A Process s Default Heap

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When a process initializes, the system creates a heap in the process's address space. This heap is called the process's default heap. By default, this heap's region of address space is 1 MB in size. However, the system can grow a process's default heap so that it becomes larger than this. You can change the default region size of 1 MB using the /HEAP linker switch when you create an application. Because a DLL does not have a heap associated with it, you should not use the /HEAP switch when you are linking a DLL. The /HEAP switch has the following syntax:

 /HEAP:reserve[,commit] 

Many Windows functions require the process's default heap. For example, the core functions in Windows 2000 perform all of their operations using Unicode characters and strings. If you call an ANSI version of a Windows function, this ANSI version must convert the ANSI strings to Unicode strings and then call the Unicode version of the same function. To convert the strings, the ANSI function needs to allocate a block of memory to hold the Unicode version of the string. This block of memory is allocated from your process's default heap. Many other Windows functions require the use of temporary memory blocks; these blocks are allocated from the process's default heap. Also, the old 16-bit Windows functions LocalAlloc and GlobalAlloc make their memory allocations from the process's default heap.

Because the process's default heap is used by many of the Windows functions, and because your application has many threads simultaneously calling the various Windows functions, access to the default heap is serialized. In other words, the system guarantees that only one thread at a time can allocate or free blocks of memory in the default heap at any given time. If two threads attempt to simultaneously allocate a block of memory in the default heap, only one thread will be able to allocate a block; the other thread will be forced to wait until the first thread's block is allocated. Once the first thread's block is allocated, the heap functions will allow the second thread to allocate a block. This serialized access causes a small performance hit. If your application has only one thread and you want to have the fastest possible access to a heap, you should create your own separate heap and not use the process's default heap. Unfortunately, you cannot tell the Windows functions not to use the default heap, so their accesses to the heap are always serialized.

A single process can have several heaps at once. These heaps can be created and destroyed during the lifetime of the process. The default heap, however, is created before the process begins execution and is destroyed automatically when the process terminates. You cannot destroy the process's default heap. Each heap is identified with its own heap handle, and all of the heap functions that allocate and free blocks within a heap require this heap handle as a parameter.

You can obtain the handle to your process's default heap by calling GetProcessHeap:

 HANDLE GetProcessHeap();