Dr. Dobb's Journal - October 2007 - (Page 37)

addresses. In Figures 1–3, some data items are associated with each tracked region. These data items include the call chain leading to the creation of the region, and a timestamp. The timestamp is used when the program runs out of memory, to pick a region that’s been unused for a long time as a target for reuse. The call chain can serve to identify the component responsible for creating the region. Listing Three (available electronically) provides a simple routine for collecting a call chain on the Intel platform. The vectored exception handler used in Listing One (available electronically) is a perfect fit for dealing with out-of-memory exceptions generated by any component in your process. With this kind of exception handling, even components you didn’t develop yourself, such as third-party modules loaded by your program, benefit from your watchdog’s protection. The DllMain() routine in Listing One sets up the module’s vectored exception handler, up front, before intercepting the target system calls. Because this handler invokes routines that depend on virtual memory tracking, as arranged by intercepting these system calls, the interception is not performed unless the handler is successfully put in place first. Vectored exception handling is available on current versions of Windows, including Vista and XP. If your operating system doesn’t support vectored exception handling, then you need to provide some means of handling out-of-memory exceptions on each thread that is started. Like the vectored handler in Listing One, your handler can invoke a routine that uses tracked data about your program’s virtual memory regions to keep the program alive, as in Figure 4, which introduces the concept of stealing regions. In Figures 4 and 5, a reserved virtual memory region is considered stolen when it’s committed for use by a component that didn’t reserve it. Similarly, an unused committed page is considered stolen when it’s unreserved and recommitted for use by a component other than the one that originally committed it. How do you know the original component won’t come along and try to use its reserved or committed space? You don’t—but your watchdog can try to head off this possibility by stealing the memory that has been unused for the longest amount of time, out of all the memory it’s tracking. The real protection involves, well, protection. On Windows, the VirtualProtect() API function can be applied to each stolen page or region to generate an exception when the region is accessed. The use of an exception handler to deal with all accesses to a region means, of course, that your program could run slowly when components start stealing regions. On the other hand, poor performance is generally more tolerable than a crash. If your program is going to stay alive for long after stealing a region that was originally Some components proactively commit virtual memory and never use it October 2007 l www.ddj.com l Dr. Dobb’s Journal 37 http://www.aspetech.com/ddj http://www.aspetech.com/ddj http://www.aspetech.com/ddj http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - October 2007

Cover Contents Hmmmm Alia Vox Developer Diaries Developer’s Notebook AI: It’s OK Again! Conversations Visual Cryptography and Bit-Plane Complexity Segmentation Inside the Windows Vista Disk Encryption Algorithm Memory-Aware Components Software and the Core Description Process Logging In C++ Effective Concurrency The Agile Edge Swaine’s Flames

Dr. Dobb's Journal - October 2007

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