COBOL Manual for TNS and TNS/R Programs

Procedure Division Verbs
HP COBOL Manual for TNS and TNS/R Programs—522555-006
9-186
READ for Sequential or Dynamic Access
•
LOCK Phrase
When the READ statement includes a LOCK phrase, the read operation also locks
the retrieved record. The concept of record locking applies only to disk files and
queued files. A successful record lock operation guarantees you exclusive access
to the record until the program executes one:
°
A REWRITE statement with the UNLOCK option for the same record
°
An UNLOCKRECORD statement for the same record
°
An UNLOCKFILE statement for the file
If the program already holds a record lock for the record in question, the read
operation notes this and preserves the lock.
If your HP COBOL process opens the same file twice (that is, treats the file as two
separate files with separate file-control entries, each with its own file-name,
executes a READ LOCK statement on one file-name and then executes
another READ LOCK statement on the other file-name ), the process
deadlocks. This problem does not occur if the process opens the file only once or if
you use time limits.
•
Interaction of LOCKFILE and READ LOCK Statements
If your process executes a READ LOCK statement on a file that it or any other
process has locked with a LOCKFILE statement, or your process executes a
LOCKFILE statement against a file that has an outstanding READ LOCK, the TIME
LIMIT phrase determines what happens.
If the second statement attempting to lock the file has a TIME LIMIT phrase, it
keeps trying to lock the file until the time limit expires. Either it fails and then times
out or it succeeds in locking the file.
If the second statement attempting to lock the file has no TIME LIMIT phrase, it
suspends execution until the statement succeeds because the contending lock is
removed or until the program is terminated by an external agency such as the
TACL command STOP.
•
INTO Phrase
When the READ statement includes an INTO phrase, the retrieved logical record is
moved from the file record area to the data item specified in the INTO phrase.
Any specified subscript or index evaluation involved occurs after the record is
retrieved and placed in the record area and just before it is moved to the data item.
The size of the sending operand in the implicit MOVE statement is the size of the
record as placed into the record area.
This move does not occur if the execution of the READ statement is unsuccessful
for any reason. See INTO Phrase.