HeapSize function (heapapi.h)

Retrieves the size of a memory block allocated from a heap by the HeapAlloc or HeapReAlloc function.


SIZE_T HeapSize(
  [in] HANDLE  hHeap,
  [in] DWORD   dwFlags,
  [in] LPCVOID lpMem


[in] hHeap

A handle to the heap in which the memory block resides. This handle is returned by either the HeapCreate or GetProcessHeap function.

[in] dwFlags

The heap size options. Specifying the following value overrides the corresponding value specified in the flOptions parameter when the heap was created by using the HeapCreate function.

Value Meaning
Serialized access will not be used. For more information, see Remarks.

To ensure that serialized access is disabled for all calls to this function, specify HEAP_NO_SERIALIZE in the call to HeapCreate. In this case, it is not necessary to additionally specify HEAP_NO_SERIALIZE in this function call.

This value should not be specified when accessing the process heap. The system may create additional threads within the application's process, such as a CTRL+C handler, that simultaneously access the process heap.

[in] lpMem

A pointer to the memory block whose size the function will obtain. This is a pointer returned by the HeapAlloc or HeapReAlloc function. The memory block must be from the heap specified by the hHeap parameter.

Return value

If the function succeeds, the return value is the requested size of the allocated memory block, in bytes.

If the function fails, the return value is (SIZE_T)-1. The function does not call SetLastError. An application cannot call GetLastError for extended error information.

If the lpMem parameter refers to a heap allocation that is not in the heap specified by the hHeap parameter, the behavior of the HeapSize function is undefined.


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 to serialization, but it must be used whenever multiple threads allocate and free memory from the same heap. Setting the HEAP_NO_SERIALIZE value eliminates mutual exclusion on the heap. Without serialization, two or more threads that use the same heap handle might attempt to allocate or free memory simultaneously, likely causing corruption in the heap. The HEAP_NO_SERIALIZE value can, therefore, be safely used only in the following situations:

  • The process has only one thread.
  • The process has multiple threads, but only one thread calls the heap functions for a specific heap.
  • The process has multiple threads, and the application provides its own mechanism for mutual exclusion to a specific heap.


Requirement Value
Minimum supported client Windows XP [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 [desktop apps | UWP apps]
Target Platform Windows
Header heapapi.h (include Windows.h)
Library Kernel32.lib
DLL Kernel32.dll

See also

Heap Functions



Memory Management Functions

Vertdll APIs available in VBS enclaves