filptr - address of BASIC file structure
Filptr returns the offset into the current memory segment of the
biotab structure associated with file number fn (see
io(inc)). If fn is zero, the current file is used (see
initio(io)). The current segment is set invisibly by this function. An
offset and seg(mach) together specify an address.
This function resets the current file.
This function implements a version of the BASIC VARPTR function.
io(inc), initio(io), seg(mach)
Error 52 (Bad file number) is generated if fn does not refer to an
Operations involving absolute memory addresses ought not to be used.
The Basmark QuickBASIC Programmerís Manual