To establish input/output (I/O) to a file or device.
OPENfilename [FORmode][ACCESSaccess][lock] AS [#]file number [LEN=reclen]
filename is the name of the file.
mode (first syntax) is a string expression with one of the following characters:
Sequential output mode
Sequential input mode
Random input/output mode
Position to end of file
mode(second syntax) determines the initial positioning within the file, and the action to be taken if the file does not exist. If the FOR mode clause is omitted, the initial position is at the beginning of the file. If the file is not found, one is created. This is the random I/O mode. That is, records may be read or written at any position within the file. The valid modes and actions taken are as follows:
Position to the beginning of the file. A "File not found" error is given if the file does not exist.
Position to the beginning of the file. If the file does not exist, one is created.
Position to the end of the file. If the file does not exist, one is created.
Specifies random input or output mode.
modemust be a string constant. Do not enclose modein double quotation marks. accesscan be one of the following:
file numberis a number between 1 and the maximum number of files allowed. The number associates an I/O buffer with a disk file or device. This association exists until a CLOSE or CLOSE file number statement is executed.
reclenis an integer expression within the range of 1-32767 that sets the record length to be used for random files. If omitted, the record length defaults to 128-byte records.
When reclenis used for sequential files, the default is 128 bytes, and reclencannot exceed the value specified by the /s switch.
A disk file must be opened before any disk I/O operation can be performed on that file. OPEN allocates a buffer for I/O to the file and determines the mode of access that is used with the buffer.
More than one file can be opened for input or random access at one time with different file numbers. For example, the following statements are allowed:
OPEN "B:TEMP" FOR INPUT AS #1
OPEN "B:TEMP" FOR INPUT AS #2
However, a file may be opened only once for output or appending. For example, the following statements are illegal:
OPEN "TEMP" FOR OUTPUT AS #1
OPEN "TEMP" FOR OUTPUT AS #2
Be sure to close all files before removing diskettes from the disk drives (see CLOSE and RESET).
A device may be one of the following:
A:, B:, C:...
Keyboard (input only)
Screen (output only)
Line Printer 1
Line Printer 2
Line Printer 3
RS-232 Communications 1
RS-232 Communications 2
For each device, the following OPEN modes are allowed:
Input, Output, or Random Only
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:
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).
Files may be restricted by the implementation of an OPEN statement which allows a process to specify an access to the successfully opened file. The access determines a guaranteed exclusivity range on that file by the process while the OPEN statement is in effect.
lock can be one of the following:
"deny none" mode. No restrictions are placed on the read/write accessibility of the the file to another process, except that the default mode is not allowed by any of the modes including SHARED.
"deny read" mode. Once a file is opened with the LOCK READ access, no other process is granted read-access to that file. An attempt to open a file with this access will be unsuccessful, if the file is currently open in default mode or with a read access.
"deny write" mode. A file successfully opened with LOCK WRITE access may not be opened for a write access by another process. An attempt to open a file with this access will be unsuccessful if the file has been opened in default mode, or with a write access by another process.
LOCK READ WRITE
"deny all" or "exclusive" mode. If a file is successfully opened with this access, the process has exclusive access to the file. A file that is currently open in this mode cannot be opened again in any mode by any process.
"compatibility" mode, in which the compatibility with other BASICs is understood. No access is specified. The file may be opened any number of times by a process, provided that the file is not currently opened by another process. Other processes are denied access to the file while it is open under default access. Therefore, it is functionally exclusive.
When an attempt is made to open a file that has been previously accessed by another process, the error "Permission Denied" will result. An example of a situation generating this error is when a process attempts to OPEN SHARED on a file that is already OPEN LOCK READ WRITE by another process.
If an OPEN statement fails because the mode is incompatible with network-installed sharing access to a device, the error generated is "Path/File Access Error." An example of this is when a process is attempting to OPEN a file for output on a directory that has been shared for read only.
For more information about using files in a networking environment, see the LOCK and UNLOCK statements.
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