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<Pushing the Limits of Windows: USER and GDI Objects - Part 1 ⁄ >

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So far in the Pushing the Limits of Windows series, I've focused on resources managed by the Windows operating system kernel, including physical and virtual memory, paged and nonpaged pool, processes, threads and handles. In this and the next post, however, I will explore two resources managed by the Windows window manager, USER and GDI objects, that represent window elements (like windows and menus) and graphics constructs (like pens, brushes and drawing surfaces). Just like for the other resources I've discussed in previous posts, exhausting the various USER and GDI resource limits can lead to unpredictable behavior, including application failures and an unusable system.

Sessions, Window Stations and Desktops

There are a few concepts that make the relationship between USER objects, GDI objects, and the system more clear. The first is the session. A session represents an interactive user logon that has its own keyboard, mouse and display and represents both a security and resource boundary.

The session concept was first introduced with Terminal Services (now called Remote Desktop Services) in Windows NT 4 Terminal Server Edition, where the physical display, keyboard and mouse concepts were virtualized for each user interactively logging on to a system remotely, and core Terminal Services functionality was built into Windows 2000 Server. In Windows XP, sessions were leveraged to create the Fast User Switching (FUS) feature that allows you to switch between multiple interactive logins on the same physical display, keyboard and mouse.

Thus, a session can be connected with the physical display and input devices attached to the system, connected with a logical display and input devices like ones presented by a Remote Desktop client application, or be in a disconnected state like exists when you switch away from a session with Fast User Switching or terminate a Remote Desktop Client connection without logging off the session.

Every process is uniquely associated with a specific session, which you can see when you add the Session column to Sysinternals Process Explorer. This screenshot, in which I've collapsed the process tree to show only processes that have no parent, is from a Remote Desktop Services (RDS - formerly Terminal Server Services) system that has four active sessions: session 0 is the dedicated session in which system processes execute on Windows Vista and higher; session 1 is the session in which I'm writing this post; Session 2 is the session of another user account that I'm concurrently logged into from another system; and finally, session 3 is one that Remote Desktop Services proactively created to be ready for the next interactive logon:

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