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Blockers are a set of processes that have acquired an asset and are waiting to receive another but is in turn blocked the same set.
A deadlock may occur in the case in which there are four conditions simultaneously:
-Mutual exclusion, meaning that data can be used by only one process at a time.
-Hold & wait in the sense that a process holds a resource that is waiting for another held by other processes
Preemprion-no in the sense that a resource can be released only by a process that uses
-Circular wait, there exists a set of processes waiting for a resource.

Managing Deadlock
To prevent deadlock and therefore decrease their likelihood must constrain the demands that can be made: to do this using several techniques.
All Request First (RAF): processes must acquire all the resources you need before you start running.
Before Release Request (RBF): all processes can acquire resources only if they have already acquired other and must release the resources already in use.
No Preemption: If a process requires a non-issue available to all those he has already made the request. The freed resources are added to the resources that a process must acquire. The process is paused and awakened when all required resources are free.
Circular wait: It imposes a total ordering relationship between resources. Each process must request resources in a certain order. If you do not find a resource must release all those previously acquired.

Avoid Deadlock
You try to simulate the allocation to decide whether it is safe. It examines the state of the graph dynamically allocated to verify that there are cycles. It requires that the system has some information is always available and is assigned a maximum number of resources. The allocation status of a resource is defined by the number of available resources.
In the moment in which it applies to the system a resource, the system checks whether that operation could lead to an unsafe state and in this case the resource is not allocated. Each application must state the maximum use of resources. In the event that this limit is exceeded, the process is blocked.

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