#OS Deadlock Necessary Conditions doubt

Question

As Galvin States we have for necessary conditions to have a deadlock as -

1) M.E

2) Hold and wait 

3) No preemption

4) Circular wait.

 

Now, for a deadlock to happen does all four these conditions need to be present/true simultaneously or if at least one of them is true then we can say there's deadlock?

I mean, say, there's no M.E therefore can we straightaway declare there's deadlock without checking for other three conditions?

Comments

These 4 conditions are necessary condition for Deadlock that means-

if (deadlock) then (ME, no preemption, hold and wait, circular wait)

But in some cases it happens that deadlock possible and no ME.. this thing should not have to happen according to if- then statement..

 

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You mentioned-

if ( (ME) && (HOLD & WAIT) && (NO PREEMPTION ON RESOURCES) && (CIRCULAR WAIT) )

         DEADLOCK

else

       NO DEADLOCK

But according to necessary conditions of Deadlock, it is implied that--

If DEADLOCK then there will be (ME)&&(HOLD&WAIT)&&(NO PREEMPTION)&&(CIRCULAR WAIT)

So i am asking what i am doing wrong??

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A deadlock happens only if all the four conditions i.e circular wait, mutual exclusion, no preemption and hold & wait are met simultaneously.

Answer 1

A deadlock happens only if all the four conditions i.e circular wait, mutual exclusion, no preemption and hold&wait are met simultaneously. 

If we can prevent the happening of either of these four conditions, then deadlock won't happen. 

http://www2.latech.edu/~box/os/ch07.pdf This link has additional details too.