OPEN - QB64 Wiki


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



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

Legacy GW-BASIC syntax

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


  • 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.


  • 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:", "COMn" 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.


  • 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.


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.
    • 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.


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

Go to Keyword Reference - Alphabetical
Go to Keyword Reference - By usage
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