OPEN Statement (File I/O) Details

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◄Syntax► ◄Details► ◄Example► ◄Contents► ◄Index► ◄Back► ───────────────────────────────────────────────────────────────────────────── OPEN enables I/O to a file, device, or ISAM table. OPEN file$ [FOR mode] [ACCESS access] [lock] AS [#]filenumber% [LEN=reclen%] OPEN database$ FOR ISAM tabletype tablename$ AS [#]filenumber% ■ The arguments file$ and database$ specify an optional device, followed by a filename or path conforming to the 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 mode is a keyword specifying one of the following file modes: ║ Output Sequential output mode. ║ Input Sequential input mode. ║ 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. ║ Random 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: ║ 1. Read/write ║ 2. Write-only ║ 3. Read-only ║ 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. If the mode argument is omitted, the default is random access. ■ ISAM is a keyword that specifies you are opening an ISAM table. ■ The argument tabletype is the name of a user-defined type that defines a subset of the table definition. See ◄TYPE►. ■ The argument tablename$ is the name of the ISAM table being opened. It follows the ◄ISAM naming conventions►. ■ The argument access is a keyword that specifies the operation performed on the opened file: ║ 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: ║ 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 processes are denied access to the file while it is opened. ■ The filenum% 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: For random-access files For sequential files ═══════════════════════ ════════════════════ reclen% sets the record reclen% specifies the length (the number of number of characters to be characters in a record). loaded into the buffer before the buffer is The default is 128 bytes. written to, or read from, the disk. A larger buffer means more room taken from BASIC, but faster file I/O. A smaller buffer means more room in memory for BASIC, but slower I/O. The default is 512 bytes. The LEN clause and reclen% are ignored if the file mode is binary. Usage Notes ■ 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 BASIC generates the error message, "Permission denied." ■ The ACCESS clause works in an OPEN statement only if you are using a version of DOS that supports networking (DOS Versions 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 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 BASIC and can be named and opened with the file argument: KYBD:, SCRN:, COMn:, LPTn:, CONS:, PIPE:. The BASIC file I/O system allows you to take advantage of user-installed devices. (See your 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 BASIC as they are for disk files. The record length for the device files is set to one. ■ BASIC sends only a carriage return at the end of a line. If the device requires a line feed, the driver must provide it. When writing device drivers, keep in mind that other 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. ■ None of the 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. ■ When you open an ISAM table, the next record is the first record in the table and the current index is the NULL index. ISAM Programming Tips ■ 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. Important ■ 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.