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DCL:MISC１:Summary:File Structures

1.5.2 UNIX Systems

The files handled by UNIX computers are all basically variable-length record files, and so the concept of blocking does not officially exist. However, it is believed that some sort of blocking is performed inside the computer, although this is not apparent on the user level for FORTRAN.

• SEQUENTIAL File

  In a formatted SEQUENTIAL file, an end-of-line symbol is added to the end of the record like a normal text file. The EOL symbol in UNIX is a single byte (LF), and two bytes (LF, CR) in MS-DOS.

  For an unformatted SEQUENTIAL file, a symbol is added to the start and end (or only at the start) of file for record recognition. This symbol is system-dependent, and there is no compatibility even among systems that use the same internal code. 

• DIRECT File

  Since the record length is specified for a DIRECT file, no symbols are added for record recognition. Thus the file will simply be a sequence of records having the same length.  (In some systems, a specific header may be added at the beginning of a file.)

  Therefore, if the internal representation of a variable is the same, there will be compatibility between most computers. Furthermore, the processing speed may be significantly faster compared to SEQUENTIAL files in some systems.

  In a UNIX computer, the DIRECT file is a convenient format to use, even when simply reading records sequentially.