FoxPro Low Level Functions

FoxPro has always had functions to read and write files at a low level. The StrToFile() and FileToStr() 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 like FREAD() 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:

  • FCREATE() creates a file.
  • FOPEN() opens a file.
  • FCLOSE() closes a file.
  • FREAD() reads bytes from a file.
  • FWRITE() 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.

Low level

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 FREAD() 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 StrToFile() and FileToStr().

File handles

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.