ACCESS: Difference between revisions

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The [[OPEN]] statement is used to open a file or [[OPEN_COM|COM]] serial communications port for program input or output.
#REDIRECT [[OPEN#File ACCESS and LOCK Permissions]]
 
 
{{PageSyntax}}
: [[OPEN]] {{Parameter|fileName$}} ['''FOR''' {{Parameter|mode}}] [{{{KW|ACCESS}}|{{{KW|LOCK}}|SHARED}} [{READ|WRITE}] [[AS]] [#]{{Parameter|fileNumber&}} [LEN = {{Parameter|recordLength}}]
 
 
===Legacy ''GW-BASIC'' syntax===
: [[OPEN]] {{Parameter|modeLetter$}}, [#]{{Parameter|fileNumber&}}, {{Parameter|fileName$}}[, {{Parameter|recordLength}}]
 
 
{{Parameters}}
* The {{Parameter|fileName$}} is a [[STRING]] variable or literal file name (path optional) in quotes.
* FOR mode can be: [[APPEND]] (write to end), [[BINARY]] (read/write), [[INPUT (file mode)|INPUT]] (read), [[OUTPUT]] (write new) or [[RANDOM]] (read/write).
* GW-BASIC's {{Parameter|modeLetter$}} is a [[STRING]] variable or the letter "A", "B", "I", "O" or "R" designating the OPEN modes above.
* {{Parameter|fileNumber&}} can be any '''positive''' [[INTEGER]] or [[LONG]] whole number value or an unused value determined by the [[FREEFILE]] function.
* [[LEN]] = or {{Parameter|recordLength}} is optional to denote the RANDOM file record byte length (default = 128) or sequential (default = 512) load buffer.
 
 
{{PageDescription}}
* '''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.'''
* {{Parameter|mode}} defaults to RANDOM if the {{Parameter|mode}} or FOR access statement is omitted. (see open modes described below)
* '''Only the {{Parameter|fileName$}}, {{Parameter|fileNumber&}} and LEN = {{Parameter|recordLength}} values can use variable values in the QBasic syntax.'''
* If [[LEN]] = is ommitted, sequential file record sizes default to 512 and [[RANDOM]] to 128 bytes in Qbasic.
* {{Parameter|fileName$}} 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 [[Keywords currently not supported by QB64|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'''.
 
 
{{PageErrors}}
* Illegal '''QB64''' Windows filename characters are ''' " * / \ | ? : < > '''. Multiple dots (periods) are allowed.
* Possible OPEN [[ERROR Codes|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 (access)|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 (file statement)|WRITE #]] to write numerical or text data or [[PRINT (file statement)|PRINT #]] for text. '''OUTPUT clears files of all data''' and clears the receive buffer on other devices such as [[ON COM(n)|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 (file statement)|WRITE #]] for numerical or text data or [[PRINT (file statement)|PRINT #]] for text as in the OUTPUT mode. '''APPEND does not remove previous data.'''
** '''INPUT''' : Sequential mode '''only reads input''' from an existing file. '''[[ERROR Codes|File error]] if file does not exist.''' Use [[INPUT (file statement)|INPUT #]] for comma separated numerical or text data and [[LINE INPUT (file statement)|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|GET #]] to read or [[PUT|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|GET #]] or [[PUT|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'''.
 
 
{{PageExamples}}
''Example 1:'' Function that displays errors and the number of errors in QBasic filenames. Returns 0 when filename is OK.
 
{{CodeStart}}
file$ = "Hello,~1.mp3"      'example call below
{{Cl|LOCATE}} 20, 30: errors% = CheckName%(file$): {{Cl|COLOR}} 14: {{Cl|PRINT}} "  Total Errors ="; errors%
 
