Sometimes a process has to reserve more than one resource. For example, a process which copies files from one tape to another generally requires two tape drives. A process which deals with databases may need to lock multiple records in a database.
In general, resources allocated to a process are not preemptable; this means that once a resource has been allocated to a process, there is no simple mechanism by which the system can take the resource back from the process unless the process voluntarily gives it up or the system administrator kills the process. This can lead to a situation called deadlock. A set of processes or threads is deadlocked when each process or thread is waiting for a resource to be freed which is controlled by another process. Here is an example of a situation where deadlock can occur.
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