Concept of Reentrancy
It is a useful, memory-saving method for multiprogrammed timesharing machines. A Reentrant method is one in which multiple clients can share a single copy of a program during the similar period. Reentrancy has 2 key functions: The program code cannot change itself, and the local data for each client process must be stored in different disk. Thus, the permanent phase is the code, and the temporary phase is the pointer back to the calling function and local variables needed by that code. Each operation instance is known as activation. It runs the code in the permanent phase, but has its permanent copy of local parameters or variables. The temporary phase associated with each activity is the activation record. usually, the activation record is store on the stack.