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Definition: re-entrant 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 re-entrant routine can be shared by any number of users or processes.

Several Chefs; One Recipe
Conceptually, re-entrant coding is as if several people were each baking a cake from a single copy of a recipe on the wall. Everyone follows the master recipe but keeps track of their individual progress by jotting down the step they are working on so they can pick up where they left off. The master recipe is never disturbed.