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

LPVOID HeapReAlloc(
HANDLE hHeap, 
DWORD dwFlags, 
LPVOID lpMem, 
DWORD dwBytes 


  • hHeap
    [in] Heap from which the memory will be reallocated. This is a handle returned by the HeapCreate or GetProcessHeap function.

  • dwFlags
    [in] Specifies several controllable aspects of heap reallocation. Specifying any of these flags overrides the corresponding flag specified in the flOptions parameter when the heap was created using the HeapCreate function. You can specify one or more of the flags in the following table.

    Value Description
    HEAP_NO_SERIALIZE Specifies that mutual exclusion is not used while HeapReAlloc 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_REALLOC_IN_PLACE_ONLY Specifies that there can be no movement when reallocating a memory block to a larger size. If this flag is not specified and the reallocation request is for a larger size, the function may move the block to a new location. If this flag is specified and the block cannot be enlarged without moving, the function fails, leaving the original memory block unchanged.
    HEAP_ZERO_MEMORY If the reallocation request is for a larger size, this flag specifies that the additional region of memory beyond the original size be initialized to zero. The contents of the memory block up to its original size are unaffected.
  • lpMem
    [in] Pointer to the block of memory that the function reallocates. This pointer is returned by an earlier call to the HeapAlloc or HeapReAlloc function.

  • dwBytes
    [in] Specifies the new size of the memory block, in bytes. A memory block's size can be increased or decreased using this function.

Return Values

If the function fails, it calls SetLastError. An application can call GetLastError for extended error information.


If HeapReAlloc 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 reallocated block, use the HeapSize function.

If HeapReAlloc fails, the original memory is not freed and the original handle and pointer are still valid.

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

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, HeapFree, HeapSize, SetLastError

 Last updated on Tuesday, July 13, 2004

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