64 bits ( __int64 or double ) the block will not be properly aligned for that.

What problems can this cause in a 32-bit Windows program?

Answer 1

On 32 bit windows it will just potentially be slower as the hardware can deal with unaligned data accesses, just more slowly.

On other operating systems / platforms it will likely cause a crash (Or VERY slow performance as the OS catches the unaligned memory access and simulates it for you in some cases)