Open statement in fortran

Question

I have question with open command of fortran.

OPEN (UNIT = , FILE=file-name, ACCESS=access, FORM=form, RECL=recl)`
Access = sequential, direct
FORM=formatted, unformatted 

recl is is the record length in bytes for a file

I tried searching a lot, but could not get what is meaning of sequential or direct access, formatted or unformatted file, record length of a file. Can someone explain me what these terms mean?

Answer 1

File access specifies how the file will be written to (or read from) after opening. Opening with one access mode, but reading/writing consistent with another access mode, often results in a runtime error.

Sequential access, naturally enough, implies reading and writing sequentially. Writing sequentially means that output is placed in the output file in the same order that the program produces it so, if X is output before Y, the file will contain X before (closer to the beginning of the file) than Y. Reading sequentially means that reading occurs from start toward end of the file. Append access is a special form of sequential access which starts at the end of the file (so write operations add to the end of the file).

Direct access means that contents of the file can be accessed in any order. This is also called random access. Essentially, when performing input or output, the program must specify the position in the file where the operation is to occur.

The position in the direct access file in Fortran is specified in terms of "records", which all have exactly the same length (specified by the RECL= clause when the file is opened). So, if a file contains 20 records and has record length equal to 30, the total size of data the program can access from the file is 600 bytes, and every read or write operation will access a record containing 30 bytes.

An unformatted file basically means the contents of the file are read and written as a stream. An unformatted sequential access file is the equivalent of a binary file in languages like C that is read from beginning to end. An unformatted direct access file is also binary, but operations can access the file in any order (under control of the program).

A formatted file essentially means that all reading and writing must involve a format specification. There are also some special treatments such as, when writing, a newline marker written to the file at the end of every write statement.

A straight text file is typically opened as a sequential access formatted file. Every Fortran read or write operation acts on a new line (so two write statements will produce two lines in the file, and two corresponding read statements will be need to read them back in).

It is possible to have a formatted direct access file. This basically means the read and write statements must specify formats to read/write the records, but records can be accessed in any order. The ends of records are typically marked with newlines.

Answer 2

It's easy to find on the web (including discussion here):

A "record" is data, usually in characters. Some files have records which are all the same length, some do not. In between, there are files which store the length of each record as part of the record. It is simplest to work with files having records which are all the same length, because (for many storage devices) you can compute the beginning of a particular record by knowing the record number and the length of the records. If the records are different lengths, it is more work to keep track of the record locations.

sequential files are accessed one record at a time, like a tape (see this page for length discussion). As a rule, tapes could be rewound, read forward, but reading at a random point was harder. Doing that is direct access. This page makes it clear that there is a distinct choice between the two - you can have one or the other.

Formatted output is just that - making the output follow some report-style format (on the level of lines), while unformatted output does not follow tidy rules. See Fortran unformatted file format for examples of discussion. On a more technical slant, this page at Oracle goes into more depth.