FreeFile

Returns a free file number

Syntax
   Declare Function FreeFile ( ) As Long

Usage
   result = FreeFile

Return Value
   The next available file number, if any, otherwise zero (0).

Description
   Returns the number of the next free file number with valid values 1 to 
   255, or 0 if there are already 255 files opened. This value is a 
   required argument to Open a file. FreeFile is useful when opening files 
   in complex programs where the programmer can't keep track of the used 
   file numbers.

   Make sure to always close files when no longer needed, otherwise you 
   will get a file number leak, and won't be able to open any files anymore 
   after 255 filenumbers are exhausted while your program is running.

   FreeFile will always return the smallest free file number. The file 
   number returned by FreeFile will not change until that file number is 
   Opened, or until a smaller file number is Closed:
      - For this reason, it is wise to use FreeFile immediately before its 
      corresponding Open, to ensure that the same file number is not 
      returned and opened elsewhere first.
      - In case of potential conflict with other threads, this non-breaking 
      'FreeFile...Open' sequence must additionally be considered as a 
      critical section of code and therefore must be protected, for example 
      by mutual exclusion (using a mutex locking).

Example
   ' Create a string and fill it.
   Dim buffer As String, f As Long
   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

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

   ' Close the file.
   Close #f

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

   When using multiple FreeFile statements, FreeFile should be used 
   immediately before the Open statement:
   Dim As Long fr, fs
   ' The CORRECT way:
   fr = FreeFile
   Open "File1" For Input As #fr

   fs = FreeFile
   Open "File2" For Input As #fs

   As opposed to:
   Dim As Long fr, fs
   ' The WRONG way:
   fr = FreeFile
   fs = FreeFile '' fs has taken the same file number as fr

   Open "file1" For Input As #fr
   Open "file2" For Input As #fs '' error: file number already opened

Platform Differences
   * 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.
   * Besides FreeBASIC's limit of 255 files per program opened at same 
     time, there is an OS limit of total amount of opened files, but 
     usually you won't touch it except in DOS, where the limit may be as 
     low as 15 files total.

Differences from QB
   * None

See also
   * Open
   * Put (File I/O)
   * Get (File I/O)
   * FileAttr

