Record Access Mode

The methods for retrieving and storing records in a file are called record access modes. Compaq COBOL supports the following three types of record access modes:

• ACCESS MODE IS SEQUENTIAL
  • With sequential files, sequential access mode retrieves the records in the same sequence established by the WRITE statements that created the file.
  • With relative files, sequential access mode retrieves the records in the order of ascending record key values (or relative record numbers).
  • With indexed files, sequential access mode retrieves records in the order of record key values.
• ACCESS MODE IS RANDOM---The value of the record key your program specifies indicates the record to be accessed in Indexed and Relative files.
• ACCESS MODE IS DYNAMIC---With relative and indexed files, dynamic access mode allows you to switch back and forth between sequential access mode and random access mode while reading a file by using the the NEXT phrase on the READ statement. For more information about dynamic access mode, refer to READ and REWRITE statements in the Compaq COBOL Reference Manual.

When you omit the ACCESS MODE IS clause in the SELECT statement, the access mode is sequential.

Example 6-17 shows sample SELECT statements for sequential files with sequential access modes.

Example 6-17 SELECT Statements for Sequential Files with Sequential Access Mode

(1) (2) FILE-CONTROL. FILE-CONTROL. SELECT LIST-FILE SELECT PAYROLL ASSIGN TO "MAIL.LIS" ASSIGN TO "PAYROL.DAT". ORGANIZATION IS SEQUENTIAL ACCESS IS SEQUENTIAL.

Sample SELECT statements for relative files with sequential and dynamic access modes are shown in Example 6-18.

Example 6-18 SELECT Statements for Relative Files with Sequential and Dynamic Access Modes

(1) (2) FILE-CONTROL. FILE-CONTROL. SELECT MODEL SELECT PARTS ASSIGN TO "ACTOR.DAT" ASSIGN TO "PART.DAT" ORGANIZATION IS RELATIVE ORGANIZATION IS RELATIVE ACCESS MODE IS SEQUENTIAL. ACCESS MODE IS DYNAMIC RELATIVE KEY IS PART-NO.

Sample SELECT statements for indexed files with dynamic and sequential access modes are shown in Example 6-19.

Example 6-19 SELECT Statements for Indexed Files with Dynamic and Default Sequential Access Modes

(1) (2) FILE-CONTROL. FILE-CONTROL. SELECT A-GROUP SELECT TEAS ASSIGN TO "RFCBA.PRO" ASSIGN TO "TEA" ORGANIZATION IS INDEXED ORGANIZATION IS INDEXED ACCESS MODE IS DYNAMIC RECORD KEY IS LEAVES. RECORD KEY IS WRITER ALTERNATE RECORD KEY IS EDITOR.

Because the default file organization is also sequential, both the relative and indexed examples require the ORGANIZATION IS clause.

Sample SELECT statements for line sequential files with sequential access modes are shown in Example 6-20.

Example 6-20 SELECT Statements for Line Sequential Files with Sequential Access Modes (Alpha)

(1) (2) FILE-CONTROL. FILE-CONTROL. SELECT MAMMALS SELECT VACATION-SPOTS ASSIGN TO "DOLPHINS" ASSIGN TO "BAHAMAS" ORGANIZATION IS LINE SEQUENTIAL ORGANIZATION IS LINE SEQUENTIAL. ACCESS MODE IS SEQUENTIAL.

6.3 Creating and Processing Files

Creating and processing sequential, line sequential, relative, and indexed files includes the following tasks:

1. Opening the file
2. Executing valid I/O statements
3. Closing the file

Sections 6.3.2 , 6.3.3 , and 6.3.4 describe the specific tasks involved in creating and processing sequential, relative, and indexed files.

6.3.1 Opening and Closing Files

A Compaq COBOL program must open a file with an OPEN statement before any other I/O or Report Writer statement can reference it. Files can be opened more than once in the same program as long as they are closed before being reopened.

Sample OPEN and CLOSE statements are shown in Example 6-21.

Example 6-21 OPEN and CLOSE Statements

. . . OPEN INPUT MASTER-FILE. OPEN OUTPUT REPORT-FILE. OPEN I-O MASTER-FILE2 TRANS-FILE OUTPUT REPORT-FILE2. CLOSE MASTER-FILE. CLOSE TRANS-FILE, MASTER-FILE2 REPORT-FILE, REPORT-FILE2. . . .

