COBOL Manual for TNS and TNS/R Programs

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1391

Disk Input and Output

HP COBOL Manual for TNS and TNS/R Programs—522555-006

28-38

Avoiding Deadlock

Deadlock is a situation in which two processes are in contention for a single-user

resource. If both processes require exclusive use of both resources, they are

deadlocked. Neither can proceed until the other surrenders control of the resource it

has under its control. If two servers get into a deadlock, their respective requesters are

also deadlocked because neither requester receives a reply from its server.

One example of deadlock is when one process has opened a file (call it ABLE) and

needs to open another file (call it BAKER), while another process has opened BAKER

and now needs to open ABLE.

Another example of deadlock is when one process has locked a record in a file (call it

record n ) and wants to read another record in the file (call it record m ), while some

other process has locked record m and wants to read record n.

The ways to avoid deadlock are:

•

Locking and Unlocking Files and Records

•

Setting Time Limits on Input-Output Operations

Locking and Unlocking Files and Records

The most complete way for a process to lock a file is to open the file for exclusive

access (open it with the exclusion mode EXCLUSIVE). This strategy prevents all other

processes from reading or writing the file.

If it is not feasible for one process to have exclusive control of a file for when it has the

file open, there are alternatives.

One alternative is protected access. When a process opens a file for protected access

(opens it with the exclusion mode PROTECTED), other processes can read and write

the file while the opening process has it open, but cannot write the file until the opening

process closes the file.

Another alternative is shared access. When a process opens a file for shared access

(opens it with the exclusion mode SHARED), other processes can read and write the

file while the opening process has it open.

When a file is open for shared access and one process needs exclusive access to the

file temporarily, that process can lock the file with a LOCKFILE statement, operate on

it, and then unlock it with an UNLOCKFILE statement. If a process only needs

temporary exclusive access to a record of the file, the process can temporarily lock the

record with a READ statement with a LOCK phrase and then unlock it with an

UNLOCKRECORD statement.