A data structure containing the important state information for a particular instance of a function call (or something that resembles a function call). May exist entirely in memory, or be partially stored in registers. Typically, an ActivationRecord contains the following:

* A pointer to the ActivationRecord of the caller (commonly known as the DynamicChain), though in a language with ContinuationPassingStyle even this isn't necessary (when the function returns, it does so via the supplied continuation; rather than the DynamicChain).
* In languages which allow nested LexicalScoping, a pointer to the ActivationRecord of the enclosing scope (aka the ReferencingEnvironment) - this pointer is called the StaticChain. In most such languages, it is generally a requirement that ''somewhere'' up the call chain, the enclosing function have been called (but in concurrent languages it may have been called in a different thread). In languages like C/C++ where nested LexicalScope''''''s are disallowed (or restricted), a StaticChain is unnecessary. Java InnerClass''''''es also seem to be designed in order to avoid a StaticChain.
* Local variables (frequently cached in registers)
* Actual parameters to the function (also frequently cached in registers)
* Space for the (eventual) return value of the function.

<add anything I forgot>

In many (most) programming languages, ActivationRecord''''''s maintain a strict LastInFirstOut discipline and are thus stored on a stack (often referred to as TheStack). For this reason, ActivationRecord''''''s are also commonly known as ''stack frames''. In the context of languages (like C) where a stack is always used; this terminology is correct. As other languages don't use stacks for this purpose (many Smalltalk implementations, StacklessPython), ActivationRecord is a more correct term when talking about languages in general.

This scheme works well, unless you have the following features in your language (which violate LIFO discipline)

* FirstClass LexicalClosure''''''s (a closure can outlive its referencing environment)
* Continuations, excluding UpwardContinuation''''''s
* CoRoutine''''''s, BlocksInSmalltalk, and a few other control structures that are found in various languages.

In languages with these, other arrangements are often used, see ContinuationImplementation for examples.

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Contributors: ScottJohnson

CategoryLanguageFeature