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1) An environment that allows more than one program to access the same set of variable.

2) The ability of a program to "sleep" until it can have (access) a varible.

3) The ability of a program to block other programs from having (accessing) a variable.

Dead Lock example:

Program GoodGosh and GoshDarn both access variables A and B by "Locking" the variables, doing some processing, and then releasing the variables. Both variables have to be obtained and locked before further processing and unlocking can occur.

While processing, Program GoodGosh acquries a lock on variable A and then attempts to acqure a lock on Varable B. Before GoodGosh can acquire variable B, Program GoshDarn acqures a lock on variable B. GoodGosh want B and owns A. GoshDarn owns A and wants B. Neither can process further until the other releases a variable. Deadlock now exists.

Note: To prevent deadlocks, all modules/programs that access the same set of variables should always acquire the variables in the same order.

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Q: What are the necessary condition for a deadlock?
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What is necessary condition for deadlock to occur?

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