This function allocates a block of memory from a heap. The allocated memory is not movable.
At a Glance
| Header file: | Winbase.h |
| Windows CE versions: | 1.0 and later |
Syntax
LPVOID HeapAlloc(HANDLE hHeap, DWORD dwFlags,
DWORD dwBytes);
Parameters
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 following flags:
| 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.
Remarks
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. Since 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:
See Also