What is a handle leak?

A handle leak is a type of resource leak, a situation where a finite computing resource, such as memory, is excessively consumed by an application. Handle leaks occur when an application opens a “handle”, a special type of reference to a system resource, and fails to close that handle when it is no longer needed. They have the potential to reduce performance, cause application or system crashes, and can be a symptom of serious bugs or sloppy coding in a software program. Many troubleshooting tools are available to programmers to help them fight leaks.

A handle leak is a resource exhaustion issue that can lead to computer system crashes.

In computer programming, an identifier is a structure that references a system resource, such as a block of memory or a file on a disk. Identifiers provide a layer of abstraction between system resources and applications and are generally managed by a computer’s operating system through a “tag table” that combines each identifier with a resource. A programmer can use an identifier to refer to a window which is then drawn to the screen by the operating system.

A well-behaved application will free up resources that are no longer needed, freeing up resources and ensuring that the operating system doesn’t end up managing a resource that is not in use. Programming errors, software bugs, and software incompatibilities can cause programs to keep handles they no longer need. As the application continues to run, it may accumulate a large number of unnecessary identifiers. This is known as a loop leak.

Dealing with leaks, like other types of resource leaks, can lead to slow performance, system instability, or, in extreme cases, a computer crash. This is because handles consume finite resources such as system memory, and if an application generates an excessive number of handles without closing them, those resources are depleted. The exact threshold at which a handle leak starts to cause problems depends on the computer’s operating system, the amount of available memory, and other factors. A leak where a few dozen handles are left open will likely not be noticed by users, while a handle leak of ten thousand or more is a sign of a serious bug in the application code.

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Programmers need to pay special attention when dealing with leaks, given their potential to cause problems. The most basic method of diagnosing a handle leak is to look at the handle count, which is the number of handles a process or application currently has open while the application is running. The Windows® Task Manager can display this information, and third-party utilities for Windows® and other operating systems can also be useful in identifying and tracking leaky identifiers.

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