| Platform SDK: Memory |
The HeapAlloc function allocates a block of memory from a heap. The allocated memory is not movable.
LPVOID HeapAlloc( HANDLE hHeap, // handle to private heap block DWORD dwFlags, // heap allocation control SIZE_T dwBytes // number of bytes to allocate );
| Value | Meaning |
|---|---|
| HEAP_GENERATE_EXCEPTIONS | Specifies that the 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 the HeapAlloc function 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_ZERO_MEMORY | Specifies that the allocated memory will be initialized to zero. Otherwise, the memory is not initialized to zero. |
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.
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 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.
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 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:
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.
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.
Memory Management Overview, Memory Management Functions, GetProcessHeap, HeapCreate, HeapDestroy, HeapFree, HeapReAlloc, HeapSize, SetLastError