Editing Memory management (section)

== Automated memory management ==

The proper management of memory in an application is a difficult problem, and several different strategies for handling memory management have been devised.

=== Automatic management of call stack variables ===
{{see also|Automatic variable|Call stack}}
In many programming language implementations, the runtime environment for the program automatically allocates memory in the [[call stack]] for non-static [[local variable]]s of a [[subroutine]], called [[automatic variable]]s, when the subroutine is called, and automatically releases that memory when the subroutine is exited. Special declarations may allow local variables to retain values between invocations of the procedure, or may allow local variables to be accessed by other subroutines. The automatic allocation of local variables makes [[Recursion (computer science)|recursion]] possible, to a depth limited by available memory.

=== Garbage collection ===
{{main|Garbage collection (computer science)}}
Garbage collection is a strategy for automatically detecting memory allocated to objects that are no longer usable in a program, and returning that allocated memory to a pool of free memory locations. This method is in contrast to "manual" memory management where a programmer explicitly codes memory requests and memory releases in the program. While automatic garbage collection has the advantages of reducing programmer workload and preventing certain kinds of memory allocation bugs, garbage collection does require memory resources of its own, and can compete with the application program for processor time.

=== Reference counting ===
{{main|Reference counting}}
Reference counting is a strategy for detecting that memory is no longer usable by a program by maintaining a counter for how many independent pointers point to the memory. Whenever a new pointer points to a piece of memory, the programmer is supposed to increase the counter. When the pointer changes where it points, or when the pointer is no longer pointing to any area or has itself been freed, the counter should decrease. When the counter drops to zero, the memory should be considered unused and freed. Some reference counting systems require programmer involvement and some are implemented automatically by the compiler. A disadvantage of reference counting is that [[circular reference]]s can develop which cause a memory leak to occur. This can be mitigated by either adding the concept of a "weak reference" (a reference that does not participate in reference counting, but is notified when the area it is pointing to is no longer valid) or by combining reference counting and garbage collection together.

=== Memory pools ===
{{main|Region-based memory management}}
A memory pool is a technique of automatically deallocating memory based on the state of the application, such as the lifecycle of a request or transaction. The idea is that many applications execute large chunks of code which may generate memory allocations, but that there is a point in execution where all of those chunks are known to be no longer valid. For example, in a web service, after each request the web service no longer needs any of the memory allocated during the execution of the request. Therefore, rather than keeping track of whether or not memory is currently being referenced, the memory is allocated according to the request or lifecycle stage with which it is associated. When that request or stage has passed, all associated memory is deallocated simultaneously.