Deadlocks

What is a deadlock?

Deadlocks are the most common scenarios occuring in a multi-threaded environment. In computing/technical terms a deadlock can be a scenario where there are two or more parties waiting for a resource but neither of them can proceed.

How can a deadlock happen?

A deadlock situation can arise if and only if the following four conditions occur at the same time in the system -

1. Mutual Exclusion: At least one resource is held in a non-sharable mode that is only one process at a time can use the resource. If another process requests that resource, the requesting process must be delayed until the resource has been released.
2. Hold and Wait:There must exist a process that is holding at least one resource and is waiting to acquire additional resources that are currently being held by other processes. 
3. No Preemption: Resouces cannot be preempted; that is, a resource can only be released voluntarily by the process holding it, after the process has completed its task. 
4. Circular Wait: There must exist a set {p₀, p₁,.....p_n} of waiting processes such that p₀ is waiting for a resource which is held by p₁, p₁ is waiting for a resource which is held by p₂,..., p_n-1 is waiting for a resource which is held by p_n and p_n is waiting for a resource which is held by p₀. 

Ways to prevent a deadlock - 
There can be many ways to prevent a deadlock -

1. Semaphore.
2. Monitors.
3. Mutex variable.