re-entrant

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)