The OPEN statement must specify one of the following four open modes:

INPUT
OUTPUT
I-O {Not for LINE SEQUENTIAL}
EXTEND

Your choice, along with the file's organization and access mode, determines which I/O statements you can use. Sections 6.3.2, 6.3.3, and 6.3.4 discuss the I/O statements for sequential, relative, and indexed files, respectively. Section 12.8.4, Case Sensitivity on Tru64 UNIX explains the importance of attention to case.

When your program performs an OPEN statement, the following events take place:

1. The I/O system builds a file specification by using the contents of the VALUE OF ID clause, if any, to alter or complete the file specification in the ASSIGN clause. Logicals and environment variables are translated.
2. The I/O system checks the file's current status. If the file is unavailable, or if it was closed WITH LOCK, the OPEN statement fails. (See Chapter 8 for information on file sharing.)
3. If the file specification names an invalid device, or contains any other errors, the I/O system generates an error message and the OPEN statement fails.
4. The I/O system takes one of the following actions if it cannot find the file:
   1. If the file's OPEN mode is OUTPUT, the file is created.
   2. If the file's OPEN mode is EXTEND, or I-O, the OPEN statement fails, unless the file's SELECT clause includes the OPTIONAL phrase. If the file's SELECT clause includes the OPTIONAL phrase, the file is created.
   3. If the file's OPEN mode is INPUT, and its SELECT clause includes the OPTIONAL phrase, the OPEN statement is successful. The first read on that file causes the AT END or INVALID KEY condition.
   4. If none of the previous conditions is met, the OPEN fails and the Declarative USE procedure (if any) gains control. If no Declarative USE procedure exists, the I/O system aborts the program.
5. If the file's OPEN mode is OUTPUT, and a file by the same name already exists, a new version is created.
6. If the file characteristics specified by the program attempting an OPEN operation differ from the characteristics specified when the file was created, the OPEN statement fails.

If the file is on magnetic tape, the I/O system rewinds the tape. (To close a file on tape without rewinding the tape, use the NO REWIND phrase.) This speeds processing when you want to write another file beyond the end of the first file, as in the following example:

CLOSE MASTER-FILE NO REWIND.

You can also close a file and prevent your program from opening that file again in the same run, as in the following example:

CLOSE MASTER-FILE WITH LOCK.

6.3.2 File Handling for Sequential and Line Sequential (Alpha) Files

Creating a sequential or (on Alpha only) line sequential file involves the following:

1. Opening the file for OUTPUT or EXTEND
2. Executing valid I/O statements
3. Closing the file

By default, Compaq COBOL assumes sequential organization and sequential access mode. (See Example 6-22.)

Example 6-22 Creating a Sequential File

IDENTIFICATION DIVISION. PROGRAM-ID. SEQ01. ENVIRONMENT DIVISION. INPUT-OUTPUT SECTION. FILE-CONTROL. SELECT TRANS-FILE ASSIGN TO "TRANS.DAT". DATA DIVISION. FILE SECTION. FD TRANS-FILE. 01 TRANSACTION-RECORD PIC X(25). PROCEDURE DIVISION. A000-BEGIN. OPEN OUTPUT TRANS-FILE. PERFORM A010-PROCESS-TRANS UNTIL TRANSACTION-RECORD = "END". CLOSE TRANS-FILE. STOP RUN. A010-PROCESS-TRANS. DISPLAY "Enter next record - X(25)". DISPLAY "enter END to terminate the session". DISPLAY "-------------------------". ACCEPT TRANSACTION-RECORD. IF TRANSACTION-RECORD NOT = "END" WRITE TRANSACTION-RECORD.

Example 6-23 Creating a Line Sequential File (Alpha)

IDENTIFICATION DIVISION. PROGRAM-ID. LINESEQ01. ENVIRONMENT DIVISION. INPUT-OUTPUT SECTION. FILE-CONTROL. SELECT LINESEQ-FILE ASSIGN TO "LINESEQ.DAT". DATA DIVISION. FILE SECTION. FD LINESEQ-FILE. 01 LINESEQ-RECORD PIC X(25). PROCEDURE DIVISION. A000-BEGIN. OPEN OUTPUT LINESEQ-FILE. CLOSE LINESEQ-FILE. STOP RUN.

By default, Compaq COBOL assumes sequential access mode when the line sequential organization is specified. (See Example 6-23.) <>

Statements for Sequential and Line Sequential (Alpha) File Processing

Processing a sequential file or line sequential file (Alpha) involves the following:

1. Opening the file
2. Processing the file with valid I/O statements
3. Closing the file

Table 6-3 lists the valid I/O statements for sequential files, and Table 6-4 lists the valid I/O statements for line sequential files. Both tables illustrate the following relationships:

• Organization determines valid access modes.
• Organization and access mode determine valid open modes.
• All three (organization, access, and open mode) enable or disable I/O statements.

