lof - length of file
FileNum is the file number used when the file was opened.
returns the number of bytes allocated to the file.
In this example, the program stops if the fileís size
exceeds 50,000 bytes:
if lof(1) > 50000 then stop
A "Bad file number" error occurs if FileNum is less than 1 or greater
than the maximum number of open files.
The Basmark QuickBASIC Programmerís Manual