If you want to validate or report the contents of the data stored in _CLIENT_BLOCK blocks, you can write a function specifically for this purpose. The function that you write must have a prototype similar to the following, as defined in crtdbg.h:

void YourClientDump(void *, size_t)

In other words, your hook function should accept a void pointer to the beginning of the user's topic of the allocation block, together with a size_t type value indicating the size of the allocation, and return void. Other than that, its contents are up to you.

Once you have installed your hook function using _CrtSetDumpClient,it will be called every time a _CLIENT_BLOCK block is dumped. You can then use _CrtReportBlockType to get information on the type or subtype of dumped blocks.

The pointer to your function that you pass to _CrtSetDumpClient is of type _CRT_DUMP_CLIENT, as defined in crtdbg.h:

typedef void (__cdecl *_CRT_DUMP_CLIENT)
   (void *, size_t);