* 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.
* '''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: