COBOL Manual for TNS/E Programs (H06.08+, J06.03+)

A file that no process has open can be opened by any single process with any exclusion
mode.
If some process already has a file open EXCLUSIVE, no other process can open the file.
A file intended for multi-user access must specify SHARED.
If some process already has a file open SHARED for any access mode except OUTPUT, another
process can open the same file SHARED for any access mode.
If some process already has a file open SHARED for OUTPUT, no other process can open the
file (because the OPEN would delete all the records in the file).
If some process already has a file open SHARED for INPUT access mode, another process
can open the same file PROTECTED for any access mode.
If some process already has a file open PROTECTED for INPUT access mode, another process
can also open the same file PROTECTED for INPUT access mode.
All other combinations of access mode and exclusion mode cause the open operation to fail.
• Record Prereading
To save execution time by overlapping reading and processing, the run-time routines perform
record prereading (starting the read for record n +1 when returning record n to the program)
when all of these conditions are true:
◦ The file access mode is SEQUENTIAL.
◦ The file is open for INPUT.
◦ The file is either a disk file opened for PROTECTED use or a disk, tape, terminal, or card
reader opened for EXCLUSIVE use.
◦ The process is not running as a process pair.
◦ The file is not opened for timed I-O.
CAUTION: Verify the OPEN statement does not meet prereading requirements when the COBOL
process does any of these, or the operation might begin with the wrong record:
• Enters a TAL routine to do input or output
• Executes the read operation as part of a TMF transaction (enters BEGINTRANSACTION,
executes a READ statement, then enters ENDTRANSACTION)
• Executes a CLOSE NO REWIND statement on a tape file and subsequently executes an OPEN
NO REWIND statement on the same file to continue from the previous position
Example 117 shows an OPEN statement with a TIME LIMITS and a SYNCDEPTH phrase and an
OPEN statement without optional phrases.
388 Procedure Division Verbs