These are proactive strategies aimed at managing deadlocks, differing in their approach to ensuring deadlock-free operation. Deadlock prevention attempts to eliminate the possibility of deadlocks by designing or operating the system in such a way that at least one of the four necessary conditions for deadlock can never simultaneously hold. By proactively violating even one of these conditions, the logical possibility of a deadlock occurring is entirely eliminated, guaranteeing a deadlock-free system, though often at the cost of reduced system utilization or increased process waiting times. Deadlock avoidance, in contrast, requires that the system be given some a priori information about the maximum resource needs of each process. With this foresight, the operating system can make dynamic decisions about whether to grant a resource request, ensuring that the system never enters an unsafe state that could lead to a deadlock. It is less restrictive than prevention, as it doesn't preclude the conditions for deadlock but manages resource allocation to bypass unsafe scenarios.

To prevent deadlocks, we must break at least one of the four necessary conditions:
1. Preventing Mutual Exclusion: The strategy here is to make resources shareable whenever possible, allowing multiple processes to access them concurrently without conflict. However, this is largely unfeasible for resources inherently requiring exclusive access, such as a printer or a writeable file, as violating mutual exclusion for these would lead to data corruption or incorrect operation. This condition is often the most difficult to circumvent.
2. Preventing Hold and Wait: Two main strategies exist. The "all-or-nothing" approach mandates that a process must request and be granted all its required resources at once, before it begins execution. If the complete set is not available, the process waits without holding any resources. While simple, this leads to low resource utilization (resources held for entire execution, even if only used briefly) and potential starvation for processes needing many popular resources. Alternatively, the "release all before requesting new" approach requires a process to release all currently held resources before it can request any new ones. This can be impractical and inefficient for multi-stage operations, potentially leading to loss of intermediate work or significant overhead for context saving and restoration.
3. Preventing No Preemption: This involves forcibly taking resources away from a process. If a process holding resources requests an additional resource that cannot be immediately allocated, then all resources currently held by the requesting process are preempted (released). These preempted resources are added to the list of resources for which the process is waiting. The process is then blocked until it can re-acquire its old resources plus the newly requested ones. This is very difficult to implement reliably, especially for resources whose state changes during use (e.g., a partially written file), as forced preemption can lead to data inconsistency or loss. It is more feasible for resources whose state can be easily saved and restored, like CPU registers.
4. Preventing Circular Wait: The most common and practical prevention method involves imposing a total linear ordering on all resource types in the system. Processes are then strictly required to request resources only in an increasing (or decreasing) order of enumeration. For example, if resources are ordered R1, R2, …, Rm, a process, once it holds Ri, can only request Rj if j>i. This strategy logically breaks the possibility of a circular dependency, as a process cannot simultaneously hold a higher-ranked resource while waiting for a lower-ranked one to complete a cycle. The drawbacks include potential inefficiencies (processes holding resources longer than needed) and the practical difficulty of defining an optimal and globally adhered-to ordering for all resources in a complex system.

Deadlock avoidance approaches operate on the principle of ensuring the system never enters an unsafe state. A system is in a safe state if there exists at least one safe sequence of all the processes () such that for each process Pi in this sequence, the resources that Pi could still request (its Need) can be satisfied by the currently available resources plus the resources currently held by all processes Pj that appear before Pi in the sequence. If such a sequence exists, no deadlock can occur. An unsafe state does not necessarily mean a deadlock exists, but it means there is no guarantee that all processes can complete, making a deadlock possible if subsequent requests are granted unwisely. The primary goal of avoidance algorithms is to prevent entry into any unsafe state. The Banker's Algorithm is the most prominent deadlock avoidance algorithm, suitable for systems with multiple instances of each resource type. It requires processes to declare their maximum resource needs (Max matrix) beforehand. The algorithm then simulates resource allocations to determine if the resulting state would be safe. If safe, the allocation proceeds; otherwise, the requesting process is made to wait.

Key data structures for the Banker's Algorithm include:
● Available: A vector indicating the number of available instances for each resource type.
● Max: An n×m matrix defining the maximum demand of each process Pi for each resource type Rj.
● Allocation: An n×m matrix showing the number of resources of each type currently allocated to each process.
● Need: An n×m matrix indicating the remaining resources needed by each process (Need[i][j] = Max[i][j] - Allocation[i][j]).