Explain the concept of Reentrancy?

It is a useful, memory-saving method for multiprogrammed timesharing systems. A Reentrant Procedure is one in which many users can share a single copy of a program during the similar period. Reentrancy has 2 key aspects: The program code cannot change itself, and the local data for each user process must be kept separately.

Therefore, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each implementation instance is called activation. It implements the code in the permanent part, but has its own copy of local variables/parameters. The temporary part associated with each activation is the activation record.