| Object Manager. The Object Manager is probably the least known of NT's Executive subsystems, is also one of the most important. An operating system's primary role is to manage a computer's physical and logical resources. Other Executive subsystems use the Object Manager to define and manage objects that represent resources. For example, through the Object Manager, the Process Manager defines a process object to track active processes.
The Object Manager performs object-management duties that include identification and reference counting. When an application opens a resource, the Object Manager either locates the associated object or creates a new object. Instead of returning an object pointer to the application that opened the resource, the Object Manager returns an opaque identifier called a handle. The handle's value is unique to the application that opened the resource, but it is not unique to the system across different applications. The application uses the handle to identify the resource in subsequent operations. When the application is finished with the object, the application closes the handle. The Object Manager uses reference counting to track how many system parts, including applications and Executive subsystems, are accessing an object that represents a resource. When the reference count goes to zero, the object is no longer in use representing the resource, and the Object Manager deletes the object (but not necessarily the resource).
The Object Manager implements NT's namespace to provide object identification. All shareable resources in NT have names that are rooted in this namespace. For example, when a program opens a file, the Object Manager parses the file's name to locate the file-system driver for the disk that stores the file. Similarly, when an application opens a Registry key, the Object Manager determines from the Registry key's name that the Configuration Manager must be called.
Most native system services that NT implements are resource related; thus, almost every system service invokes Object Manager functions. For example, services that open an existing resource call on the Object Manager to look up the resource name in the Object Manager namespace, ensure the caller has sufficient rights to open the resource, and allocate and return a handle to identify the open instance. Services that require a handle to a previously opened resource call the Object Manager to translate the handle to the object it represents.
The Object Manager calls other Executive subsystems when necessary. Every object type has functions that execute when NT performs particular operations on objects of that type. Thus, when the Object Manager creates a file object to represent an open file, the Object Manager invokes the I/O Manager's function for opening files. Similarly, the Object Manager creates an associated process object for an open process and invokes the Process Manager's function for opening processes.
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