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

When the execution of the START statement is successful, the key of reference established for
the start operation is also used for any subsequently executed READ NEXT statements for the
file, until the execution of some statement explicitly establishes a different key of reference.
Whenever the start operation is successful, if a NOT INVALID KEY phrase is specified, control
passes to the imperative statement in that phrase. If that imperative statement does not transfer
control elsewhere using a GO TO statement, control then passes to the end of the START
statement.
• Result of Unsuccessful Start Operation
When the execution of a START statement is unsuccessful for any reason, the file position
indicator is set to indicate that no valid next record has been established, and the key of
reference for the file is undefined.
• START Statement Without the POSITION Phrase
Execution of the START statement establishes the specified key as the key of reference for the
file:
◦ For sequential files, an alternate key
◦ For relative files, the relative key item or an alternate key
◦ For indexed files, a prime key or alternate key
◦ For queue files, a prime key
The start operation searches the file for the first record whose value for the key of reference
satisfies the specified relation with respect to the comparison data item.
When the specified key is the prime record key or an alternate record key, the comparison
data item is handled as if it were an alphanumeric data item (regardless of its actual
description) and the normal nonnumeric comparison rules (using the ASCII collating sequence)
apply. When the size of the comparison data item in character positions is less than that of
the file key, the comparison proceeds as if the record’s key value were truncated on the right
to reduce its size to the equivalent number of characters.
When the file does not contain a qualifying record, the invalid-key condition exists, and the
execution of the START statement is unsuccessful.
When the search succeeds, the file position indicator for the file is set to point to the record
found.
• START Statement With the POSITION Phrase
A START statement that includes a POSITION phrase also serves to establish a key of reference
and to establish a value for the file position indicator. If more than one record has the same
alternate key, the POSITION phrase determines which record is read. If the program is reading
according to a partial alternate key that has duplicate values in the file, but the complete
alternate key values are unique, the POSITION phrase has no effect (even if the alternate key
is specified with the DUPLICATES phrase).
Suppose a requester asks for a set of records from a member of a context-free server class:
all the records that have a certain alternate key value, or partial value. If the message buffer
cannot hold all such records, the server can return the first group of records plus the value of
the unique key with the reply. This value is that of the prime record key, the relative key, or
the entire alternate key declared as having no duplicates. Then the requester can ask for
another group, supplying the value of both the partial alternate key and the unique key, and
the server can resume with the last record it had sent or (by using the AFTER phrase) the record
beyond that. The server can remain context-free, because the information necessary to do the
456 Procedure Division Verbs