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.