Table 6-3 Valid I/O Statements for Sequential Files

			Open Mode
File Organization	Access Mode	Statement	INPUT	OUTPUT	I/O	EXTEND
SEQUENTIAL	SEQUENTIAL	READ	Yes	No	Yes	No
		REWRITE	No	No	Yes	No
		WRITE	No	Yes	No	Yes
		UNLOCK	Yes	Yes	Yes	Yes

Writing a Sequential File

Each WRITE statement appends a logical record to the end of an output file, thereby creating an entirely new record in the file. The WRITE statement appends records to files that are OPEN for the following modes:

• OUTPUT---Output mode can create the following two kinds of files:
  • Storage files---A storage file remains on tape or disk for future reference or processing.
  • Print-control files---The Data Division LINAGE clause, the Environment Division APPLY PRINT-CONTROL clause, the Procedure Division ADVANCING phrase (in the WRITE statement), or Report Writer statements and phrases designates a file as a print-control file.
    On OpenVMS Alpha, each record in a print-control file contains a header that performs line spacing. On Tru64 UNIX, line spacing is done with blank records in print-control files.
• EXTEND---Extend mode permits new records to be added in sequence after the last record of an existing file (see Extending a Sequential File or Line Sequential File (Alpha) in Section 6.5.1).

Table 6-4 Valid I/O Statements for Line Sequential Files (Alpha)

			Open Mode
File Organization	Access Mode	Statement	INPUT	OUTPUT	EXTEND
LINE SEQUENTIAL	SEQUENTIAL	READ	Yes	No	No
		WRITE	No	Yes	Yes
		UNLOCK	Yes	Yes	Yes

Writing a Line Sequential File (Alpha)

Each WRITE statement appends a logical record to the end of an output file, thereby creating an entirely new record in the file. The WRITE statement appends records to files that are OPEN for the following modes:

• OUTPUT---Output mode creates a new file or overwrites an already existing file.
• EXTEND---Extend mode permits new records to be added in sequence after the last record of an existing file (see Extending a Sequential File or Line Sequential File (Alpha). <>

Writing a Record

You can write records in the following two ways:

• WRITE record-name FROM source-area
• WRITE record-name

The first way provides easier program readability with multiple record types. For example, statements (1) and (2) in the following example are logically equivalent:

FILE SECTION. FD STOCK-FILE. 01 STOCK-RECORD PIC X(80). WORKING-STORAGE SECTION. 01 STOCK-WORK PIC X(80). ----------------(1)---------------- --------------(2)--------------- WRITE STOCK-RECORD FROM STOCK-WORK. MOVE STOCK-WORK TO STOCK-RECORD. WRITE STOCK-RECORD.

When you omit the FROM phrase, you process the records directly in the record area or buffer (for example, STOCK-RECORD).

The following example writes the record PRINT-LINE to the device assigned to that record's file, then skips three lines. At the end of the page (as specified by the LINAGE clause), it causes program control to transfer to HEADER-ROUTINE.

WRITE PRINT-LINE BEFORE ADVANCING 3 LINES AT END-OF-PAGE PERFORM HEADER-ROUTINE.

For a WRITE FROM statement, if the destination area is shorter than the file's record length, the destination area is padded on the right with spaces; if longer, the destination area is truncated on the right. This follows the rules for a group move.

6.3.3 File Handling for Relative Files

Creating a relative file involves the following tasks:

1. Specifying ORGANIZATION IS RELATIVE in the Environment Division SELECT clause
2. Specifying ACCESS MODE IS SEQUENTIAL (or RANDOM) in the Environment Division SELECT clause
   Each of these two access modes requires a different processing technique. (Refer to the Creating a Relative File in Sequential Access Mode and Creating a Relative File in Random Access Mode sections in this chapter for information about those techniques.)
3. Opening the file for OUTPUT or I-O
4. Initializing the relative key data name for each new record
5. Executing a WRITE statement for each new relative record
6. Closing the file

Creating a Relative File in Sequential Access Mode

When your program creates a relative file in sequential access mode, the I/O system does not use the relative key. Instead, it writes the first record in the file at relative record number 1, the second record at relative record number 2, and so on, until the program closes the file. If you use the RELATIVE KEY IS clause, the compiler moves the relative record number of the record being written to the relative key data item. Example 6-24 writes 10 records with relative record numbers 1 to 10.

