HeapReAlloc

The HeapReAlloc function reallocates a block of memory from a heap. This function enables you to resize a memory block and change other memory block properties. The allocated memory is not movable.

LPVOID HeapReAlloc(
  HANDLE hHeap,   // handle to heap block
  DWORD dwFlags,  // heap reallocation options
  LPVOID lpMem,   // pointer to memory to reallocate
  SIZE_T dwBytes  // number of bytes to reallocate
);

Parameters

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 a value overrides the corresponding value specified in the flOptions parameter when the heap was created by using the HeapCreate function. This parameter can be one or more of the following values.

Value	Meaning
HEAP_GENERATE_EXCEPTIONS	Specifies that the operating-system raises an exception to indicate a function failure, such as an out-of-memory condition, instead of returning NULL.
HEAP_NO_SERIALIZE	Specifies that mutual exclusion is not used while HeapReAlloc is accessing the heap. 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.
HEAP_REALLOC_IN_PLACE_ONLY	Specifies that there can be no movement when reallocating a memory block to a larger size. If this value is not specified and the reallocation request is for a larger size, the function may move the block to a new location. If this value 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 value 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] New size of the memory block, in bytes. A memory block's size can be increased or decreased by using this function.

If the heap specified by the hHeap parameter is a "non-growable" heap, dwBytes must be less than 0x7FFF8. You create a non-growable heap by calling the HeapCreate function with a nonzero value.

Return Values

If the function succeeds, the return value is a pointer to the reallocated memory block.

If the function fails and you have not specified HEAP_GENERATE_EXCEPTIONS, the return value is NULL.

If the function fails and you have specified HEAP_GENERATE_EXCEPTIONS, the function may generate the following exceptions:

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

Remarks

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 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.

Note  To guard against an access violation, use structured exception handling to protect any code that writes to or reads from a heap. For more information on structured exception handling with memory accesses, see Reading and Writing and Structured Exception Handling.

Windows 95/98: The heap managers are designed for memory blocks smaller than four megabytes. If you expect your memory blocks to be larger than one or two megabytes, you can avoid significant performance degradation by using the VirtualAlloc or VirtualAllocEx function instead.

Requirements

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winbase.h; include Windows.h.
  Library: Use Kernel32.lib.

See Also

Memory Management Overview, Memory Management Functions, GetProcessHeap, HeapAlloc, HeapCreate, HeapDestroy, HeapFree, HeapSize, SetLastError