What is a Re-entrant procedure?

What is a re entrant procedure and can you give an example scenario of when it is used?

Edit: Also, can multiple processes access a re entrant procedure in parallel?

Please prov开发者_运维知识库ide a different way of explaining than wikipedia as I don't totally understand their description hence my question here

The idea behind re-entrancy is that the routine may be called while it is in the middle of executing already and it will still work right.

Generally this is achieved by it using only parameters and local variables declared on the stack (in C terms, no static locals). It would also be important that it not lock any global resources during execution.

Now, you may ask, "How would such a weird thing as a routine being run multiple times at once happen?" Well, some ways this could happen are:

• The routine is recursive (or mutually-recursive with some other set of routines).
• It gets called by another thread.
• It gets called by an interrupt.

If any of these happen, and the routine is modifying a global (or C static local), then the new execution could potentially wipe out the changes the first execution made. As an example, if that global was used as a loop control variable, it might cause the first execution, when it finally gets to resume, to loop the wrong number of times.

It is a subroutine which can be called when it is already active. For instance recursive functions are often reentrant. Functions which are called from signal handlers must be reentrant as well. A reentrant function is thread-safe but not all thread-safe one are reentrant.

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 procedure call