The Coffman conditions, which are necessary for a deadlock to occur, include mutual exclusion, hold and wait, no preemption, and circular wait. These conditions were formulated by Edward G. Coffman Jr., an American computer scientist, and they help to identify situations where deadlocks can occur.
Mutual Exclusion: A resource can only be owned by one process at a time if it is not shareable. For instance, if one process uses a printer, the other processes must wait for the printer to become available. This exclusivity is shared by some physical and logical resources. Deadlock can result from several circumstances, including the potential for dispute when a resource is mutually exclusive.
Hold and Wait: In a system that frequently experiences deadlocks, processes might make incremental resource requests without surrendering the resources they now possess. Multiple processes can become interdependent as a result of a process’s ability to hold onto specific resources (such as memory blocks, locks, or I/O devices) while it waits for other resources. For example, while Process B keeps Resource Y and waits for Resource X, Process A may hold Resource X and wait for Resource Y. Because it lays the groundwork for cyclic waiting, this dependence chain is necessary for the deadlock to occur.
No Preemption: In a system with no preemption, once a process acquires a resource, it cannot be removed until it decides to release it. Without preemption, if one process is waiting for a resource that another process has, it cannot simply “take over” the resource. Instead, it has to wait till the other process has finished using it. For example, if Process A has a file lock and Process B needs it, Process B must wait until Process A has completed its task and released the file. This lack of preemption prolongs the waiting period, increasing the likelihood of a deadlock if the other Coffman requirements are present.
Circular Wait: There is a circular chain of processes, with each process waiting for a resource held by the one that comes after it. The final process in the chain is waiting for a resource held by the first, resulting in a closed loop.
All four of the Coffman requirements must be met at the same time for a deadlock to occur in any system. This implies that deadlock is impossible in systems where even one of these requirements is not met. By looking at these conditions, system designers can put strategies in place to prevent deadlocks by making sure that at least one of them cannot hold. Some of these strategies include enabling resource preemption, ordering resources to prevent circular waits, or requesting all resources upfront to avoid hold-and-wait scenarios.