SIMSCRIPT II.5 Programming Language

Programming Language Concepts
3.5 Miscellaneous Input/Output Statements and Facilities
3.5.1 Logical File Assignment: The OPEN Statement
Physical device or data characteristics, specified by execution control commands to the operating
system, are associated with SIMSCRIPT II.5 logical unit numbers through a logical file name. As
mentioned previously, this is usually constructed from the logical unit number to give a name of the
form SIMUnn.
To allow the specification of device or file characteristics within a program, the SIMSCRIPT II.5
language supports an open statement. The open statement provides a means of explicitly specifying
a logical filename and some of the associated data characteristics at program level rather than
at execution command level. Consult the SIMSCRIPT II.5 user manual for specific details of the
parameters.
The general form of the statement is:
open unit device for input (or output), data characteristics
where device is an arithmetic expression evaluating to the logical unit number. The clause for
input or for output designates whether the device is to be used as an input or output device. The
word for is optional. The data characteristics may describe required characteristics of the data
stream. An example of the use is:
open unit 2 for output, file name is "OUTFILE"
use 2 for output
which will open a file, referred to in the future as unit 2, using the logical name "OUTFILE". This
logical file name must be a text variable. If the file name is not specified, the default logical file
name, simu02, is created as described above.
Many file and I/O device data characteristics may be specified at program level in the open statement.
Some of these are common across machines, while some are particular to one machine or
language implementation. In general, however, the characteristics consist of logical filename assignment
and file format and file-type description.
File format parameters contain information on how the file is structured internally. The basic physical
unit of the data file is the record, which usually corresponds to a line of input or output. File
format parameters that may usually be specified are the recordsize, or length of each record, and
whether this size is fixed or indicates the maximum of variable length records. For example:
open unit 2 for output,
file name is "OUTFILE", recordsize = 80, fixed
assigns unit 2, in the output mode, to a file or device whose logical filename is "OUTFILE",
which is composed of 80-character fixed length records.
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