reentrant code

(computer science) A subprogram in a time-sharing or multiprogramming system that can be shared by a number of users, and can therefore be applied to a given user program, interrupted and applied to some other user program, and then reentered at the point of interruption of the original user program. Also known as reentrant code.

A programming routine that can be used by multiple programs simultaneously. It is used in operating systems and other system software as well as in multithreading, where concurrent events are taking place. It is written so that none of its code is modifiable (no values are changed) and it does not keep track of anything. The calling programs keep track of their own progress (variables, flags, etc.), thus one copy of the reentrant routine can be shared by any number of users or processes.

Conceptually, it is as if several people were each baking a cake from a single copy of a recipe on the wall. Everyone looks at the master recipe, but keeps track of their own progress by jotting down the step they are at on their own scratchpad so they can pick up where they left off. The master recipe is never disturbed.

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