Deadlock Characterization

Deadlock can arise if four conditions hold simultaneously.

• Mutual exclusion: only one process at a time can use a (non-sharable) resource.
• Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes.
• No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task.
• Circular wait: there exists a set {P0, P1, …, P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and P0 is waiting for a resource that is held by P0.