This function allocates a block of memory from a heap. The allocated memory is not movable.

LPVOID HeapAlloc(
HANDLE hHeap, 
DWORD dwFlags, 
DWORD dwBytes 


  • hHeap
    [in] Handle to the heap from which the memory will be allocated. This parameter is a handle returned by the HeapCreate or GetProcessHeap function.

  • dwFlags
    [in] Specifies several controllable aspects of heap allocation. Specifying any of these flags will override the corresponding flag specified when the heap was created with HeapCreate. You can specify one or more of the flags listed in the following table.

    Value Description
    HEAP_NO_SERIALIZE Specifies that mutual exclusion will not be used while the HeapAlloc function 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.
    HEAP_ZERO_MEMORY Specifies that the allocated memory will be initialized to zero. Otherwise, the memory is not initialized to zero.
  • dwBytes
    [in] Specifies the number of bytes to be allocated. If the heap specified by the hHeap parameter is a nongrowable heap, dwBytes must be less than 0x7FFF8. You create a nongrowable heap by calling the HeapCreate function with a nonzero value.

Return Values

A pointer to the allocated memory block indicates success. NULL indicates failure.

If the function fails, it does not call SetLastError. An application cannot call GetLastError for extended error information.


If HeapAlloc succeeds, it allocates at least the amount of memory requested. If the actual amount allocated is greater than the amount requested, the process can use the entire amount. To determine the actual size of the allocated block, use the HeapSize function.

To free a block of memory allocated by HeapAlloc, use the HeapFree function.

Memory allocated by HeapAlloc is not movable. Because the memory is not movable, it is possible for the heap to become fragmented.

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    

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, HeapCreate, HeapDestroy, HeapFree, HeapReAlloc, HeapSize, SetLastError

 Last updated on Tuesday, July 13, 2004

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