FoxPro Low Level Functions
FoxPro has always had functions to read and write files at a low
functions introduced in Visual FoxPro gave us an easy way of handling
text files but there are still times when you have to drop back to working
with bytes and unprintable control characters and that's when functions
become very useful.
The low level commands all start with "F" and if you look up any of their entries in Help
then you'll find links to all the others. The five most important are:
creates a file.
opens a file.
closes a file.
reads bytes from a file.
writes bytes to a file.
These commands date back to the earliest days of FoxPro and it
somehow seems more natural to write them in upper case. Even
VFP9 Help still uses this convention.
These are referred to as 'low level' functions because there is very
little between you and the contents of the file. For example, if you use
to read ten bytes from the file then it will try to read ten bytes
starting from the current position in the file and will move its
pointer ten bytes further down the file. This might make the pointer
fall off the end of the file but that's your problem as a programmer.
The advantage of these functions and commands is that they allow you to read every
byte of the file and and you can manipulate binary and control characters that
defeat the text-based commands
These functions all rely on the concept of a file handle. This is
an integer which is returned when you create or open a file and
which you then use in all further operations on that file. You can
use the DISPLAY STATUS command to see
which files are associated with which file handles:
User-opened files: D:\DEV\FRED.TXT Handle=12 Pos=0 Read=Yes Write=Yes
This shows that I have a file named fred.txt open, it is associated with
file handle 12, I'm at the start of the file (Position 0) and I have read
and write access to the file.