Deadlock

Deadlock occurs when each process in a set is waiting for an event that can only be caused by some other process in that set. Basically, deadlock is an operating system issue that reared its head with the advent of multiprogramming environments. First and foremost, a deadlock cannot occur unless there are at least two processes trying to run concurrently. There are actually four distinct circumstances that must be met for deadlock to occur, which will be discussed shortly, but it is worth noting that it is fundamentally a multiprocessing problem. Mono-processing systems do not have to worry about deadlock. The reason is that deadlock involves resource allocation, and if there is only one process, it has uncontested access to all resources. Only certain types of resources are associated with deadlock, and they are of the exclusive-use, non-preemptible type. That is to say, only one process can use the resource at any given time, and once allocated the resource cannot be unallocated by the operating system, but rather the process has control over the resource until it completes its task. Excellent examples of such resources are printers, plotters, tape drives, etc.. Resources that do not fit the criteria are memory and the CPU. While it is convenient to discuss deadlock in terms of hardware resources, there are software resources that are equally good candidates for deadlock, such as records in a data base system, slots in a process table, or spooling space. Hardware or software, all that matters is that the resources are non-preemptible and serially reusable.
The four conditions that must exist for deadlock are as follows: The first two are described above - mutually exclusive use of resources by the processes and non-preemption (resources cannot be removed from the processes). The third condition is called the 'hold and wait' or 'wait for' condition. This means that processes can maintain possession of a resource and simultaneously request or wait for other