Question
List and briefly explain the four necessary conditions for a deadlock to occur in a
system.
system.
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GPT-4o mini
A deadlock is a situation in a computer system where a set of processes become stuck in a state where they cannot proceed because each process is waiting for a resource that is held by another process in the set. For a deadlock to occur, four necessary conditions must be present simultaneously:
1. **Mutual Exclusion**:
- At least one resource must be held in a non-sharable mode. This means that if one process is using a resource, other processes must be prevented from using it. For instance, if a printer is assigned to one process, no other process can access that printer until the first process releases it.
2. **Hold and Wait**:
- Processes holding resources are allowed to request additional resources without releasing the resources they currently hold. This means a process can hold onto its resources while waiting for another resource to become available. For example, a process might hold onto a printer while waiting to acquire a scanner.
3. **No Preemption**:
- Resources cannot be forcibly taken from the processes holding them; they must be voluntarily released by the process holding the resource. In other words, if another process requires a resource that is currently allocated to another process, it cannot preempt it. The process must wait until the resource is released.
4. **Circular Wait**:
- There exists a circular chain of processes, each of which is waiting for a resource held by the next process in the chain. For example, Process A is waiting for a resource held by Process B, Process B is waiting for a resource held by Process C, and Process C is waiting for a resource held by Process A. This creates a closed loop that prevents any of the processes from proceeding.
These four conditions together create the perfect setup for a deadlock situation, allowing processes to block each other indefinitely if none of the conditions is addressed or resolved.
1. **Mutual Exclusion**:
- At least one resource must be held in a non-sharable mode. This means that if one process is using a resource, other processes must be prevented from using it. For instance, if a printer is assigned to one process, no other process can access that printer until the first process releases it.
2. **Hold and Wait**:
- Processes holding resources are allowed to request additional resources without releasing the resources they currently hold. This means a process can hold onto its resources while waiting for another resource to become available. For example, a process might hold onto a printer while waiting to acquire a scanner.
3. **No Preemption**:
- Resources cannot be forcibly taken from the processes holding them; they must be voluntarily released by the process holding the resource. In other words, if another process requires a resource that is currently allocated to another process, it cannot preempt it. The process must wait until the resource is released.
4. **Circular Wait**:
- There exists a circular chain of processes, each of which is waiting for a resource held by the next process in the chain. For example, Process A is waiting for a resource held by Process B, Process B is waiting for a resource held by Process C, and Process C is waiting for a resource held by Process A. This creates a closed loop that prevents any of the processes from proceeding.
These four conditions together create the perfect setup for a deadlock situation, allowing processes to block each other indefinitely if none of the conditions is addressed or resolved.