What Is a Heap?
 
A heap is a form of memory manager that an application can use when it needs to allocate and free memory dynamically. Common situations that call for the use of a heap are when the size of the memory needed is not known ahead of time and the size of the memory is too large to neatly fit on the stack (automatic memory). Even though the heap is the most common facility to accommodate dynamic memory allocations, there are a number of other ways for applications to request memory from Windows. Memory can be requested from the C runtime, the virtual memory manager, and even from other forms of private memory managers. Although the different memory managers can be treated as individual entities, internally, they are tightly connected. Figure 6.1 shows a simplified view of Windows-supported memory managers and their dependencies.
 
 
 
 Figure 6.1 An overview of Windows memory management architecture
 
As illustrated in Figure 6.1, most of the high-level memory managers make use of the Windows heap manager, which in turn uses the virtual memory manager. Although high-level memory managers (and applications for that matter) are not restricted to using the heap manager, they most typically do, as it provides a solid foundation for other private memory managers to build on. Because of its popularity, the primary focal point in this chapter is the Windows heap manager.
 
When a process starts, the heap manager automatically creates a new heap called the default process heap. Although some processes use the default process heap, a large number rely on the CRT heap (using new/delete and malloc/free family of APIs) for all their memory needs. Some processes, however, create additional heaps (via the HeapCreate API) to isolate different components running in the process. It is not uncommon for even the simplest of applications to have four or more active heaps at any given time.
 
The Windows heap manager can be further broken down as shown in Figure 6.2.
 
 
 
 Figure 6.2 Windows heap manager