A deadlock is a situation in computer science and operating systems where two or more processes are unable to proceed because each is waiting for the other to release a resource. For a deadlock to occur, four necessary conditions, known as the Coffman conditions, must be satisfied simultaneously. These conditions were identified by computer scientist Edward G. Coffman, Jr. Here are the four necessary conditions for a deadlock to occur:
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Mutual Exclusion (Mutex):
- The first condition for a deadlock is that at least one resource must be held in a non-sharable (exclusive) mode. This means that once a process acquires the resource, no other process can access it until the first process releases it. Mutual exclusion is a fundamental property of certain types of resources, such as printers, certain sections of memory, and hardware devices.
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Hold and Wait (Hold-and-Wait):
- The second condition is that a process must be holding at least one resource and waiting to acquire additional resources held by other processes. In other words, a process cannot release any resources while waiting for another resource. This condition creates a situation where processes may be holding resources and waiting indefinitely for additional resources to become available.
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No Preemption:
- The third condition is that resources cannot be preempted, meaning that once a process holds a resource, it cannot be forcibly taken away. Resources can only be released voluntarily by the process holding them. Preemption would involve forcibly taking a resource away from a process, which is not allowed in systems that satisfy the no preemption condition.
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Circular Wait:
- The fourth condition is that there must be a circular waiting chain of processes, where each process in the chain is waiting for a resource held by the next process in the chain. This circular dependency creates a situation where no process can proceed, as each is waiting for a resource that is held by another process in the cycle.
For a deadlock to occur, all four of these conditions—Mutual Exclusion, Hold and Wait, No Preemption, and Circular Wait—must be simultaneously satisfied. Breaking any one of these conditions can prevent or resolve deadlocks in a system. Deadlock prevention, avoidance, and recovery strategies are used in operating systems to manage and mitigate the impact of deadlocks when they are detected or anticipated.
