Many programs are interactive and need to access the user's terminal. Although
the terminal is a file which can be connected with an OPEN statement, its name is
system-dependent. Fortran solves the problem by providing two special files usually
called the standard input file and the standard output file. These files are
pre-connected, i.e. no OPEN statement is needed (or permitted). They are both
formatted sequential files and, on interactive systems, handle input and
output to the terminal. You can READ and WRITE from these files simply by
having an asterisk "*" as the unit identifier. These files make terminal I/O
simple and portable; examples of their use can be found throughout this
book.
When a program is run in batch mode most systems arrange for standard output to be diverted to a log file or to the system printer. There may be some similar arrangement for the standard input file.
The asterisk notation has one slight drawback: the unit numbers is often specified by an integer variable so that the source of input or destination of output can be switched from one file to another merely be altering the value of this integer. This cannot be done with the standard input or output files.
In order to retain compatibility with Fortran66, many systems provide other
pre-connected files. It used to be customary to have unit 5 connected to the
card-reader, and unit 6 to the line printer. Other units were usually connected to disc
files with appropriate names: thus unit 39 might be connected to a file called
FTN039.DAT or even TAPE39. These pre-connections are completely obsolete and
should be ignored: an OPEN statement can supersede a pre-connection on any
numbered unit. Unfortunately these obsolete pre-connections can have unexpected
side effects. If you forget to open an output file you may find that your program will
run without error but that the results will be hidden on a file with one of these
special names.
