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This function returns the size, in bytes, of a memory block allocated from a heap by the HeapAlloc or HeapReAlloc function.

DWORD HeapSize(
  HANDLE hHeap,
  DWORD dwFlags,


[in] Handle to the heap in which the memory block resides.

This handle is returned by the HeapCreate or GetProcessHeap function.


[in] Controllable aspects of accessing the memory block.

Only one flag is currently defined; however, all other flag values are reserved for future use.

Specifying HEAP_NO_SERIALIZE overrides the corresponding flag specified in the flOptions parameter if the heap was created using the HeapCreate function.

Specifies that mutual exclusion is not used while HeapSize is accessing the heap.

Do not specify this flag when accessing the process heap. The system might create additional threads within the application's process that simultaneously access the process heap.

This flag is ignored.


[in] Valid pointer to the memory block whose size the function will obtain.

This is a pointer returned by the HeapAlloc or HeapReAlloc function.

In the debug configuration, where heap sentinels are enabled, the return value is the exact allocation size for the heap item.

In the retail configuration, where debug information is not included, the return value is the allocation size aligned to the block size (32 bytes).

0xFFFFFFFF indicates failure. The function does not call SetLastError.

Serialization ensures mutual exclusion when two or more threads attempt to simultaneously allocate or free blocks from the same heap. There is a small performance cost, but it must be used when multiple threads allocate and free memory from the same heap.

A critical section is always used to serialize access to an individual heap. There is a critical section per heap to protect access to each heap.

An attempt to grab a critical section that is not owned is a fast path operation that incurs little overhead. This is similar to using the HEAP_NO_SERIALIZE flag if only one thread was ever accessing a particular heap.

If there is contention for the critical section, and therefore the heap, a new thread request to allocate heap space will serialize.

Windows Embedded CEWindows CE 1.0 and later
Windows MobileWindows Mobile Version 5.0 and later

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