This function frees a memory block allocated from a heap by the HeapAlloc or HeapReAlloc function.

BOOL HeapFree(
HANDLE hHeap, 
DWORD dwFlags, 


  • hHeap
    [in] Handle to the heap whose memory block is freed by the function. This parameter is a handle returned by the HeapCreate or GetProcessHeap function.

  • dwFlags
    [in] Specifies several controllable aspects of freeing a memory block. Only one flag is currently defined; however, all other flag values are reserved for future use. Specifying this flag will override the corresponding flag specified in the flOptions parameter when the heap was created using the HeapCreate function.

    Value Description
    HEAP_NO_SERIALIZE Specifies that mutual exclusion will not be used while HeapFree is accessing the heap. This flag should not be specified when accessing the process heap. The system may create additional threads within the application's process that simultaneously access the process heap.
  • lpMem
    [in] Pointer to the memory block to be freed. This pointer is returned by the HeapAlloc or HeapReAlloc function.

Return Values

Nonzero indicates success. Zero indicates failure. To get extended error information, call GetLastError.


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 flag 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 flag 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.


Runs on Versions Defined in Include Link to
Windows CE OS 1.0 and later Winbase.h   Lmem.lib

Note   This API is part of the complete Windows CE OS package as provided by Microsoft. The functionality of a particular platform is determined by the original equipment manufacturer (OEM) and some devices may not support this API.

See Also

GetLastError, GetProcessHeap, HeapAlloc, HeapCreate, HeapDestroy, HeapReAlloc, HeapSize

 Last updated on Tuesday, July 13, 2004

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