Editing Reentrancy (computing) (section)

==Background==
Reentrancy is not the same thing as [[idempotence]], in which the function may be called more than once yet generate exactly the same output as if it had only been called once. Generally speaking, a function produces output data based on some input data (though both are optional, in general). Shared data could be accessed by any function at any time. If data can be changed by any function (and none keep track of those changes), there is no guarantee to those that share a datum that that datum is the same as at any time before.

Data has a characteristic called [[Scope and extent|scope]], which describes where in a program the data may be used. Data scope is either [[global variable|global]] (outside the [[variable scope|scope]] of any function and with an indefinite extent) or [[Local variables, recursion and reentrancy|local]] (created each time a function is called and destroyed upon exit).

Local data is not shared by any routines, re-entering or not; therefore, it does not affect re-entrance. Global data is defined outside functions and can be accessed by more than one function, either in the form of [[global variable]]s (data shared between all functions), or as [[static variable]]s (data shared by all invocations of the same function). In [[object-oriented programming]], global data is defined in the scope of a class and can be private, making it accessible only to functions of that class. There is also the concept of [[instance variable]]s, where a class variable is bound to a class instance. For these reasons, in object-oriented programming, this distinction is usually reserved for the data accessible outside of the class (public), and for the data independent of class instances (static).

Reentrancy is distinct from, but closely related to, [[thread-safety]]. A function can be [[thread-safe]] and still not reentrant. For example, a function could be wrapped all around with a [[mutex]] (which avoids problems in multithreading environments), but, if that function were used in an interrupt service routine, it could starve waiting for the first execution to release the mutex. The key for avoiding confusion is that reentrant refers to only ''one'' thread executing. It is a concept from the time when no multitasking operating systems existed.