{{Cl|FUNCTION}} CheckName% (Filename$)
  '{{Cl|NOT}}E: Function also displays filename errors so {{Cl|LOCATE}} on screen before call!
  {{Cl|DIM}} L {{Cl|AS}} {{Cl|INTEGER}}, DP {{Cl|AS}} {{Cl|INTEGER}}, XL {{Cl|AS}} {{Cl|INTEGER}}
  L = {{Cl|LEN}}(Filename$): DP = {{Cl|INSTR}}(Filename$, "."): {{Cl|IF...THEN|IF}} DP {{Cl|THEN}} XL = L - DP 'extension
  {{Cl|IF...THEN|IF}} L = 0 {{Cl|OR (boolean)|OR}} L > 12 {{Cl|OR (boolean)|OR}} DP > 9 {{Cl|OR (boolean)|OR}} XL > 3 {{Cl|THEN}}
    CheckName% = -1: {{Cl|COLOR}} 12: {{Cl|PRINT}} "Illegal format!"; : {{Cl|EXIT FUNCTION}}
  {{Cl|END IF}}
  {{Cl|FOR...NEXT|FOR}} i% = 1 {{Cl|TO}} L      'check each filename character"
    code% = {{Cl|ASC}}({{Cl|MID$}}(Filename$, i%, 1)): {{Cl|COLOR}} 10      ' see ASCII codes
    {{Cl|SELECT CASE}} code%      'check for errors and highlight in red
'{{Cl|CASE}} 34, 42 {{Cl|TO}} 44, 47, 58 {{Cl|TO}} 63, 91 {{Cl|TO}} 93, 124: E% = E% + 1: {{Cl|COLOR}} 12 ' '''QBasic errors'''
        {{Cl|CASE}} 34, 42, 47, 58, 60, 62, 92, 124: E% = E% + 1: {{Cl|COLOR}} 12 ' '''QB64 errors'''
        {{Cl|CASE}} 46: dot% = dot% + 1: {{Cl|IF...THEN|IF}} dot% > 1 {{Cl|THEN}} E% = E% + 1: {{Cl|COLOR}} 12
    {{Cl|END SELECT}}   
    {{Cl|PRINT}} {{Cl|CHR$}}(code%);  'use {{Cl|LOCATE}} before {{Cl|FUNCTION}} call to place print
  {{Cl|NEXT}} 
  CheckName% = E%
{{Cl|END FUNCTION}} '' ''
{{CodeEnd}}
''Note: The QBasic character error list is commented out and the function will return invalid filenames under QB64.
 
{{OutputStart}}
                        {{text|Hello|#54FC54}}{{text|,|red}}{{text|~1.mp3|#54FC54}}  {{text|Total Errors|yellow}}<nowiki> = </nowiki>{{text|1|yellow}}
{{OutputEnd}}
:''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:
{{CodeStart}} '' ''
f% = {{Cl|FREEFILE}} 'should always be 1 at program start
{{Cl|OPEN}} "SCRN:" {{Cl|FOR...NEXT|FOR}} {{Cl|OUTPUT}} {{Cl|AS}} #f%
g% = {{Cl|FREEFILE}} 'should always be 2 after 1
{{Cl|OPEN}} "temp.txt" {{Cl|FOR...NEXT|FOR}} {{Cl|OUTPUT}} {{Cl|AS}} #g%
 
{{Cl|FOR...NEXT|FOR}} i = 1 {{Cl|TO}} 2
    {{Cl|PRINT (file statement)|PRINT}} #i, "Hello World, Screen and File version"
NEXT '' ''
{{CodeEnd}}{{small|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.
{{CodeStart}}
{{Cl|CLS}}
 
{{Cl|OPEN}} "test.tst" {{Cl|FOR (file statement)|FOR}} {{Cl|OUTPUT}} {{Cl|AS}} #1
{{Cl|PRINT (file statement)|PRINT}} #1, "If test.tst didn't exist:"
{{Cl|PRINT (file statement)|PRINT}} #1, "A new file was created named test.tst and then deleted."
{{Cl|PRINT (file statement)|PRINT}} #1, "If test.tst did exist:"
{{Cl|PRINT (file statement)|PRINT}} #1, "It was overwritten with this and deleted."
{{Cl|CLOSE}} #1
 
{{Cl|OPEN}} "test.tst" {{Cl|FOR (file statement)|FOR}} {{Cl|INPUT (file mode)|INPUT}} {{Cl|AS}} #1
{{Cl|DO}} {{Cl|UNTIL}} {{Cl|EOF}}(1)
{{Cl|INPUT (file statement)|INPUT}} #1, a$
{{Cl|PRINT}} a$
{{Cl|LOOP}}
{{Cl|CLOSE}} #1
 
{{Cl|KILL}} "test.tst"
 
{{Cl|END}}
 
{{CodeEnd}}
 
{{OutputStart}}
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.
{{OutputEnd}}
:'''Warning:''' Make sure you don't have a file named test.tst before you run this or it will be overwritten.
 
 
{{PageSeeAlso}}
* [[PRINT (file statement)]], [[INPUT (file statement)]]
* [[GET]], [[PUT]], [[WRITE (file statement)]]
* [[INPUT$]], [[LINE INPUT (file statement)]]
* [[CLOSE]], [[LOF]], [[EOF]], [[LOC]]
* [[SEEK (statement)]], [[SEEK]]
* [[OPEN COM]], [[LEN]], [[RESET]]
* [[FIELD]], [[TYPE]]
* [[_FILEEXISTS]], [[_DIREXISTS]]
* [[_OPENCLIENT]], [[_OPENHOST]], [[_OPENCONNECTION]] {{text|(TCP/IP)}}
* [[_SNDOPEN]], [[_LOADIMAGE]]
 
 
{{PageNavigation}}

Latest revision as of 22:02, 8 August 2022

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