what are necessary conditions for Deadlock explain with RAG & example.

Deadlock is a situation where a set of processes are blocked because each process is holding a resource and waiting for another resource acquired by some other process. There are four necessary conditions for a deadlock to occur, also known as Coffman conditions:

1. Mutual Exclusion: At least one resource must be held in a non-shareable mode. Only one process can use the resource at any given instant of time.

2. Hold and Wait: A process holding at least one resource is waiting to acquire additional resources held by other processes.

3. No Preemption: A resource can be released only voluntarily by the process holding it.

4. Circular Wait: A set of processes {p0, p1, ..., pn} exists in such a way that p0 is waiting for a resource held by p1, p1 is waiting for a resource held by p2, ..., pn is waiting for a resource held by p0.

A Resource Allocation Graph (RAG) is used to check the occurrence of deadlock. In this graph, each process and each resource type is represented as a node. If a process requests a resource, a request edge is drawn from the process to the resource. If a process holds a resource, an assignment edge is drawn from the resource to the process.

Example: Let's consider a system with three processes {P1, P2, P3} and three resources {R1, R2, R3}. P1 holds R1 and requests for R2, P2 holds R2 and requests for R3, and P3 holds R3 and requests for R1. This situation can be represented as a circular wait in the RAG, which is a clear indication of deadlock.