OPEN Statement (File I/O) Details
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OPEN file$ [FOR mode] [ACCESS access] [lock] AS [#]filenumber% [LEN=reclen%]
OPEN database$ FOR ISAM tabletype tablename$ AS [#]filenumber%
Usage Notes
■ The arguments file$ and database$ specify an optional device, followed
by a file name or path conforming to the MS-DOS naming conventions.
The argument database$ must be the name of an ISAM file. If file$ or
database$ do not exist, they will be created.
■ The argument tabletype is the name of a user-defined type that defines
a subset of the table definition. The argument tablename$ is the name
of the ISAM table being opened. It follows the ISAM naming conventions.
See: ◄TYPE Statement► ◄ISAM Naming Conventions►
■ The argument mode is a keyword specifying one of the following
file modes:
Keyword Description
═══════ ═════════════════════════════════════════════════════════
APPEND Sequential output mode and sets the file
pointer to the end of file and the record number to
the last record of the file. A PRINT # or WRITE #
statement then extends (appends to) the file.
BINARY Specifies binary-file mode. In binary mode,
you can read or write information to any byte
position in the file using GET and PUT.
In binary mode, if no ACCESS clause is present,
three attempts are made to open the file. The
attempts follow the same order as those for random
files.
INPUT Sequential input mode
OUTPUT Sequential output mode
RANDOM (Default) Specifies random-access file mode, the
default mode. In random mode, if no ACCESS clause
is present, three attempts are made to open the
file when the OPEN statement is executed. Access is
attempted in the following order:
• Read/write
• Write-only
• Read-only
■ The argument access is a keyword that specifies the operation performed
on the opened file:
Type Description
══════════ ═════════════════════════════════════════════════════
READ Opens the file for reading only.
WRITE Opens the file for writing only.
READ WRITE Opens the file for both reading and writing.
This mode is valid only for random and binary
files and files opened for append.
■ The lock argument works in a multiprocessing environment to restrict
access by other processes to an open file. It can be one of the
following keywords specifying the lock type:
Keyword Description
═══════════════ ═════════════════════════════════════════════════
SHARED Any process on any machine can read from or write
to this file. Do not confuse the shared lock type
with the SHARED statement or the shared attribute
that appears in other statements.
LOCK READ No other process is granted read access to this
file. This access is granted only if no other
process has a previous read access to the file.
LOCK WRITE No other process is granted write access to this
file. This lock is granted only if no other
process has a previous write access to the file.
LOCK READ WRITE No other process is granted either read or write
access to this file. This access is granted only
if read or write access has not already been
granted to another process, or if a lock read or
lock write is not already in place.
■ If you do not specify a lock type, the file can be opened for reading
and writing any number of times by this process, but other processses
are denied access to the file while it is opened.
■ The filenum% argument is an integer expression whose value is between
1 and 255, inclusive. When an OPEN is executed, the file number is
associated with the file as long as it is open. Other I/O statements can
use the number to refer to the file.
■ The argument reclen% is an integer expression less than or equal to
32,767 bytes. It specifies different settings for random-access or
sequential files:
File Mode Description
═════════════ ═══════════════════════════════════════════════════
Random access reclen% sets the record length (the number of
characters in a record); default is 128 bytes.
Sequential reclen% specifies the number of characters to be
loaded into the buffer before the buffer is written
to, or read from, the disk. A larger buffer means
more room taken from Visual Basic, but faster file
I/O; a smaller buffer means more room in memory for
Visual Basic, but slower I/O. Default is 512 bytes.
■ The LEN clause and reclen% are ignored if the file mode is binary.
■ You must open a file before any I/O operation can be performed on it.
OPEN allocates a buffer for I/O to the file or device and determines the
mode of access used with the buffer.
■ If the file is already opened by another process and the specified type
of access is not allowed, the OPEN fails and Visual Basic generates the
error message, "Permission denied."
■ The ACCESS clause works in an OPEN statement only if you are using a
version of MS-DOS that supports networking (DOS version 3.0 or later).
In addition, you must run the SHARE.EXE program (or the network startup
program must run it) to perform any locking operation. If ACCESS is used
with OPEN, earlier versions of MS-DOS return the error message,
"Advanced feature unavailable."
■ For sequential files, reclen% need not correspond to an individual
record size, because a sequential file may have records of different
sizes.
■ The following devices are supported by Visual Basic and can be named
and opened with the file argument:
KYBD:, SCRN:, COMn:, LPTn:, CONS:, PIPE:.
■ The Visual Basic file I/O system allows you to take advantage of user-
installed devices. (See your MS-DOS manual for information on character
devices.)
■ Character devices are opened and used in the same manner as disk files.
However, characters are not buffered by Visual Basic as they are for
disk files. The record length for the device files is set to one.
■ Visual Basic sends only a carriage return at the end of a line. If the
device requires a linefeed, the driver must provide it. When writing
device drivers, keep in mind that other Visual Basic users will want to
read and write control information. Writing and reading of device-
control data is handled by the IOCTL statement and IOCTL$ function.
See:
■ None of the Visual Basic devices directly supports binary mode. However,
line printer devices can be opened in binary mode by adding the BIN
keyword:
OPEN "LPT1:BIN" FOR OUTPUT AS #1
■ Opening a printer in BIN mode eliminates printing a carriage return at
the end of a line.
■ In input, random, and binary modes you can open a file under a different
file number without first closing the file. In output or append mode,
you must close a file before opening it with a different file number.
Usage Notes (ISAM-specific)
■ When you open an ISAM table, the next record is the first record in
the table and the current index is the NULL index.
■ Any ISAM operation that closes a table causes transactions to be
committed. For example, if a type mismatch occurs while you are
opening an ISAM table, the table is closed and a pending transaction
is committed.
■ You may wish to code your programs so they first open all tables,
then perform all transactions, then close tables. Make sure any
operation that can close a table occurs outside a transaction.