• A set of process is in a deadlock state if each process in the group is waiting for an event that can be caused by only another process in the set. In other words, each member of the group of deadlock processes is waiting for a resource that can be released only by a deadlock process. Not any of the processes can run, none of them can release any resources, and not any of them can be awakened. It is significant to note that the number of processes and the number and kind of resources possessed and requested are unimportant.
• The simplest instance of deadlock is where process 1 has been allocated non-shareable resources A, say, a tap drive, and process 2 has be owed non-sharable resource B, say, a printer. At present, if it turns out that process 1 needs resource B (printer) to proceed and process 2 needs resource A (the tape drive) to proceed and these are the only two processes in the system, each one is blocked the other and all valuable work in the system stops. This state is termed deadlock. The system is in deadlock state since each process holds a resource being requested by the other process neither process is willing to release the resource it holds.
• The resources may be either physical or logical. Instances of physical resources are Printers, Tape Drivers, Memory Space, and CPU Cycles. Instances of logical resources are Files, Semaphores, and Monitors.
• Preemptable and Nonpreemptable Resources
Resources come in two types: preemptable and nonpreemptable.
A preemptable resource is one that can be taken away from the process with no ill effects. Memory is an instance of a preemptable resource. Alternatively, a nonpreemptable resource is one that cannot be taken away from process (with no causing ill effect). For instance, CD resources are not preemptable at an arbitrary instant.
Reallocating resources can determine deadlocks that involve preemptable resources. Deadlocks that occupy nonpreemptable resources are hard to deal with.