The Is data file locked command returns True if the data file of the open database or at least one of its segments is locked — i.e. write protected. 
 Placed, for instance, in the On Startup database method, this command enables the prevention of any risk of accidental opening of a locked data file. 
This method will prevent the opening of the database if the data file is locked: 
  If(Is data file locked)
    ALERT("The data file is locked. Impossible to open database.")
    QUIT 4D
 End if