OPEN

From QB64 Wiki
Jump to navigation Jump to search
The printable version is no longer supported and may have rendering errors. Please update your browser bookmarks and please use the default browser print function instead.

The OPEN statement is used to open a file or COM serial communications port for program input or output.


Syntax

OPEN fileName$ [FOR mode] [{ACCESS|{LOCK|SHARED}} [{READ|WRITE}] AS [#]fileNumber& [LEN = recordLength]


Legacy GW-BASIC syntax

OPEN modeLetter$, [#]fileNumber&, fileName$[, recordLength]


Parameters

  • The fileName$ is a STRING variable or literal file name (path optional) in quotes.
  • FOR mode can be: APPEND (write to end), BINARY (read/write), INPUT (read), OUTPUT (write new) or RANDOM (read/write).
  • GW-BASIC's modeLetter$ is a STRING variable or the letter "A", "B", "I", "O" or "R" designating the OPEN modes above.
  • fileNumber& can be any positive INTEGER or LONG whole number value or an unused value determined by the FREEFILE function.
  • LEN = or recordLength is optional to denote the RANDOM file record byte length (default = 128) or sequential (default = 512) load buffer.


Description

  • QB64 can open as many files as your computer memory can handle. QBasic could only open about 15 at a time.
  • QB64 will allocate 4 bytes of memory for every possible file number up to the highest number used in a program.
  • mode defaults to RANDOM if the mode or FOR access statement is omitted. (see open modes described below)
  • Only the fileName$, fileNumber& and LEN = recordLength values can use variable values in the QBasic syntax.
  • If LEN = is ommitted, sequential file record sizes default to 512 and RANDOM to 128 bytes in QBasic.
  • fileName$ can be up to 255 characters with no limit on file name extension length in QB64.
  • Once a file or port is opened, it can be used in any program procedure using the assigned file number.
  • The "SCRN:" device is supported in version 1.000 and up (see Example 3).
  • Devices such as "KYBD:", "CONS:", and "LPTn:" are not supported in QB64..
Note: OPEN "LPTn" is not supported by QB64, but may be supported directly by your operating system.
  • OPEN COM can also be used for serial port access in QB64.


Errors

  • Illegal QB64 Windows filename characters are " * / \ | ? : < > . Multiple dots (periods) are allowed.
  • Possible OPEN errors include "Bad file name or number", "Bad File Mode", "File Not Found" or "Path Not Found".
    • An OPEN file not found error may occur if CHR$(0) to (31) are used in a Windows file name.
  • QB64 does not have DOS file name limitations.


Details

File ACCESS and LOCK Permissions

  • ACCESS clause limits file access to READ, WRITE or READ WRITE on a network.
  • LOCK clause can specify SHARED or a LOCK READ or LOCK WRITE file lock in an OPEN statement working on a network.
  • A separate LOCK statement can lock or UNLOCK file access on a network using a format that can lock specific records.
  • If another process already has access to a specified file, program access is denied for that file OPEN access. A "Permission Denied" error 70 will be returned. A network program must be able to handle a denial of access error.

File Access Modes

  • FOR mode can be:
    • OUTPUT: Sequential mode creates a new file or erases an existing file for new program output. Use WRITE # to write numerical or text data or PRINT # for text. OUTPUT clears files of all data and clears the receive buffer on other devices such as COM ports.
    • APPEND: Sequential mode creates a new file if it doesn't exist or appends program output to the end of an existing file. Use WRITE # for numerical or text data or PRINT # for text as in the OUTPUT mode. APPEND does not remove previous data.
    • INPUT : Sequential mode only reads input from an existing file. File error if file does not exist. Use INPUT # for comma separated numerical or text data and LINE INPUT # or INPUT$ to only read text data. Use _FILEEXISTS or _DIREXISTS to avoid errors.
    • BINARY: Creates a new file when it doesn't exist or reads and writes to an existing binary file. Use GET # to read or PUT # to write byte positions simultaneously. LEN = statements are ignored in this mode.
    • RANDOM: Creates a new file when it doesn't exist or reads or writes to an existing random file record. Use GET # or PUT # to read or write to file records. A LEN = statement can define the byte size of a record (no LEN statement defaults to 128 bytes)
    • Modes INPUT, BINARY and RANDOM allow a file to be concurrently opened in a different mode and number.


GW-BASIC modes

  • Mode letter is a variable or literal STRING letter value as one of the following:
    • "A" = APPEND.
    • "B" = BINARY.
    • "I" = INPUT.
    • "O" = OUTPUT.
    • "R" = RANDOM.


Examples

Example 1: Function that displays errors and the number of errors in QBasic filenames. Returns 0 when filename is OK.

file$ = "Hello,~1.mp3" 'example call below LOCATE 20, 30: errors% = CheckName%(file$): COLOR 14: PRINT " Total Errors ="; errors% FUNCTION CheckName% (Filename$) 'NOTE: Function also displays filename errors so LOCATE on screen before call! DIM L AS INTEGER, DP AS INTEGER, XL AS INTEGER L = LEN(Filename$): DP = INSTR(Filename$, "."): IF DP THEN XL = L - DP 'extension IF L = 0 OR L > 12 OR DP > 9 OR XL > 3 THEN CheckName% = -1: COLOR 12: PRINT "Illegal format!"; : EXIT FUNCTION END IF FOR i% = 1 TO L 'check each filename character" code% = ASC(MID$(Filename$, i%, 1)): COLOR 10 ' see ASCII codes SELECT CASE code% 'check for errors and highlight in red 'CASE 34, 42 TO 44, 47, 58 TO 63, 91 TO 93, 124: E% = E% + 1: COLOR 12 ' QBasic errors CASE 34, 42, 47, 58, 60, 62, 92, 124: E% = E% + 1: COLOR 12 ' QB64 errors CASE 46: dot% = dot% + 1: IF dot% > 1 THEN E% = E% + 1: COLOR 12 END SELECT PRINT CHR$(code%); 'use LOCATE before FUNCTION call to place print NEXT CheckName% = E% END FUNCTION

Note: The QBasic character error list is commented out and the function will return invalid filenames under QB64.

Hello,~1.mp3 Total Errors = 1

Note: The screen output displays filename characters in green except for red comma QBasic error.


Example 2: When OPEN "SCRN:" FOR OUTPUT AS #f is used, PRINT #f will print the text to the screen instead of to a file:

f% = FREEFILE 'should always be 1 at program start OPEN "SCRN:" FOR OUTPUT AS #f% g% = FREEFILE 'should always be 2 after 1 OPEN "temp.txt" FOR OUTPUT AS #g% FOR i = 1 TO 2 PRINT #i, "Hello World, Screen and File version" NEXT

code by Steve McNeill
Note: Linux or Mac file names can use a path destination such as ".\SCRN:" to use SCRN: as an actual file name.


Example 3: Showcasing different file modes.

CLS OPEN "test.tst" FOR OUTPUT AS #1 PRINT #1, "If test.tst didn't exist:" PRINT #1, "A new file was created named test.tst and then deleted." PRINT #1, "If test.tst did exist:" PRINT #1, "It was overwritten with this and deleted." CLOSE #1 OPEN "test.tst" FOR INPUT AS #1 DO UNTIL EOF(1) INPUT #1, a$ PRINT a$ LOOP CLOSE #1 KILL "test.tst" END

If test.tst didn't exist: A new file was created named test.tst and then deleted. If test.tst did exist: It was overwritten with this and deleted.

Warning: Make sure you don't have a file named test.tst before you run this or it will be overwritten.


See also



Navigation:
Keyword Reference - Alphabetical
Keyword Reference - By usage
Main WIKI Page