A deadlock is a situation wherein two or more competing actions are waiting for the other to finish, and thus neither ever does. It is often seen in a paradox like 'the chicken or the egg'.
“ When two trains approach each other at a crossing, both shall come to a full stop and neither shall start up again until the other has gone. ”

In computer science, deadlock refers to a specific condition when two or more processes are each waiting for another to release a resource, or more than two processes are waiting for resources in a circular chain (see Necessary conditions). Deadlock is a common problem in multiprocessing where many processes share a specific type of mutually exclusive resource known as a software, or soft, lock. Computers intended for the time-sharing and/or real-time markets are often equipped with a hardware lock (or hard lock) which guarantees exclusive access to processes, forcing serialization. Deadlocks are particularly troubling because there is no general solution to avoid (soft) deadlocks.

This situation may be likened to two people who are drawing diagrams, with only one pencil and one ruler between them. If one person takes the pencil and the other takes the ruler, a deadlock occurs when the person with the pencil needs the ruler and the person with the ruler needs the pencil, before he can give up the ruler. Both requests can't be satisfied, so a deadlock occurs.

The telecommunications description of deadlock is a little stronger: deadlock occurs when none of the processes meet the condition to move to another state (as described in the process's finite state machine) and all the communication channels are empty. The second condition is often left out on other systems but is important in the telecommunication context.

Necessary conditions:
There are four necessary conditions for a deadlock to occur, known as the Coffman conditions from their first description in a 1971 article by E. G. Coffman.

1. Mutual exclusion condition: a resource that cannot be used by more than one process at a time
2. Hold and wait condition: processes already holding resources may request new resources
3. No preemption condition: only a process holding a resource may release it
4. Circular wait condition: two or more processes form a circular chain where each process waits for a resource that the next process in the chain holds

Deadlock can only occur in systems where all 4 conditions hold true.

for details,please visit:
http://en.wikipedia.or g/wiki/Deadlock

Answered by jaivir , an ibibo Master, at 8:01 PM on July 30, 2008

Deadlock refers to a specific condition when two or more processes are each waiting for another to release a resource, or more than two processes are waiting for resources in a circular chain (see Necessary conditions). Deadlock is a common problem in multiprocessing where many processes share a specific type of mutually exclusive resource known as a software, or soft, lock. Computers intended for the time-sharing and/or real-time markets are often equipped with a hardware lock (or hard lock) which guarantees exclusive access to processes, forcing serialization. Deadlocks are particularly troubling because there is no general solution to avoid (soft) deadlocks.

Answered by Shobhit , an ibibo Master, at 7:38 PM on July 30, 2008

Conditions for deadlock to occur

* Mutual Exclusion : At least one unsharable resource - processes claim exclusive control of resources they need
* Hold and Wait : Process holds one resource while waiting for another
* No Preemption : Resources only released voluntarily - no interruption possible (i.e. cannot be forcefully withdrawn by another process)
* Circular Wait : Circular chain of processes - each waiting for a resource held by another

Deadlock is a SERIOUS problem that can occur in concurrent systems,

Four strategies - we shall look at each in term :

* Prevention
* Avoidance
* Detection
* Recovery

for more you can see the site http://users.cs.cf.ac.uk/O.F.R ana/os/lectureos12/node2.html

Answered by Sharad Singh , an ibibo Master, at 2:04 PM on July 30, 2008

When only all the below given conditions holds true, deadlock can occur in systems.

- Mutual Exclusion: At least one unsharable resource - processes claim exclusive control of resources they need

- Hold and Wait: Process holds one resource while waiting for another

- No Preemption: Resources only released voluntarily - no interruption possible (i.e. cannot be forcefully withdrawn by another process)

- Circular Wait: Circular chain of processes - each waiting for a resource held by another

For comprehensive elaboration on these conditions, refer to: en.wikipedia.org/wiki/Deadlock

H ope it would help.

Answered by A.R. , an ibibo Master, at 12:47 PM on July 30, 2008

Deadlock refers to a specific condition when two or more processes are each waiting for another to release a resource, or more than two processes are waiting for resources in a circular chain (see Necessary conditions). Deadlock is a common problem in multiprocessing where many processes share a specific type of mutually exclusive resource known as a software, or soft, lock. Computers intended for the time-sharing and/or real-time markets are often equipped with a hardware lock (or hard lock) which guarantees exclusive access to processes, forcing serialization. Deadlocks are particularly troubling because there is no general solution to avoid (soft) deadlocks.

This situation may be likened to two people who are drawing diagrams, with only one pencil and one ruler between them. If one person takes the pencil and the other takes the ruler, a deadlock occurs when the person with the pencil needs the ruler and the person with the ruler needs the pencil, before he can give up the ruler. Both requests can't be satisfied, so a deadlock occurs.

Answered by msg4hk , an ibibo Master, at 11:01 AM on July 30, 2008

well mohan anand,There are four necessary conditions for a deadlock to occur, known as the Coffman conditions from their first description in a 1971 article by E. G. Coffman.

1. Mutual exclusion condition: a resource that cannot be used by more than one process at a time
2. Hold and wait condition: processes already holding resources may request new resources
3. No preemption condition: only a process holding a resource may release it
4. Circular wait condition: two or more processes form a circular chain where each process waits for a resource that the next process in the chain holds


Deadlock can only occur in systems where all 4 conditions hold true.
en.wikipedia.org/wiki/D eadlock - 40k

Answered by Debashis , an ibibo Master, at 10:58 AM on July 30, 2008

Well Mohan , There are four necessary conditions for a deadlock to occur, known as the Coffman conditions from their first description in a 1971 article by E. G. Coffman.

1. Mutual exclusion condition: a resource that cannot be used by more than one process at a time
2. Hold and wait condition: processes already holding resources may request new resources
3. No preemption condition: only a process holding a resource may release it
4. Circular wait condition: two or more processes form a circular chain where each process waits for a resource that the next process in the chain holds


Deadlock can only occur in systems where all 4 conditions hold true.

For more details , Please visit the source site :

http://en.wikipedia.org/wiki/D eadlock

Answered by Alok Gupta , an ibibo Guru, at 10:13 AM on July 30, 2008

there are diffrernt kind dead lock available

Answered by selva kumar at 9:41 AM on July 30, 2008