第882页 IPC object identification and handles for open objects

In order to access an IPC object, a process must have some means of identifying the object, and once the object has been “opened,” the process must use some type of handle to refer to the open object. Table 43-1 summarizes these properties for
the various types of IPC facilities.
IPC终究是一些共享的变量和内存区域，因为不同进程间的地址空间是不一样的，所以不能简单地全部用全局变量来标识这些共享的变量和内存区域。