HeapAlloc

The HeapAlloc function allocates a block of memory from a heap. The allocated memory is not movable.

LPVOID HeapAlloc(

HANDLE hHeap,	// handle to the private heap block
DWORD dwFlags,	// heap allocation control flags
DWORD dwBytes	// number of bytes to allocate
);

Parameters

hHeap

Specifies the heap from which the memory will be allocated. This parameter is a handle returned by the HeapCreate or GetProcessHeap function.

dwFlags

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:

Flag	Meaning
HEAP_GENERATE_EXCEPTIONS	Specifies that the operating system will raise 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 will not be used while this function is accessing the heap. For more information about HEAP_NO_SERIALIZE, see the Remarks section of HeapCreate.
HEAP_ZERO_MEMORY	Specifies that the allocated memory will be initialized to zero.

dwBytes

Specifies 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 Values

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 the following exceptions:

Value	Meaning
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.

Note that heap corruption can lead to either exception. It depends upon the nature of the heap corruption.

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.

Note that if HEAP_ZERO_MEMORY is not specified, the allocated memory will not be initialized to zero.

See Also

GetProcessHeap, HeapCreate, HeapDestroy, HeapFree, HeapReAlloc, HeapSize, SetLastError

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