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The following is necessary condition for deadlock :
1) Mutual Exclusive
Mutual Exclusive means only one process will access or take resource at a time.
2) Hold and Wait
Hold and Wait means a process is holding some resource and waiting for another resource and it does not allow another process to execute.
3) Non pre-emption
Non pre-emption means No any running process is allow to pre-empted means process is executed until that process is not completed.
4) Circular Wait
For circular wait condition there must be any circular wait condition like
P1,P2,P3 are the process and then P1 wants the resource to P2 and P2 wants to resource to P3 and P3 wants the resource to P1 this is the condition for circular wait.
If all the case satisfy then there is deadlock.