ANS:C - reentrant procedures

A reentrant procedure is one in which a single copy of the program code can be shared by multiple users during the same period of time. Re-entrance has two key aspects: The program code cannot modify itself and the local data for each user must be stored separately.

In a shared system, reentrancy allows more efficient use of main memory: One copy of the program code is kept in main memory, but more than one application can call the procedure. Thus, a reentrant procedure must have a permanent part (the instructions that make up the procedure) and a temporary part (a pointer back to the calling program as well as memory for local variables used by the program).

Each execution instance, called activation, of a procedure, will execute the code in the permanent part but must have its own copy of local variables and parameters. The temporary part associated with a particular activation is referred to as an activation record.

The most convenient way to support reentrant procedures is by means of a stack. When a reentrant procedure is called, the activation record becomes part of the stack frame that is created on a procedure call.