Deadlock occurs when two or more competing processes are each waiting for resources held by the other, resulting in all processes waiting indefinitely. There are four conditions required for deadlock: mutual exclusion, hold and wait, no preemption, and circular wait. Techniques to prevent deadlock include attacking each condition: allowing some resources to be shared, requiring processes request all resources at start, allowing preemption of resources, and imposing a global numbering on resource requests.

1. S.V.I.T.,Vasad
 NAME: RAJAN SHAH  TOPIC: DEADLOCK:
DEFINITION,
CHARACTERISTICS &
PREVENTION.

2. DEADLOCK
 Definition: A deadlock is a situation in which two computer
programs sharing the same resource are effectively
preventing each other from accessing the resource, resulting
in both programs ceasing to function.
 The reason, why earliest computer OS ran only one program
at a time.
 A set of processes is deadlocked if each process in the set is
waiting for an event that only another process can trigger.

3. Continue...
 As all the processes in the set are waiting for an another
process to trigger an event that could wake up any of the
other members of the set, all the processes would wait for
ever.
 None of the processes can run, none of them can release any
resource, and thus none can be awakened.
 The no. of processes and the no. and kind of resources
processed and requested are unimp.. This kind of request
holds for any kind of resource and is called Resource
Deadlock.

4. Characteristics Of Deadlock
 A deadlock situation can arise if the following four conditions
hold simultaneously in a system:
1. Mutual Exclusion
2. Hold & Wait
3. No Pre-emption
4. Circular wait.

5. Prevention Techniques
 The methods to prevent the Deadlock are:
1. Attacking the Mutual Exclusion Condition.
2. Attacking the Hold & Wait Condition.
3. Attacking the No Pre-emption Condition.
4. Attacking the Circular Wait Condition.

6. Attacking the Mutual Exclusion
Condition
 Mutual Exclusion: For two processes, if any one process is
accessing the common resource then the another process is
excluded from accessing the resource.
 Thus, each resource is either currently allocated to exactly
one process or it is available for other process to access it.
Two processes can’t simultaneously access the common
resource.

7. Example
 It is clear that by allowing two processes to write on the printer
simultaneously will lead to chaos.
 Actually, the only process which requests the physical printer is
the printer daemon.
 Printer daemon never requests any other resource, so the
deadlock can be eliminated and also it prints only after the
complete o/p file is available.
 But this could in itself lead to deadlock for two processes having
finished their part, but not all, of their o/p, and can’t continue.
 Thus, avoiding to assign a resource unless not absolutely
necessary and try to make less processes request for the
resource.

8. Attacking the Hold & Wait
Condition
 Hold and Wait: In this scenario, processes currently holding
resource can request new resource to complete its task.
 If the process requests all the resources before starting the
execution, deadlocks can be eliminated.
 I.E. if everything is available, process will be allocated
whatever it needs and can run to completion.
 Whereas, if one or more of the requested resource is/are
busy, then nothing will be allocated and the process would
just wait for the resource to become free and enter the
critical region.
 The problem that how many resources does a process may
need can’t be known until it has started running.

9. Example
 Consider a process, that reads data from an i/p tape, analyzes
it for an hour, and then writes an o/p tape as well as plotting
the results.
 Thus, if all resources are requested in adv, the process will tie
up the o/p tape and the plotter for an hour.
 Thus, this method wastes the resources as for an hour no
other process would be able to use the plotter and the o/p
tape.

10. Attacking the No Pre-emption
Condition
 No Pre-emption: Once a process holds a resource, it can’t be
taken away by another process or the kernel and resource
can only be voluntarily released by the process.
 Example: if a process is assigned a printer and is in the middle
of printing its output, forcibly taking away the printer because
the plotter needed isn’t available is not possible.
 Thus, a resource can be used only if the process holding it,
releases it voluntarily.

11. Attacking the Circular Wait
Condition
 Circular wait: Each process is waiting to obtain a resource
which is held by another process.
 The circular wait can be eliminated by a rule that if a process
is holding a resource at the moment then it can’t request for
other resource, and if it wants to access the other resource
then it must release the first one.
 Another way to avoid the circular wait is to provide a global
numbering of all the resources.
 A process can request whenever it wants, but all the request
must be made in numerical order.

12. Example
 Consider the following resources numbered as below:
1. Image setter
2. Scanner
3. Plotter
4. Tape-Drive
5. CD-ROM drive.
A process may request first a printer and then a tape drive, but
it can’t request first a plotter and then a printer.

13. Example
 Here, if A requests for j ;
i>j, then A is not allowed to
request j because that is
lower than what it already
has.
 Similarly, if B requests for i;
i<j, then B is not allowed to
request for i.
 Thus, either way deadlock
is not possible.
A
i
B
j

14. THANK YOU