A data file is what type of file when each record is read in order of its position in the file?

Chapter 3: COBOL File Organizations

This chapter explains the available COBOL file organizations and gives examples of using them.

3.1 Data File Organizations

Using only COBOL syntax, COBOL programs can create, update and read files of four different organizations:

• Line sequential

  Line Sequential files are a special type of sequential file. They correspond to simple text files as produced by the standard editor provided with your operating system.

• Record sequential

  Sequential files are the simplest form of COBOL file. Records are placed in the file in the order they are written, and can only be read back in the same order.

• Relative Files

  Every record in a relative file can be accessed directly without having to read through any other records. Each record is identified by a unique ordinal number both when it is written and when it is read back.

• Indexed

  Indexed files are the most complex form of COBOL file which can be handled directly by COBOL syntax. Records in an indexed file are identified by a unique user-defined key when written. Each record can contain any number of user-defined keys which can be used to read the record, either directly or in key sequence.

3.1.1 Sequential Files

A sequential file is a file in which the records can only be accessed sequentially. Records are always added to the end of the file.

3.1.1.1 Line Sequential Files

In line sequential files, each record in the file is separated from the next by a record delimiter. On DOS, Windows and OS/2 this is a carriage return (x"0D") and a line feed (x"0A") character. On UNIX it is just the line feed (x"0A") character. These characters are inserted after the last non-space character in each record so line sequential files always contain variable-length records.

Report files are line sequential, since most PC printers require the carriage return and/or line feed characters at the end of each record.

Most PC editors produce line sequential files, and these files can therefore be edited with almost any PC editor.

The primary use of line sequential files is for display-only data.

Caution: We recommend that you do not use line sequential files for binary or packed data fields.

Line sequential files are also known as text files, or flat ASCII files.

When you declare a file as line sequential in COBOL, you do so through the SELECT clause.

Example

Creating a line sequential file:

3.1.1.2 Record Sequential Files

Record sequential files are nearly always simply called sequential files, since record sequential is the default for a sequential file.

Records in a record sequential file can be either fixed or variable in length.

Variable-length records save disk space. There are many applications that can benefit from the use of variable-length records. A common example is where your application generates many small records, with occasional large ones. If you make the record length as long as the largest record, you waste a lot of disk space. The way to prevent this waste is to use variable-length records.

When you declare a file as record sequential in COBOL, you do so through the SELECT clause.

Example

Creating a record sequential file with fixed-length records.

Note: In place of the ORGANIZATION clause above, you could use:

organization is sequential

Or, you could simply omit the ORGANIZATION clause, as record sequential is the default file organization (if the SEQUENTIAL directive is not set).

Example

Creating a record sequential file with variable-length records.

3.1.2 Relative Files

With relative file organization, you can access records sequentially or randomly. For sequential access, you simply do a sequential READ to get the next record in the file. For random access, you specify the ordinal number of the record in the file.

Relative files have a fixed-length file format. You can declare that you want the records to have a recording mode of "variable" but even if you do this, the system assumes the maximum record length for all WRITE statements to the file, and pads the unused character positions. So, when you are in a situation where you have a lot to gain by using variable-length records, you should avoid relative files because they are always fixed format.

Relative files have the fastest access time of all the file types used by this COBOL system so, if speed of access is the most important consideration, you should consider using relative files.

With relative files, you can have numeric keys, but you cannot key on fields. If you need to access data randomly based on certain fields, you must use indexed files.

Example

Creating a relative file with a record length of 80 characters:

Note: The relative key field is relfil-key. When you are randomly accessing this file, there is no KEY IS field on the READ statement. The number in relfil-key determines which record is read. (For sequential access, a simple READ statement gets the next record.)

3.1.3 Indexed Files

Indexed file access enables you to access records either randomly or sequentially, using one or more key fields in the individual records.

Key comparisons are made on a byte-by-byte basis from right to left using the ASCII collating sequence.

COBOL indexed files are actually made up of two physical files: a data file and an index file. The index file is created automatically, and has an extension of .idx; the data file can have any other extension, although .dat is very common.

Records in indexed files can be either fixed or variable in length.

Whenever you need to provide users with many different views of a file, you need indexed files. In your programs, this implies the need for random access, keyed on one or more fields in the records.

Example

Creating an indexed file with fixed-length 80-byte records keyed on the first five bytes of each record:

Creating an indexed file with variable-length records, varying in length from 5 to 80 bytes:

Note: The keys defined for the file must all lie in the fixed part of the record.

3.2 External Files

When using COBOL, you can open data files in one program, and perform file operations (such as READ and WRITE) in another, as long as all the programs are in the same run unit. This is called using external files. There are several reasons for using external files:

• To better structure your system
• To prevent the resetting of a record pointer between programs (since you do not have to close the file at the end of a program, but should do so at the end of the application)
• To avoid the overhead of the OPEN statement in every program (the file only needs to be opened once, even if it is read, or written to, in many programs)

3.2.1 How to Use External Files

To use external files, you must:

• Include the FD and the SELECT clause for the external files in every program in your run unit that accesses those files
• Code the EXTERNAL clause in the FD
• Declare all Working-Storage items for these files (that you want to be accessible to every program that uses the files) as EXTERNAL

Example

The following programs illustrate how to use external files in COBOL programs. The main program opens a file, which is written to, and read from, in two separate subprograms before the file is closed at the end of the main program. For the sake of brevity, no file status checking is performed.

The programs are included in your cobol\demo directory, if you want to compile and run them yourself. They are in a file named extfile.cbl. The subprograms are also part of this file - they are named readfile and writefil.

You can animate these programs to familiarize yourself with the way external files work. The source listing that follows is the one you get if you compile them. To compile and animate or run the sample program, use the commands:

If you run this suite of programs more than once, delete isamfil.dat and isamfil.dat.idx after each run.

Here is the source for the extfile.cbl program (also containing the readfile and writefil subprograms):

In extfile, note the SELECT clause (starting on line 20) and the FD clause (starting on line 26). For other programs to access file isamfil.dat, these two clauses must be present, and described identically as they are here. Typically, a copyfile is used for the SELECT and FD clauses. Here, they are simply duplicated in the readfile and writefil subprograms.

In the main-line paragraph (line 35), you see the simple operations performed by extfile: the file is opened, written to, read, and closed. When you look in the write-to-the-file and read-the-file paragraphs, you can see that the read and write operations take place in the subprograms readfile and writefil.

When writefil (beginning on line 113) is called, isamfil.dat is already open, and writefil can access it. Two records are written in writefil.

In readfile, the isamfil.dat file is read (again, without the need for the open), some trivial calculations are performed, and a line of data is displayed.

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