Open filename For Input [encoding_type] [lock_type] As [#]filenumber
Open filename For Output [encoding_type] [lock_type] As [#]filenumber
Open filename For Append [encoding_type] [lock_type] As [#]filenumber

Open filename For Binary [access_type] [lock_type] As [#]filenumber
Open filename For Random [access_type] [lock_type] As [#]filenumber [Len = record_length]

result = Open( filename[,] For {Input|Output|Append}[,] As filenumber )
result = Open( filename[,] For Binary[,] Access {Read|Write}[,] As filenumber )
result = Open( filename[,] For Random[,] Access {Read|Write}[,] As filenumber [[,] Len = record_length] )
Open filename For {Input|Output|Append} As filenumber
Open filename For Binary Access {Read|Write} As filenumber
Open filename For Random Access {Read|Write} As filenumber [Len = record_length]

filename
encoding_type

• Encoding "ascii" (ASCII encoding is used, default)
  
• Encoding "utf8" (8-bit Unicode encoding is used)
  
• Encoding "utf16" (16-bit Unicode encoding is used)
  
• Encoding "utf32" (32-bit Unicode encoding is used)
  

access_type

• Access [Read] [Write] (both read and write access can be used, which is the default)
  

lock_type

• Shared (the file can be freely accessed by other processes)
  
• Lock [Read] [Write] (both read and write access can be denied to other processes)
  

filenum
record_length

In the first usage, Open returns zero (0) on success and a non-zero error code otherwise.

Opens a disk file for reading and/or writing. The file number file_num is bound to the file on disk, for use in subsequent file operations, such as Input # and Lock. The next available file number can be retrieved with FreeFile.

The Input, Output and Append file modes open disk files for sequential text I/O, useful for reading or writing plain text files.
When the Input mode is specified, only reading file operations can be used, like Line Input # and Get #. If the disk file does not exist a runtime error will be thrown.
The Append mode specifies that only writing operations can be used, like Print # and Put #. Writing operations will take place at the end of the disk file if it exists, preserving the existing data.
The Output mode is like the Append mode, except that if the file exists then its contents are deleted and its length reset to zero before writing.

The Input, Output and Append file modes also allow selection of a character encoding to be used when reading from or writing text to the disk file. ASCII or a Unicode encoding may be specified (see the description of the encoding_type parameter above).

The Binary and Random file modes open disk files for random-access reading or writing of arbitrary sized binary data. The Binary file mode allows reading and writing of simple data type values, like Byte or LongInt, using binary read or write operations, like Get #. LOC and Seek are among the procedures used for reading and writing to arbitrary locations in the disk file. The Random file mode is similar to Binary, except that file I/O operations always use a constant data size when reading or writing.

By default, the Binary and Random file modes allow both reading and writing operations on the opened disk file, but this can be changed by specifying an access type (see the description for the access_type parameter above).

For any file mode, access to the opened disk file can be restricted or granted to other threads or programs by specifying a lock type (see the description for the lock_type parameter above). If no lock type is specified, other threads of the current program can freely open the disk file (Shared), while other programs cannot (Lock Read Write). Lock and Unlock can be used to temporarily restrict access to parts of a file.

The error code returned by Open can be checked using Err in the next line. The function version of Open returns directly the error code as an integer.

' Create a string and fill it.
Dim buffer As String, f As Integer
buffer = "Hello World within a file."

' Find the first free file number.
f = FreeFile

' Open the file "file.ext" for binary usage, using the file number "f".
Open "file.ext" For Binary As #f
If Err>0 Then Print "Error opening the file":End

' Place our string inside the file, using number "f".
Put #f, , buffer

' Close all open files.  
Close

' End the program. (Check the file "file.ext" upon running to see the output.)
End

'OPEN A COM PORT
Open Com "COM1:9600,N,8,1" As #1
If Err>0 Then Print "The port could not be opened."

'COM1, 9600 BAUD, NO PARITY BIT, EIGHT DATA BITS, ONE STOP BIT

'function version of OPEN
If Open("file.ext" For Binary Access Read As #1) = 0 Then

    Print "Successfully opened file"

    '' ...

    Close #1

Else

    Print "Error opening file"

End If

• Linux requires the filename case matches the real name of the file. Windows and DOS are case insensitive.
  
• Path separators in Linux are forward slashes /. Windows uses backward slashes \ but it allows for forward slashes /. DOS uses backward slashes \.
  
• On Windows, a file number used in a dynamic link library is not the same as an identical file number used in the main program. File numbers can not be passed or returned and then used between a DLL and an executable.
  
• If you try to open a directory on Linux, the Open command will succeed.
  
  

• Using MS-DOS device names to open streams or hardware devices ("LPT:", "SCR:", etc.) is supported only in the -lang qb dialect; for other modes FreeBASIC's new composite keywords must be used: see Open Com, Open Cons, Open Err, Open Pipe, Open Lpt, Open Scrn.
  
• Open can be called as a function that returns an error code.
  
  

• The -lang qb dialect supports the old GW-BASIC-style syntax OPEN mode_string, #filenum, filename [length] with mode_string being "I" for input, "O" for output, "A" for append, "R" for random, "B" for binary.
  
  

• Err (and a list of error codes)
  
• Close
  
• FreeFile
  
• Open Cons, Open Err, Open Pipe, Open Lpt, Open Com, Open Scrn