Allocation Hook Functions
Applies to: 
Visual Studio 
Visual Studio for Mac
Note
This article applies to Visual Studio 2017. If you're looking for the latest Visual Studio documentation, see Visual Studio documentation. We recommend upgrading to the latest version of Visual Studio. Download it here
An allocation hook function, installed using _CrtSetAllocHook, is called every time memory is allocated, reallocated, or freed. You can use this type of hook for many different purposes. Use it to test how an application handles insufficient memory situations, such as to examine allocation patterns, or log allocation information for later analysis.
Note
Be aware of the restriction about using C run-time library functions in an allocation hook function, described in Allocation Hooks and C Run-Time Memory Allocations.
An allocation hook function should have a prototype like the following example:
int YourAllocHook(int nAllocType, void *pvData,
size_t nSize, int nBlockUse, long lRequest,
const unsigned char * szFileName, int nLine )
The pointer that you pass to _CrtSetAllocHook is of type _CRT_ALLOC_HOOK, as defined in CRTDBG.H:
typedef int (__cdecl * _CRT_ALLOC_HOOK)
(int, void *, size_t, int, long, const unsigned char *, int);
When the run-time library calls your hook, the nAllocType argument indicates what allocation operation is about to be made (_HOOK_ALLOC, _HOOK_REALLOC, or _HOOK_FREE). In a free or in a reallocation, pvData has a pointer to the user article of the block about to be freed. However for an allocation, this pointer is null, because the allocation hasn't occurred. The remaining arguments contain the size of the allocation in question, its block type, the sequential request number associated with it, and a pointer to the file name. If available, the arguments also include the line number in which the allocation was made. After the hook function performs whatever analysis and other tasks its author wants, it must return either TRUE, indicating that the allocation operation can continue, or FALSE, indicating that the operation should fail. A simple hook of this type might check the amount of memory allocated so far, and return FALSE if that amount exceeded a small limit. The application would then experience the kind of allocation errors that would normally occur only when available memory was very low. More complex hooks might keep track of allocation patterns, analyze memory use, or report when specific situations occur.