Question

Visual C++ debug runtime library features so-called allocation hooks. Works this way: you define a callback and call _CrtSetAllocHook() to set that callback. Now every time a memory allocation/deallocation/reallocation is done CRT calls that callback and passes a handful of parameters.

I successfully used an allocation hook to find a reproduceable memory leak – basically CRT reported that there was an unfreed block with allocation number N (N was the same on every program run) at program termination and so I wrote the following in my hook:

int MyAllocHook( int allocType, void* userData, size_t size, int blockType, 
    long requestNumber, const unsigned char* filename, int lineNumber)
{
     if( requestNumber == TheNumberReported ) {
         Sleep( 0 );// a line to put breakpoint on
     }
     return TRUE;
}

since the leak was reported with the very same allocation number every time I could just put a breakpoint inside the if-statement and wait until it was hit and then inspect the call stack.

What other useful things can I do using allocation hooks?

Answer 1

You could also use it to find unreproducible memory leaks:

• Make a data structure where you map the allocated pointer to additional information
• In the allocation hook you could query the current call stack (StackWalk function) and store the call stack in the data structure
• In the de-allocation hook, remove the call stack information for that allocation
• At the end of your application, loop over the data structure and report all call stacks. These are the places where memory was allocated but not freed.