HeapReAlloc function (heapapi.h)

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.

Syntax

DECLSPEC_ALLOCATOR LPVOID HeapReAlloc(
  [in] HANDLE                 hHeap,
  [in] DWORD                  dwFlags,
  [in] _Frees_ptr_opt_ LPVOID lpMem,
  [in] SIZE_T                 dwBytes
);

Parameters

[in] hHeap

A handle to the heap from which the memory is to be reallocated. This handle is a returned by either the HeapCreate or GetProcessHeap function.

[in] dwFlags

The heap reallocation options. 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.

[in] lpMem

A pointer to the block of memory that the function reallocates. This pointer is returned by an earlier call to the HeapAlloc or HeapReAlloc function.

[in] dwBytes

The 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 value

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 either of the exceptions listed in the following table. For more information, see GetExceptionCode.

The alignment of memory returned by HeapReAlloc is MEMORY_ALLOCATION_ALIGNMENT in WinNT.h:

#if defined(_WIN64) || defined(_M_ALPHA)
#define MEMORY_ALLOCATION_ALIGNMENT 16
#else
#define MEMORY_ALLOCATION_ALIGNMENT 8
#endif

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

Remarks

If HeapReAlloc succeeds, it allocates at least the amount of memory requested.

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

HeapReAlloc is guaranteed to preserve the content of the memory being reallocated, even if the new memory is allocated at a different location. The process of preserving the memory content involves a memory copy operation that is potentially very time-consuming.

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.

Requirements

See also

Heap Functions

HeapAlloc

HeapFree

Memory Management Functions

Vertdll APIs available in VBS enclaves