- **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 { P<sub>0</sub>, P<sub>1</sub>, …, P<sub>0</sub> } of waiting processes such that P<sub>0</sub> is waiting for a resource that is held by P<sub>1</sub>, P<sub>1</sub> is waiting for a resource that is held by P<sub>2</sub>, …, P<sub>n –1</sub> is waiting for a resource that is held by P<sub>n</sub>, and P<sub>0</sub> is waiting for a resource that is held by P<sub>0</sub>.
### Methods for Handling Deadlocks
- Ensure that the system will never enter a deadlock state.
- Allow the system to enter a deadlock state and then recover.
- Ignore the problem and pretend that deadlocks never occur in the system; **used by most operating systems, including UNIX**.
### Deadlock Prevention
- **Mutual Exclusion** – It is not required for sharable resources; must hold for nonsharable resources.
- **Hold and Wait** – It must guarantee that whenever a process requests a resource, it does not hold any other resources.
- Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none.
- If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released.
- **Circular Wait** – It imposes a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration.
- [Operating System | Process Management | Deadlock Introduction](https://www.geeksforgeeks.org/operating-system-process-management-deadlock-introduction/)