HeapAlloc function (heapapi.h)

Allocates a block of memory from a heap. The allocated memory is not movable.


  [in] HANDLE hHeap,
  [in] DWORD  dwFlags,
  [in] SIZE_T dwBytes


[in] hHeap

A handle to the heap from which the memory will be allocated. This handle is returned by the HeapCreate or GetProcessHeap function.

[in] dwFlags

The heap allocation options. Specifying any of these values will override the corresponding value specified when the heap was created with HeapCreate. This parameter can be one or more of the following values.

Value Meaning
The system will raise an exception to indicate a function failure, such as an out-of-memory condition, instead of returning NULL.

To ensure that exceptions are generated for all calls to this function, specify HEAP_GENERATE_EXCEPTIONS in the call to HeapCreate. In this case, it is not necessary to additionally specify HEAP_GENERATE_EXCEPTIONS in this function call.

Serialized access will not be used for this allocation.

For more information, see Remarks.

To ensure that serialized access is disabled for all calls to this function, specify HEAP_NO_SERIALIZE in the call to HeapCreate. In this case, it is not necessary to additionally specify HEAP_NO_SERIALIZE in this function call.

This value should not be specified when accessing the process's default heap. The system may create additional threads within the application's process, such as a CTRL+C handler, that simultaneously access the process's default heap.

The allocated memory will be initialized to zero. Otherwise, the memory is not initialized to zero.

[in] dwBytes

The number of bytes to be allocated.

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 allocated 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. The particular exception depends upon the nature of the heap corruption. For more information, see GetExceptionCode.

Exception code Description
STATUS_NO_MEMORY The allocation attempt failed because of a lack of available memory or heap corruption.
STATUS_ACCESS_VIOLATION The allocation attempt failed because of heap corruption or improper function parameters.

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


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

To allocate memory from the process's default heap, use HeapAlloc with the handle returned by the GetProcessHeap function.

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

Memory allocated by HeapAlloc is not movable. The address returned by HeapAlloc is valid until the memory block is freed or reallocated; the memory block does not need to be locked. Because the system cannot compact a private heap, it can become fragmented.

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

#if defined(_WIN64) || defined(_M_ALPHA)

Applications that allocate large amounts of memory in various allocation sizes can use the low-fragmentation heap to reduce heap fragmentation.

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.


For an example, see AWE Example.


Requirement Value
Minimum supported client Windows XP [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 [desktop apps | UWP apps]
Target Platform Windows
Header heapapi.h (include Windows.h)
Library Kernel32.lib
DLL Kernel32.dll

See also

Heap Functions



Memory Management Functions

Vertdll APIs available in VBS enclaves