Define deadlock? Explain the necessary conditions for deadlock to occur. (5) Ans: Deadlock is a situation, in which processes never finish executing and system resources are tied up, preventing other jobs from starting. A process requests resources; if the resources are not available at that time, the process enters a wait state. Waiting processes may never again change state, because the resources they have requested are held by other waiting processes, thereby causing deadlock.Necessary conditions for deadlock to occur are: i. Mutual exclusion: At least one resource must be held in a nonsharable mode; that is, only one process at a time can use the resource. If another process requeststhat resource, the requesting process must be delayed until the resource has been released. ii. Hold and wait: A process must be holding at least one resource and waiting to acquire additional resources that are currently being held by other processes. iii. No pre-emption: Resources cannot be pre-empted; that is, a resource can be released only voluntarily by the process holding it, after the process holding it has completed its task. iv. Circular wait: A set{P0, P1,……, Pn) of waiting processes must exist such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, ……., Pn-1 is waiting for a resource that is held by Pn and Pn is waiting for a resource that is held by P0. All four conditions must hold simultaneously for a deadlock to occur and conditions are not completely independent. For example, the circular-wait implies the hold-andwait conditio