Explain the concept of Reentrancy in Operating systems.?-Os interview questions

Explain the concept of Reentrancy in Operating systems.?-Os interview questions
It is a useful, memory-saving technique for multiprogrammed timesharing systems. A Reentrant Procedure in O.S is one in which multiple users can share a single copy of a program during the same period. Reentrancy has 2 key aspects: The program code cannot modify itself, & the local data for each user process must be stored separately.
Thus, the permanent part is the code, & the temporary part is the pointer back to the calling program & local variables used by that program. Each execution instance is called activation. It executes 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. Generally, the activation record is kept on the stack.

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