Open PL/I also provides an I/O method for accessing consecutive files by a numeric key representing the relative record numbers of the records in the file. This is called relative record I/O. It can be used only with files of fixed-length records, and it cannot be used with indexed (VSAM) files.
Relative record I/O files are regarded as keyed files, but the keys are not stored in the file. They are just the ordinal numbers of the records in the file. The keys must be positive integers. Any value used in a KEY or KEYFROM option of an I/O statement must be an integer or be capable of conversion to an integer.
The ENVIRONMENT attribute is not used in the declaration of a relative record I/O file. Instead, such a file is specified by using an extension of the TITLE option of the OPEN statement. The format of this TITLE option is as follows:
TITLE('filename -DAM n')
where n is the length of the records in the file. The length n includes two bytes that are normally added to the beginning of each record. This halfword is used to record special information about the file.(See the section TITLE Options.)
In general, the same set of PL/I I/O operations can be used with relative record I/O files as with indexed files.
A relative record I/O file can be created using an additional extension in the TITLE option, as follows:
TITLE('filename -DAM n -NOSIZE')
The -NOSIZE option indicates that the two-byte special field is not to be added to the data records. If -NOSIZE is specified for a file, the DELETE statement cannot be used with the file.