How hold and wait is different from circular wait?

Question

I am not able to figure out how two conditions for deadlock are different from each other. Hold and wait is look like a subset of the circular wait.

Answer 1

Hold and Wait and Circular Wait are conditions that are met when deadlocks occur.

This means that if these two conditions are not met, you will not be in a deadlock.

The hold and wait condition states that the process is holding onto a resource/s that may (or may not) be required by other processes. The key point here is that the process is holding onto those resources and will not release them until it gets access to the requested resources (which are being held by other processes).

The circular wait condition states that there exists a chain of processes where each process is waiting for a resource that is being held by another process. In this condition it does not necessarily mean that the process is holding onto a resource that the other is requesting, but rather that all processes are waiting for a resource that is being held by another process.

See these articles for more information:

http://www.cs.yale.edu/homes/aspnes/pinewiki/Deadlock.html http://nob.cs.ucdavis.edu/classes/ecs150-1999-02/dl-cond.html

Answer 2

For Hold And Wait, There must exist a process that is holding a resource already allocated to it while waiting for additional resource that are currently being held by other processes. Whereas, for Circular Wait to occur The processes in the system form a circular list or chain where each process in the list is waiting for a resource held by the next process in the list.

Point to be noted: the formation of chain (Circular list) is must for Circular Wait which is not the case with Hold and Wait.

Reference: http://www.personal.kent.edu/~rmuhamma/OpSystems/Myos/deadlockCondition.htm