< programming > Used to describe code which can have multiple simultaneous, interleaved, or nested invocations which will not interfere with each other. This is important for parallel processing , recursive functions or subroutines, and interrupt handling .
It is usually easy to arrange for multiple invocations (e.g. calls to a subroutine) to share one copy of the code and any read-only data but, for the code to be re-entrant, each invocation must use its own copy of any modifiable data (or synchronised access to shared data). This is most often achieved using a stack and allocating local variables in a new stack frame for each invocation. Alternatively, the caller may pass in a pointer to a block of memory which that invocation can use (usually for outputting the result) or the code may allocate some memory on a heap , especially if the data must survive after the routine returns.
Re-entrant code is often found in system software, such as operating systems and teleprocessing monitors . It is also a crucial component of multithreaded programs where the term "thread-safe" is often used instead of "re-entrant".
(1996-12-21)