Look at "Chapter 14 - Testing and Debugging Applications" in the VFP online help file. Click on "displaying output" after searching for "DEBUGOUT".

>Thanks John, the mystery is solved. I was opening a file with:
>nArch = fopen(etc,etc,2), that is with buffer and it always returns -1.
>
>Anyway, im very interested in the asserts and debugout rutines you stated, where can i get information about that?
>
>Thanks very much
>Héctor L.