garbage collection

Garbage collection is the systematic recovery of pooled computer storage that is being used by a program when that program no longer needs the storage. This frees the storage for use by other programs (or processes within a program). It also ensures that a program using increasing amounts of pooled storage does not reach its quota (in which case it may no longer be able to function).

Garbage collection is an automatic memory management feature in many modern programming languages, such as Java and languages in the .NET framework. Languages that use garbage collection are often interpreted or run within a virtual machine like the JVM. In each case, the environment that runs the code is also responsible for garbage collection.

In older programming languages, such as C and C++, allocating and freeing memory is done manually by the programmer. Memory for any data that can't be stored within a primitive data type, including objects, buffers and strings, is usually reserved on the heap. When the program no longer needs the data, the programmer frees that chunk of data with an API call. Because this process is manually controlled, human error can introduce bugs in the code. Memory leaks occur when the programmer forgets to free up memory after the program no longer needs it. Other times, a programmer may try to access a chunk of memory that has already been freed, leading to dangling pointers that can cause serious bugs or even crashes.

Programs with an automatic garbage collector (GC) try to eliminate these bugs by automatically detecting when a piece of data is no longer needed. A GC has two goals: any unused memory should be freed, and no memory should be freed unless the program will not use it anymore. Although some languages allow memory to be manually freed as well, many do not.

This was last updated in March 2008

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