GWBASIC User's Manual

GW-BASIC User's Guide
COM2: Input, Output, or Random Only
Disk files allow all modes.
When a disk file is opened for APPEND, the position is initially at the end of the file, and the
record number is set to the last record of the file (LOF(x)/128). PRINT, WRITE, or PUT then
extends the file. The program may position elsewhere in the file with a GET statement. If this is
done, the mode is changed to random and the position moves to the record indicated.
Once the position is moved from the end of the file, additional records may be appended to the
file by executing a GET #x, LOF(x)/reclen statement. This positions the file pointer at the end
of the file in preparation for appending.
Any values entered outside of the ranges given result in "Illegal function call" errors.
The files are not opened.
If the file is opened as INPUT, attempts to write to the file result in "Bad file mode" errors.
If the file is opened as OUTPUT, attempts to read the file result in "Bad file mode" errors.
Opening a file for OUTPUT or APPEND fails, if the file is already open in any mode.
Since it is possible to reference the same file in a subdirectory via different paths, it is nearly
impossible for GW-BASIC to know that it is the same file simply by looking at the path. For this
reason, GW-BASIC does not let you open the file for OUTPUT or APPEND if it is on the same
disk, even if the path is different. For example if mary is your working directory, the following
statements all refer to the same file:
OPEN "REPORT"
OPEN "\SALES\MARY\REPORT"
OPEN "..\MARY\REPORT"
OPEN "..\..\MARY\REPORT"
At any one time, it is possible to have a particular diskette filename open under more than one
file number. Each file number has a different buffer, so several records from the same file may be
kept in memory for quick access. This allows different modes to be used for different purposes;
or, for program clarity, different file numbers to be used for different modes of access.
If the LEN=reclen option is used, reclen may not exceed the value set by the /s:reclen
switch option in the command line.
In a network environment, the use of the OPEN statement is based upon two different sets of
circumstances:
● Devices may be shared on a network for specific purposes only, so that OPEN statements
may be restricted to specific modes among those which might be requested, such as:
INPUT, OUTPUT, APPEND, and default (random).
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