| .OPEN (File I/O) Statement Details. |
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| OPEN (File I/O) Statement Details |
|
| Syntax |
| OPEN file [FOR mode1] [ACCESS access] [lock] AS [#]filenum [LEN=reclen] |
| |
| The file is a string expression that specifies an optional device, followed |
| by a file name or path name conforming to the DOS file-naming conventions. |
| |
| 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. |
| |
| Syntax 1 |
| |
| In the first syntax, mode1 is one of the following: |
| |
| Mode Description |
| OUTPUT Specifies sequential output mode. |
| |
| INPUT Specifies sequential input mode. |
| |
| APPEND Specifies 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 may 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 mode1 is omitted, the default random-access mode is assumed. |
| |
| The access expression specifies the operation performed on the opened |
| file. If the file is already opened by another process and the |
| specified type of access is not allowed, the OPEN fails and an error |
| message is generated that reads "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 an error message that reads "Advanced feature |
| unavailable." |
| |
| The access argument can be one of the following: |
| |
| Access 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 clause works in a multiprocessing environment to restrict |
| access by other processes to an open file. The lock types are as |
| follows: |
| |
| Lock Type Description |
| default If locktype is not specified, the file may 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. |
| |
| SHARED Any process on any machine may read from or write |
| to this file. Do not confuse the SHARED lock type |
| with the SHARED statement or the SHARED attribute |
| appearing 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. |
| |
| |
| When the OPEN is restricted by a previous process, it generates |
| error 70, "Permission denied," under DOS. |
| |
| The filenum (file number) argument is an integer expression whose value |
| is between 1 and 255. When an OPEN is executed, the file number is |
| associated with the file as long as it is open. Other I/O statements |
| may use the number to refer to the file. |
| |
| The reclen (record length) argument is an integer expression that, if |
| included, sets the record length (number of characters in one record) |
| for random-access files. For sequential files, the default length for |
| records is 512 bytes; for random-access files, the default is 128 bytes. |
| The value of reclen cannot exceed 32,767 bytes. If the file mode is |
| binary, then the LEN clause is ignored. |
| |
| For sequential files, reclen need not correspond to an individual record |
| size, since a sequential file may have records of different sizes. When |
| used to open a sequential file, 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 BASIC, but faster file |
| I/O. A smaller buffer means more room in memory for BASIC, but slower I/O. |
| The default buffer size is 512 bytes. |
| |
| Syntax 2 |
| |
| OPEN mode2,[#]filenum,file[,reclen] |
| |
| In the second form of the OPEN syntax, mode2 is a string expression the |
| first character of which must be one of the following: |
| |
| Mode Description |
| O Specifies sequential output mode. |
| I Specifies sequential input mode. |
| R Specifies random-access file input/output mode. |
| B Specifies binary file mode. |
| A Specifies sequential output mode and sets the file pointer |
| to the end of the file and the record number to the last |
| record of the file. A PRINT # or WRITE # statement extends |
| (appends to) the file. |
| |
| Note: The second OPEN syntax does not support any of the access and |
| file-sharing options found in the first syntax and is supported |
| for compatibility with programs written in earlier versions of |
| BASIC. |
| |
| The following devices are supported by BASIC and can be named and |
| opened with the file argument: |
| |
| KYBD:, SCRN:, COMn:, LPTn:, CONS:. |
| |
| 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 only sends 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, the line |
| printer devices (LPT1:, LPT2:) 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. |
| |
| Note: 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. |
