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 LexicalScopes are disallowed (or restricted), a StaticChain is unnecessary. Java InnerClasses 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,
ActivationRecords maintain a strict
LastInFirstOut discipline and are thus stored on a stack (often referred to as
TheStack). For this reason,
ActivationRecords 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)
In languages with these, other arrangements are often used, see
ContinuationImplementation for examples.
Contributors:
ScottJohnson
CategoryLanguageFeature