Database development and training for Birmingham and the midlands
Specialists in Microsoft Access and Visual Foxpro database training and development
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FoxPro low level file handling
FoxPro has always had functions to read and write files at a low level but we do not have to use them quite so often in Visual FoxPro. 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 directly with bytes. The older FoxPro functions like FREAD() then become very useful.
FoxPro's 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 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 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.
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:
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.