File Access Interruption Recovery
You can use the GetPosition or SetPosition methods to minimize the loss of work in the event of a system failure during file access. To use them and recover from access interruptions, you must access the file in Update mode using the GetFile built-in function or the Open method, specifying the mode parameter "U". You need to use this mode in anticipation of possible interruptions if you want to recover from them later.
To start reading or writing from the beginning of a file, use SetPosition to set a read/write position of 0, immediately after you open the file in Update mode.
Warning! In Update mode, any write operation clears the file of all data that follows the position you set.
When reading from or writing to a file, use GetPosition periodically to determine your current byte position in the file. This establishes a checkpoint to which you can return after a failure. You must save the checkpoint value in a separate file or a database so you can retrieve it later. How often you save a checkpoint depends on your requirements: the less work you want to redo, the more frequent your checkpoints should be. You may also want to save information identifying the data you’re reading or writing at the time.
After a failure interrupts your file access, reopen the file in Update mode. You can then retrieve the last checkpoint value you saved, and use SetPosition to apply that value. Your read/write position in the file is the position of the last checkpoint, and you can continue reading or writing from there.
Use the Update mode with GetPosition and SetPosition only for recovering from interruptions as described here, or for starting at the beginning of the file again.
Note: For fixed-width file layouts, the field start positions and lengths are computed in 8-bit bytes, except in the case of UCS2 or UTF-16 where they are computed in 16-bit double-bytes. This may create complications when using variable width encodings (for example, UTF-8 or Shift-JIS), when the data is elsewhere calculated in characters (rather than bytes), because the data involved may contain characters of varying byte-width.