"So J has to have a description of that structure contained in the file."
so you'd suggest recreating? I had a hunch that if the file size was increased, that there'd just be some empty space on the end and then the ammend syntax (which is by-reference) would be able to continue doing its work... which would then update the the header area which describes the structure contained. There must be a mismatch on an internal file length count somewhere (a guess). ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