Example 6-24 Creating a Relative File in Sequential Access Mode

IDENTIFICATION DIVISION. PROGRAM-ID. REL02. ENVIRONMENT DIVISION. INPUT-OUTPUT SECTION. FILE-CONTROL. SELECT FLAVORS ASSIGN TO "BRAND" ORGANIZATION IS RELATIVE ACCESS MODE IS SEQUENTIAL. DATA DIVISION. FILE SECTION. FD FLAVORS. 01 KETCHUP-MASTER. 02 FILLER PIC X(14). 02 REC-NUM PIC 9(05). 02 FILLER PIC X(31). 02 FILLER PIC X(31). WORKING-STORAGE SECTION. 01 REC-COUNT PIC S9(5) VALUE 0. PROCEDURE DIVISION. A000-BEGIN. OPEN OUTPUT FLAVORS. PERFORM A010-WRITE 10 TIMES. CLOSE FLAVORS. STOP RUN. A010-WRITE. MOVE "Record number" TO KETCHUP-MASTER. ADD 1 TO REC-COUNT. MOVE REC-COUNT TO REC-NUM. WRITE KETCHUP-MASTER INVALID KEY DISPLAY "BAD WRITE" STOP RUN.

Creating a Relative File in Random Access Mode

When a program creates a relative file using random access mode, the program must place a value in the RELATIVE KEY data item before executing a WRITE statement. Example 6-25 shows how to supply the relative key. It writes 10 records in the cells numbered: 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20. Record cells 1, 3, 5, 7, 9, 11, 13, 15, 17, and 19 are also created, but contain no valid records.

Example 6-25 Creating a Relative File in Random Access Mode

IDENTIFICATION DIVISION. PROGRAM-ID. REL03. ENVIRONMENT DIVISION. INPUT-OUTPUT SECTION. FILE-CONTROL. SELECT FLAVORS ASSIGN TO "BRAND" ORGANIZATION IS RELATIVE ACCESS MODE IS RANDOM RELATIVE KEY IS KETCHUP-MASTER-KEY. DATA DIVISION. FILE SECTION. FD FLAVORS. 01 KETCHUP-MASTER. 02 FILLER PIC X(14). 02 REC-NUM PIC 9(05). 02 FILLER PIC X(31). WORKING-STORAGE SECTION. 01 KETCHUP-MASTER-KEY PIC 99. 01 REC-COUNT PIC S9(5) VALUE 0. PROCEDURE DIVISION. A000-BEGIN. OPEN OUTPUT FLAVORS. MOVE 0 TO KETCHUP-MASTER-KEY. PERFORM A010-CREATE-RELATIVE-FILE 10 TIMES. DISPLAY "END OF JOB". CLOSE FLAVORS. STOP RUN. A010-CREATE-RELATIVE-FILE. ADD 2 TO KETCHUP-MASTER-KEY. MOVE "Record number" TO KETCHUP-MASTER. ADD 2 TO REC-COUNT. MOVE REC-COUNT TO REC-NUM. WRITE KETCHUP-MASTER INVALID KEY DISPLAY "BAD WRITE" STOP RUN.

Statements for Relative File Processing

Processing a relative file involves the following:

1. Opening the file
2. Setting the relative record number
3. Processing the file with valid I/O statements
4. Closing the file

Table 6-5 lists the valid I/O statements and illustrates the following relationships:

• Organization determines valid access modes.
• Organization and access mode determine valid open modes.
• All three (organization, access, and open mode) enable or disable I/O statements.

Table 6-5 Valid I/O Statements for Relative Files

			Open Mode
File Organization	Access Mode	Statement	INPUT	OUTPUT	I-O	EXTEND
RELATIVE	SEQUENTIAL	DELETE READ REWRITE START WRITE UNLOCK	No Yes No Yes No Yes	No No No No Yes Yes	Yes Yes Yes Yes No Yes	No No No No Yes Yes
	RANDOM	DELETE READ REWRITE WRITE UNLOCK	No Yes No No Yes	No No No Yes Yes	Yes Yes Yes Yes Yes	No No No No No
	DYNAMIC	DELETE READ READ NEXT REWRITE START WRITE UNLOCK	No Yes Yes No Yes No Yes	No No No No No Yes Yes	Yes Yes Yes Yes Yes Yes Yes	No No No No No No No

Writing a Relative File

Each WRITE statement places a record into a cell that contains no valid data. If the cell does not already exist, the I/O system creates it. To change the contents of a cell that already contains valid data, use the REWRITE statement.